Battery bank minimum charge current

phoenixfire

Hello,
This is my first post to this forum. I have had a functional PV system for about 18 months now. Originally it consisted of (3) KC130TM panels, 8 Sam's Club (interstate) GC2 210AH batteries. and a Xantrex C40 Charge controller. Here is a summarized list of what it consists of now.

(3) KC130TM PV panels
(2) KD135SX-UPU PV Panels

Xantrex C40 Charge Controller

10 Trojan L16H-AC Batteries (brand new in jan)

Tripp Lite 2000watt Inverter/Charger

Go Power 1000 Watt Pure Sine Inverter

John Deere 3200 Watt Inverter Generator

This winter I noticed our original GC2 batteries were lacking in capacity so we bought the 10 Trojan batteries. These batteries are rated at 435AH/C20. We have a 12v system so the batteries are arranged in five strings of two. Total battery bank capacity: 2175AH.

The first question most are going to ask is why two inverters. The Tripp Lite runs our Freezer (in the root cellar, 41deg ambient) and a chest freezer conversion to refrigerator. Originally this was supposed to be our main and only inverter however after finding out it has a 2.5 amp no load current I changed my mind. It now runs in standby mode until either the freezer of refrigerator kick on. The Go Power inverter runs everything else in the house. Laptops, lights etc.

Our approximate power usage is 1.5kwh per day. One third of which goes to laptops, one third to the freezer and refrigerator, and one third to everything else.

This usage ~121AH per day.

My array total is 660w which should equate to about 120AH or charge per day.

My concern is after reading these forums; I have seen in more than one post that the minimum charge current to charge batteries is 5% of the c/20. According to the literature from Trojan they only list the maximum current. There is no minimum current requirement in any of their literature that I have found so far. I realize that my peak charge current is about 2.29% which falls way below the stated "minimum" I have seen quoted here.

My question is: Is this a hard minimum? or is all that matters is the batteries receive a full charge cycle as often as possible? I have had completions of the charge cycle many times but normally only on very good sunny days. (on other days I will start the generator for about 1 hour to help the bulk charge process along) I have the C40 set to 14.8v as the absorption voltage so the controller takes the bank to 14.8 and holds it there for one hour then drops down to the float voltage. I certainly do recognize I need to add some more panels as I am hovering way to close to the bare minimum to charge the batteries, however the 5% rule I have seen quoted would require a bare minimum of 11 panels to achieve. More than doubling my current system, that would also require either replacement or addition of charge controller(s) since the 5% charge current is 108amps at 12V. This is an off-grid home with most things requiring 12v (rv pumps, booster pump for reverse osmosis, Marine macerating toilet) with exception of lighting and the appliances already mentioned.

Advice/concerns/questions welcome

Scott

niel

Re: Battery bank minimum charge current

welcome to the forum.
i do believe i saw trojan state 10-13% for their l-16s, but i don't remember where. anyway, you are coming up way short and this could cause some capacity loss and sulfation. it's good that you have the generator to bring things up to speed and you may want to do that early in the day so that the final charge stages may be done with the pvs providing there isn't too much loading going on at the same time because a load will subtract from the charge going to the batteries and that's low enough as it is.
if you want to keep the batteries a reasonable amount of time then more than double your pv system and get yourself a good mppt controller regardless for it will recoup some of the power that the c40 will normally lose (may average about 10% or so more than the c40 and you may want to keep the c40 for backup to your system). do not add any more loads until you can meet the batteries' needs better. you will still need to run the generator even when you reach past the 5% charge rate for the sun she does not always shine. ideally, if you could try to attain a 10% rate this would help you tremendously, but we understand that the $ for pvs does not grow on trees. do what you can, but make the mppt controller a definite.

BB.

Re: Battery bank minimum charge current

Scott,

phoenixfire wrote: »

This winter I noticed our original GC2 batteries were lacking in capacity so we bought the 10 Trojan batteries. These batteries are rated at 435AH/C20. We have a 12v system so the batteries are arranged in five strings of two. Total battery bank capacity: 2175AH.

Batteries with falling capacity (I don't know how old your batteries are--premature or not?)--Many times is the result of operating the batteries for significant lengths of time below 75% state of charge. An indication of "deficit charging".

A good place to start is reading a couple of battery FAQs:

Deep Cycle Battery FAQ
www.batteryfaq.org

Batteries are the only part of the system that are really sensitive to how you use them. Knowing the requirements will help you avoid many of the common mistakes that take out batteries early (in months or a couple of years, vs 5-10+ years for good quality batteries with reasonable use).

All of the rules of thumb (5-13% rate of charge, etc.)--are to help design a system that the owner will be happy with from the start.

In your case, you already have the system, so we need to gather some data and see what can be improved upon.

So, I would be interested in the temperature compensated specific gravity of your batteries. You should check all cells a few times over a few days, then just check one pilot cell as needed to ensure the bank is properly charged.

The first question most are going to ask is why two inverters. The Tripp Lite runs our Freezer (in the root cellar, 41deg ambient) and a chest freezer conversion to refrigerator. Originally this was supposed to be our main and only inverter however after finding out it has a 2.5 amp no load current I changed my mind. It now runs in standby mode until either the freezer of refrigerator kick on. The Go Power inverter runs everything else in the house. Laptops, lights etc.

Good solution and good catch on designing your system properly. Picking the right hardware and setting up for conservation of power is a great start.

Our approximate power usage is 1.5kwh per day. One third of which goes to laptops, one third to the freezer and refrigerator, and one third to everything else.

I was going to say that sounds really low for daily power usage--but the conversion fridge is a real power saver if done right. And the laptops are another good move.

So far so good.

This usage ~121AH per day.

My array total is 660w which should equate to about 120AH or charge per day.

Now, you have not said where the system is installed... If it is somewhere near Bolder Colorado, we can use the PV Watts Program to give you seasonal (or even daily) estimated power collection:

A good place to start is assume ~0.52 derating (solar panel to charge controller to flooded cell battery to inverter to AC load). Start with 1kW of solar panels (sorry, minimum program will accept). And a fixed array:

"Station Identification"
"City:","Boulder"
"State:","Colorado"
"Lat (deg N):", 40.02
"Long (deg W):", 105.25
"Elev (m): ", 1634
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 40.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.4 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.43, 71, 5.96
2, 4.89, 70, 5.88
3, 6.05, 94, 7.90
4, 6.09, 88, 7.39
5, 5.99, 87, 7.31
6, 6.08, 83, 6.97
7, 6.06, 83, 6.97
8, 6.24, 86, 7.22
9, 6.25, 86, 7.22
10, 5.67, 84, 7.06
11, 4.60, 69, 5.80
12, 4.29, 68, 5.71
"Year", 5.56, 967, 81.23

So, based on 1,000 watts of solar panel, we would expect around 68 to 94 kWhrs per month depending on season. Toss out the lowest three months (assume you use genset) we get 71 kWHrs per month in January. Or for 660 Watts of panel:

• 71 kWH per 1kW of panel per month * 1/30 days * 0.66 kW of panels = 1.56 kWH per day = 1,560 Watt*Hours per day in January.
• 1,560 WH / 13 volts = 120 Amp*Hours

So, that is pretty close to what you estimate your useful harvest is.

My concern is after reading these forums; I have seen in more than one post that the minimum charge current to charge batteries is 5% of the c/20. According to the literature from Trojan they only list the maximum current. There is no minimum current requirement in any of their literature that I have found so far. I realize that my peak charge current is about 2.29% which falls way below the stated "minimum" I have seen quoted here.

5% is recommended for several reasons... One is that for equalization of a battery bank is a good compromise for mixing electrolyte. However, equalization is only needed once a month +/- (for flooded cell banks). So, you can do the equalization with a generator/battery charger.

Another reason is that batteries have self discharge--that tends to get worst the older the batteries are. You can maintain a battery on a 1% rate of charge, but much of that energy goes into keeping up with the self discharge rate. If you are storing batteries (such as a cabin in the winter), 1% can be fine.

One of the big killers of batteries is storing/operating them for long periods of times (days, weeks, months) below 75% state of charge. With a low rate of charge (2-3%), it can take a long time for the battery to be recharged back up above > ~90% state of charge. The longer the batteries spend <75% state of charge, the more the sulfates "harden" and the more capacity loss you will experience.

My question is: Is this a hard minimum? or is all that matters is the batteries receive a full charge cycle as often as possible? I have had completions of the charge cycle many times but normally only on very good sunny days. (on other days I will start the generator for about 1 hour to help the bulk charge process along) I have the C40 set to 14.8v as the absorption voltage so the controller takes the bank to 14.8 and holds it there for one hour then drops down to the float voltage. I certainly do recognize I need to add some more panels as I am hovering way to close to the bare minimum to charge the batteries, however the 5% rule I have seen quoted would require a bare minimum of 11 panels to achieve. More than doubling my current system, that would also require either replacement or addition of charge controller(s) since the 5% charge current is 108amps at 12V. This is an off-grid home with most things requiring 12v (rv pumps, booster pump for reverse osmosis, Marine macerating toilet) with exception of lighting and the appliances already mentioned.

Monitor your electrolyte Specific Gravity (crank up the genset if you are below 75% state of charge and bad weather ahead--which it sounds like you do). Also monitor your water usage---too little water usage may mean that you are not charging enough (in your case, you may want to put the Absorb cycle up to 2+ hours)... A lot of water usage, you may want to crank down the voltage/absorption times).

In the end, you are probably going to mix generator run time and $$$ for solar panels.

You can possibly reduce genset run time by starting the genset in the morning (when needed) and "bulk charging" the bank (to 80%+ state of charge) and then let the solar take over for the rest of the day.

Another rule of thumb we use for sizing a battery bank is the 3 days of no sun and a 50% maximum discharge level... For 120 AH per day:

• 120 AH * 3 days * 1/0.50 discharge = 720 AH of 12 volt battery bank

Your current bank is 2,175 AH--Or almost 3x larger than this rule of thumb.

While it is nice to have a large battery bank (run it ~75-90% state of charge) -- it does require more charging current to keep the batteries mixed/equalized.

If you had a smaller 720 AH battery bank, you solar panels would be charging at a ~7-9% rate--which is right in the middle.

I would monitor your specific gravity and water usage (I also like a battery monitor if you don't have one, much easier to read a XX% gauge vs taking SG samples--very nice for spouse, kids, guests who may not pay as much attention)--and if you are using reasonable amounts of water, don't have cell s.g. balance issues, are keeping the battery bank mostly in the 75-90%+ state of charge, and are getting reasonable battery life--I would not 2-3x your array size just to match the 5% rate of charge for you battery bank.

If your battery bank is running below 75% state of charge (for days or longer) and/or you are spending too much for generator fuel (and noise+fumes+etc.)--then look at adding solar panels to your system.

My 2 cents worth of comments--Over all, it sounds like you have a good handle on your system.

-Bill

BB.

Re: Battery bank minimum charge current

A couple other points to ponder (possibly you have already addressed):

• Paralleling battery bank: Personally, I think that more than about 3 parallel battery strings are difficult to ensure battery current is shared among the strings. Are all of the strings connected with the same length wire and all sharing equal current? Or are some batteries closer to the load/chargers than others--Not a good thing.
• Paralleling battery banks also means that a fuse/breaker per string should be installed to prevent inter-bank shorts from causing fires.
• And, fuses/breakers should be used on all wiring leaving the + bus/bank connection (proper fuses/breakers sized for wire gauge).
• Next time, look at getting larger cells (for example, a few vendors offer large 2 or 4 volt high capacity cells---allows you to build one or two string banks).
• Your loads sound reasonably small--but for such a large AH/Capacity bank, one would wonder if 24 or 48 volt battery setups are in your future? Helps keep battery wiring current lower (generally, if you peak continuous draw is over ~1,200 watts, you should look at a higher voltage bank).
• Why such a large bank? Are you looking at adding loads in the future?
• How long did your last bank last? Too large of bank ties up lots of money unless you have special needs (like a deep well water pump, shop tools, etc. where you need surge current and/or are looking to add a small A/C system in the future, etc.).

-Bill

nsaspook

Re: Battery bank minimum charge current

BB. wrote: »

Scott,

You can possibly reduce genset run time by starting the genset in the morning (when needed) and "bulk charging" the bank (to 80%+ state of charge) and then let the solar take over for the rest of the day.

-Bill

I want to say thanks for that idea. I just included your morning boost logic in my DIY monitor code to check for low SOC in the morning and turn on the charger with a (timer or for SOC >80) as a selectable option.

tallgirl

Re: Battery bank minimum charge current

BB. wrote: »

A couple other points to ponder (possibly you have already addressed):

• Paralleling battery bank: Personally, I think that more than about 3 parallel battery strings are difficult to ensure battery current is shared among the strings. Are all of the strings connected with the same length wire and all sharing equal current? Or are some batteries closer to the load/chargers than others--Not a good thing.

-Bill

With the proper tools, this isn't that hard to figure out. For each battery, in each string, measure the voltage DIFFERENCE between that battery and its sibling. Not only will that tell you what's going on between strings, it will also tell you immediately where the problem is. From there, the typical solution is cleaning the connection between the cable and terminal -- that's where the majority of the voltage drop happens anyway, not the minuscule fractions of ohms difference because one wire is two or three inches longer than the other.

icarus

Re: Battery bank minimum charge current

Sounds like a classic too much battery, not enough charge current situation.

I would suggest that you get a good battery monitor, more closely couple your load/charging capacity/battery bank size.

FYI,

We have ~400 watts of PV, and we routinely draw ~600 wh out of 4 T-105s (~450ah) everyday. We routinely charge at ~25 amps,, a bit on the low side, but works pretty well.

A pretty common misconception is that bigger battery banks are better. (If I have 20 kwh capacity but only draw 1 kwh/day, it should last forever right? Wrong!)

Since your l-16's are nearly new, you might consider taking a number of them off line, keeping them in monthly float, and then run the bank more efficiently with a smaller bank, holding the others in reserve. (or sell them to someone,, even if you only get $.50 on the dollar, it might be "cheaper" than having all of them fail prematurely).

Tony

phoenixfire

Re: Battery bank minimum charge current

Batteries with falling capacity (I don't know how old your batteries are--premature or not?)--Many times is the result of operating the batteries for significant lengths of time below 75% state of charge. An indication of "deficit charging".

There were not old just crappy batteries. The original batteries were the interstate GC2 from Sam’s club that were poor performers from the beginning. I usually start the generator when the batteries are at or before 75% actually when the voltage at the batteries shows 12.4 or below even under load I start the genset unless there is a very high current draw (like freezer and fridge running at the same time)

So, I would be interested in the temperature compensated specific gravity of your batteries. You should check all cells a few times over a few days, then just check one pilot cell as needed to ensure the bank is properly charged.

Last night I did a thorough charge of the batteries via the genset. After the Trip Lite went to float (14.4v) I switched to my black and decker 40 amp auto charger (which takes the batteries to 14.8 for a few hours under no load) I ran that for about 2 hours until the DC load almost equaled the charge current. Then I checked the Specific gravity and got 1.274 +/- .001 per cell.

I routinely (about once per week) do the above procedure minus the Specific gravity (about once per month) the rest of the time I charge with the genset and the trip Lite in the morning and let the PV do the work the rest of the day.

I was going to say that sounds really low for daily power usage--but the conversion fridge is a real power saver if done right. And the laptops are another good move

That fridge uses less than 200wh per day unless it gets really hot. It’s quite amazing really. All I did was buy a industrial DIN rail thermostat from Newark Electronics and install it inside a fan circulated cage inside the unit insulated from the interior walls. I set the thermostat to about 39deg (chest freezers continue to cool quite a bit after they turn off) this thing stays between 32.5 and 40 and runs very little and for very short bursts of time. I think everyone should use one of these.

Now, you have not said where the system is installed

In the mountains of Montana with solid view of the sun from 10am until about 1 hrs before sunset.

And a fixed array

Actually I built a Manual tracker top of pole mount for them. I set the declination about one per month or two. And swing the array usually three times per day. (about 30 deg east of south, south, and 30 deg west of south) then after sunset back to the start position to be ready for the AM. I need to get the meter for the C40 to see what I am actually putting out when I track them.

% is recommended for several reasons... One is that for equalization of a battery bank is a good compromise for mixing electrolyte. However, equalization is only needed once a month +/- (for flooded cell banks). So, you can do the equalization with a generator/battery charger.

This is exactly what I have been doing. I equalize using the Tripp Lite about once per month.

Monitor your electrolyte Specific Gravity (crank up the genset if you are below 75% state of charge and bad weather ahead--which it sounds like you do). Also monitor your water usage---too little water usage may mean that you are not charging enough (in your case, you may want to put the Absorb cycle up to 2+ hours)... A lot of water usage, you may want to crank down the voltage/absorption times).

I do monitor the Specific gravity however once I got a feel for the voltage to gravity relationship based on current load I was able to determine when the bank needed charged based on the current voltage and current load on the bank. Honestly I am probably being too picky about charging them as I know from experience that when I start the generator when the voltage read 12.35 and load is 14A but in that same case if I removed all loads and waited an hour the voltage would return to 12.55 or higher. But I figure short cycling with batteries is bad unless you severely overcharge them which is almost impossible with the currents I can provide for the size of the bank. What constitutes too little or too much water usage? These batteries are only 4 months old and I have yet to see any change in the water level. However I understand that L16s have a huge water capacity, so not sure how to gauge it. I wish the C40 had time settings. It is hard coded for 1 hour absorption.

Another rule of thumb we use for sizing a battery bank is the 3 days of no sun and a 50% maximum discharge level... For 120 AH per day:

120 AH * 3 days * 1/0.50 discharge = 720 AH of 12 volt battery bank

Being an engineer and oddly into overkill (unusual combo for my profession)

I used the same basic calculation but changed a few things.

150AH * 4 days * 1/0.25 Discharge = 2400AH

The 150AH is what I was getting in the peak of the summer months, 4 days seemed better than 3, and 25-30% DOD seemed way better than 50% especially since my understanding is sulfation doesn’t become a big problem until battery state falls below 75%

The thing I missed is the 5-13% charge current or I would not have done it this way.

Paralleling battery bank: Personally, I think that more than about 3 parallel battery strings are difficult to ensure battery current is shared among the strings. Are all of the strings connected with the same length wire and all sharing equal current? Or are some batteries closer to the load/chargers than others--Not a good thing.

I actually use an inductive DC current meter on the series leads on each string. I also check the voltage on each segment of each string. This lets me see if current is being equally applied/removed. If I find (never with this bank so far) a set with a disparity I remove them from the bank and repeatedly charge/equalize the set and put back in service.

Paralleling battery banks also means that a fuse/breaker per string should be installed to prevent inter-bank shorts from causing fires.

This is something I understand the "need" for but I can’t say I have ever seen it actually done in practice.

Next time, look at getting larger cells (for example, a few vendors offer large 2 or 4 volt high capacity cells---allows you to build one or two string banks).

Originally I had ordered the surrette 4v cells but when they were put on backorder until may (keep in mind this was at the end of December) and battery prices were going up almost 10% on Jan 1 I opted to go with the Trojans. Sometimes I think I should have waited but doesn’t matter now. I really like the surrette batteries but sometimes availability can be a nightmare with them.

Your loads sound reasonably small--but for such a large AH/Capacity bank, one would wonder if 24 or 48 volt battery setups are in your future? Helps keep battery wiring current lower (generally, if you peak continuous draw is over ~1,200 watts, you should look at a higher voltage bank).

I originally wanted to goto 48v but the issue of my 12v loads became a serious issue as the DC-DC converters out there have relatively small current capacities. And pulling of individual sets of batteries within a string causes lots or problems as well. Not to mention the limited availability of 48v inverters compared to 12 or 24v. With my current system the current isn’t bad but if I must increase my solar to above 1.5kw then it starts to become harder to keep efficiency up at those high currents.

Why such a large bank? Are you looking at adding loads in the future?
How long did your last bank last? Too large of bank ties up lots of money unless you have special needs (like a deep well water pump, shop tools, etc. where you need surge current and/or are looking to add a small A/C system in the future, etc.).

My old bank is still here but not used. It was ok, I just felt I started with junk batteries and wanted to get it right the second time. No new loads hopefully. Actually I hope to cut my loads some in the future. Currently I am a consulting engineer so I use allot of computer time. I am hoping to move toward a more agrarian lifestyle drastically reducing the computer time and possibly lowering my loads by as much as 25%

After taking all this in. I am thinking of splitting my bank in two. I already have 400amp battery selector switches on both the bank and the charge controller. I could split the bank into 1305AH and 870AH banks. Alternate them weekly to make sure they get exercised properly. Literally all it would take to do this is to take off one set of parallel cables and attach the second bank cables (already in place) to the second half of the bank.

Do you think this would be beneficial? At least until I can muster some more Panels. I am thinking maybe of adding a second charge controller and second array that I can switch between banks. Making it possible to put the banks together. Separate. And change them together. Or separate in any configuration that makes sense that day.

As far as MPPT controllers go. Is it really worth it? 10% is something but not allot. Seems adding an additional panel would give more than 10% for the same money spent on the MPPT.

I was considering getting a FM80 so I could bring all five panels in as a series array. Making wire losses lower especially if I have to increase the array power. What is the efficiency like for the flex80 charging a 12 volt bank at 60 amps with incoming voltage at say 90 or so? Basically I am wondering if it is worth it to series all 5 (and maybe 6) panels running into the flex80 vs trying to bring upwards of 50-60 amps in on a cable. I do understand this is what it is designed for but this seems like the fringe of its abilities so it concerns me if it is worth it.

Thanks again and looking forward to responses.

Scott

phoenixfire

Re: Battery bank minimum charge current

icarus wrote: »

Sounds like a classic too much battery, not enough charge current situation.

I would suggest that you get a good battery monitor, more closely couple your load/charging capacity/battery bank size.

FYI,

We have ~400 watts of PV, and we routinely draw ~600 wh out of 4 T-105s (~450ah) everyday. We routinely charge at ~25 amps,, a bit on the low side, but works pretty well.

A pretty common misconception is that bigger battery banks are better. (If I have 20 kwh capacity but only draw 1 kwh/day, it should last forever right? Wrong!)

Since your l-16's are nearly new, you might consider taking a number of them off line, keeping them in monthly float, and then run the bank more efficiently with a smaller bank, holding the others in reserve. (or sell them to someone,, even if you only get $.50 on the dollar, it might be "cheaper" than having all of them fail prematurely).

Tony

This really must be a VERY common issue. As I know at least 10 people with solar and not a single one of them meet the 5-13% charge current requirement. I am guessing the only reason they function is they do not use that much power so their huge battery banks get recharged and basically trickle charged every day.

For example I know one person who has 1kw in panels. (old ones that are not very efficient) and 4800AH of battery. This is similar to my situation but i beleive he uses less than half my load per day. His reasoning is he has up to a month of capacity without sun. He really doesn’t even own a generator.

Another guy has about the same capacity as me but consumes at least 2.5-3 times my loads per day. and has about 1850w of array. he does run his generator ALOT. (the difference lots of kids makes)

Here is the funniest part. The guy just above is a solar installer. So I am guessing even the pros make this mistake quite a bit.

Scott

icarus

Re: Battery bank minimum charge current

Personally, I think a MPPT controller is worth it for two reasons. The first is to be able to run the PV at higher Voltages and reduce wiring losses. Second, I routinely see 10-30% gain, depending on SOC and temps. You are right, that adding a number of PV watts would achieve the same result, but you lose a great percentage of that potential as well.

The one caveat about MPPT is if you wire wire all your PV into a series array, you are much more subject to partial shading issues than if you wired them in other combinations of series/parallel. For example I wire all my panels @ 12 vdc parallel array simply because I have shading issues that would more adversely effect output if I wired them all in series, or even series/parallel.

Good luck, and welcome to the forum,

Tony

icarus

Re: Battery bank minimum charge current

Sorry for the second post, they passed one another!

The misconception is, (among others!) if you are only drawing ~5% of ah capacity, you are in effect only drawing off the surface charge. You can do this for months or years, but it sort of defies the logic of a big battery bank. And then if you had a big load it would go dead quickly. Let's say you have a $1000 battery bank that you draw 5% daily, and because you never really charge it, it lasts (useful) 5 years, so a cost of $200/year.

On the other hand, you have $250 battery bank that you draw (same load,, all else being equal) ~20% daily, but charge properly and it lasts the same five years, your battery cost would be $50/year.

Just to illustrate the point,, when we built our new system, the choice was 4 T105s @ $400 or 4 L-16 @ $600 (don't remember the real numbers anymore)

The calculation was if the T-105's lasted 5 years, as opposed to 7 for the L-16s then it was cheaper net net to buy the smaller, less expensive battery bank, and replace it sooner. Of course if the L-16's ran 10 years and the t-105s only five I would lose, but one has to consider the time value of the money as well. We just figured we would be way ahead with the smaller/cheaper bank. So far, in year three, they are testing like new.

Tony

PS I am beginning to become a firm believer that there is a delicate balance to be had between loads/charge ability/battery bank size. After a number of months or years one can get a good handle on the average daily/weekly loads and average daily and weekly PV current available, (which is much more useful than annual numbers IMHO). Once one gets a good handle on loads, the other two parts then solve themselves. If I had to err, I would err on too much PV, since loads always have a way of growing. Excess Pv capacity can easily be manage (within reason) by load shifting, using the excess to charge lap tops, or power tool batteries, or pump extra water and so forth when you have more than enough sun, and yet have enough, or nearly enough when you have less than perfect sun.

As to your friend who never runs a gennie, I would guess at some point along the line he passed a point of diminishing returns. For example if I had a system big enough to provide 100% of my electrical needs 365, it would be way too big 350 days per year. For example, by making the choice to power the shop with the gennie, reduced my Pv costs by ~75% net/net, ad the cost of a few gallons of gasoline per month.

T

niel

Re: Battery bank minimum charge current

"The 150AH is what I was getting in the peak of the summer months, 4 days seemed better than 3, and 25-30% DOD seemed way better than 50% especially since my understanding is sulfation doesn’t become a big problem until battery state falls below 75%

The thing I missed is the 5-13% charge current or I would not have done it this way."

as to sulfation, that can occur with higher socs too. it is vital to get the batteries fully charged up ever few days maximally and ideally every day. anytime the batteries are not full this sulfation process starts up. the longer a battery stays undercharged the further along that sulfation damage is and a lower capacity may be seen as a symptom. that 75% figure bb cites is an arbitrary number and is meaningless if you cause damages by not bringing the battery to full charge often enough. this means that even bringing it to 95% for a week will still cause damage due to the 5% deficit charge.
unfortunately there are inefficiencies to overcome and even the self discharge of the batteries so that 5% charge rate is important unless you can dump it into the batteries from the generator (assuming it has a high enough charge rate and you can stand the noise and long run times), but it would be more often and more time in running it. the dod not dipping too low will allow more cycle life, but it has to reach full charge and is not a goal to keep a battery's dod at.

phoenixfire

Re: Battery bank minimum charge current

The one caveat about MPPT is if you wire wire all your PV into a series array, you are much more subject to partial shading issues than if you wired them in other combinations of series/parallel. For example I wire all my panels @ 12 vdc parallel array simply because I have shading issues that would more adversely effect output if I wired them all in series, or even series/parallel.

I am not against the MPPT controller. But if I were to buy one I would most definately want to take advantage of being able to series my array to reduce losses/cableing needed to bring in such high currents.

My question still remains is the efficiency of the MPPT worth the conversion to changing the array to series. For example if the controller was losing 5% due to conversion loss from 120vdc down to 14.8vdc then using heavier wiring and bringing in the array at 18v would be more beneficial by spending the money on wiring to keep losses below 2%

I understand that all charge controllers have losses simply in the charge process.
here is an example:

a. Xantrex C40 bulk charging @ 40A with the array all in parallel

b. Outback FM80 bulk chaging @40A with the array all in parallel

c. Outback FM80 bulk charing @40A with the array in series

both a and b using heavier gauge wiring to keep losses below 2%
c would not require heavier cable and the current would only be 10A max (130w panels)

I understand the concept of MPPT converting excess voltage to the minimum voltage required to get maximum current. I also understand that the losses in DC-DC can offset at least some of the benfit.

The concensus seems to be that the difference between a nd b would be about 10% on averge.

C is what I am most interested in. Is the DC-DC loss greater than or equal to the benefit of using higher voltages? The answer would appear to lie in the length of your cable runs. as my total run from array to controller is only about 30ft i am going to guess it would be cheaper to just buy some heavier cable and subvert the DC-DC losses. The formulas for DC-DC efficiency are greatly rooted in the difference between input and output voltages. It would appear the overall efficiency of the FM80 would be greatly affected by 120v-12v conversions unless your cable runs were so long it was impossible to reduce losses without the conversion.

obviously this is all hypothetical based on imformation provided by the manufacturer. I am really interested to see peoples individual actual experience with this as the manufacturer data is unlikely to be accurate.

Currently My cableing from my array is 1/0 but if I need to increase my array size I am expecting to have to increase it to 4/0 and move to copper instead of aluminum.

icarus

Re: Battery bank minimum charge current

"Currently My cableing from my array is 1/0 but if I need to increase my array size I am expecting to have to increase it to 4/0 and move to copper instead of aluminum."t


Back to the MPPT issue for a minute. The biggest advantage of MPPT is taking the excess voltage (that is higher than battery voltage) and converting it into useable current at battery voltage.

For example 17.4 volts @ 10 amps from the PV would be ~174 watts.

That same 10 amps in to a 12.0v battery would be 120 watts,, a 54 watt loss.

Now understanding that nothing is 100%, but if the MPPT controller turned that 174 watts into 12.0 vdc it might put is something like 13 amps. (Example only, your mileage may vary!)

I know that my Rogue controller (as verified by the tri-metric routinely converts PV voltage of ~17.4, into the battery ~13-14.6 vdc at a rate of ~98%. If the temp is cold and the batteries are low, I can see huge increases for a while in the morning. Fact is I seldom let the batteries get low enough to really see the huge % increase,, say from 17.4 to 12.0.

Tony

niel

Re: Battery bank minimum charge current

mppt controllers are not just hype from manufacturers and they do harvest on average about 10%, but i've seen it around 25-30% at times and that is more apt to happen when socs are lower and no they don't have to be real low to achieve that much, but lower than around 80-90% soc obviously. there are too many of us on this forum that have mppt and can verify this and it is worth it for your system size and especially for what you technically need in the way of pv. i won't delve too much on the subject of mppt as there are too many threads that cover it and yes, some controllers have their little problems so that is your decision. i can definitely say it is justifiable in your case and would be more cost effective than running more pwm controllers.

phoenixfire

Re: Battery bank minimum charge current

niel wrote: »

mppt controllers are not just hype from manufacturers and they do harvest on average about 10%, but i've seen it around 25-30% at times and that is more apt to happen when socs are lower and no they don't have to be real low to achieve that much, but lower than around 80-90% soc obviously. there are too many of us on this forum that have mppt and can verify this and it is worth it for your system size and especially for what you technically need in the way of pv. i won't delve too much on the subject of mppt as there are too many threads that cover it and yes, some controllers have their little problems so that is your decision. i can definitely say it is justifiable in your case and would be more cost effective than running more pwm controllers.

As stated before I really wasnt trying to say that MPPT was 'hype' but the claims of the manufacturers are the extreme not the norm in most cases. I do have experience with the solar boost controllers from Bluesky and I can tell you that they definately DO NOT get the benefits that the average salespoerson will tell you you will. Maybe for 5 minutes on the perfect day in january but the honest actual (average) seems to be closer to 10% I am not disputing this fact. The thing I am trying to figure out is if using the High voltage input ability of the FM80 is worth the DC-DC loss. As stated in my previous post This makes a big difference in my decision as to wether or not to series my array or buy larger more expensive cables to offset the loss at higher currents.

My appologies if I wasnt being clear.

PS: I searched the forums for MPPT but I havnt yet found the info I am refering to above. Hence why I am asking the question.

Thanks again,

Scott

BB.

Re: Battery bank minimum charge current

Scott,

To better search this forum--Use Google and type in:

• search terms site:wind-sun.com

The "site:wind-sun.com" will limit results to this forum--much better searching overall.

A couple of PWM/MPPT FAQs from our host:

All About Charge Controllers
Read this page about power tracking controllers

Regarding "extra harvest" from MPPT's--In many cases, a small system with standard panels and battery banks will work just fine with PWM too.

Where MPPT controller really stand out (in my humble opinion) is that you can run a 100 VDC between the solar array and the battery bank and the MPPT controller will efficiently down convert from 100 VDC to your battery bank voltage (12-48 volts).... This can save you a lot of money for copper wiring and/or allow you to put the PV array in good sun, while the house/battery shed may be in trees or on the back side of a hill.

In general, PWM controllers are nice for arrays under 400 watts. And MPPT controllers are nice for arrays over 400 watts (because you can run them at a higher voltage).

-Bill

phoenixfire

Re: Battery bank minimum charge current

OK after reading all this. Sorry I posted a wall of text back near the begining and it appears noone saw my questions there. So I will revise those questions with my ideas for changing my system.

Split my battery bank into two parts
Bank A: 870AH
Bank B: 1305AH

I already have 400 amp battery selector swtiches on my system to allow for this setup so this is as easy as taking off a set of parallel cables and attaching the bank2 cables. My charge controller aslso has a selector switch to allow my to charge 1, 1+2, or 2 More on that later.

Purchase an Outback FM80

connect my existing 5 panels to the FM80 through the the Controller selector switch.

This should bring my charge current % to about 4% for the 1305AH bank and about 6% for the 870AH bank.

The selector switch on my Charge controller will allow me to select what bank to charge independent of which bank is providing loads with power.

This setup would allow me to charge the offline bank without a load being on it. then when the charge is complete to send the remaining power to the online bank. Each morning I would switch which bank is running loads and repeat the process of charging the offline bank and then switching to the online bank. This configuration should ensure proper complete charge of each set of batteries since there would be no load during the charge process.

My intent then would be to (in the future) buy additional panels to augment this process. The additional array/charge controller would allow me allot of flexibility on where the power is going based on current sun conditions.

I am a very hand-ons person. I actually prefer manual systems to automatic ones most of the time so none of this is a bother to me. And if i need to leave my wife alone for a few days for whatever reason I can just set the charge controllers to charge the online bank and forget about it until I get home.

As always your input is desired.

Thanks,

Scott

icarus

Re: Battery bank minimum charge current

It sounds like a or idea in theory. That said, if you are net/net using more power on average than you are able to put back in net/net you will still be getting behind. Managing your PV is going to be a bit tricky, but it should work until you get some more PV. (especially if you are prepared to use the generator)

Tony

BB.

Re: Battery bank minimum charge current

In the end, I am not sure switching the battery banks weekly (or whatever) is really going to gain you much for the extra effort.

Monitor the whole bank, and if you find cells drifting apart in Specific Gravity -- Fire up the Genset and do an equalization. It would be good exercise for the genset anyway.

-Bill

lamplight

Re: Battery bank minimum charge current

icarus wrote: »

The misconception is, (among others!) if you are only drawing ~5% of ah capacity, you are in effect only drawing off the surface charge. You can do this for months or years, but it sort of defies the logic of a big battery bank. And then if you had a big load it would go dead quickly.

as you guys know, i also have a new bank of rolls batteries that i am not quite at 5% minimum charge rate for. This statement from Tony coincides with what the manual says, that you actually SHOULD discharge it relatively low now and again. quoted text below:

If the battery bank is large in relation to the PV array (C/20 min) and loads are large then the batteries will require a higher voltage setting. Also the battery should be cycled deeply (i.e. to 50%) before starting an auxiliary charge source such as a generator. Once every three months the bank should be discharged to the low voltage set point before starting the generator. This is usually dependent on the cut-off of the inverter which is usually 11 volts on a 12V system. The batteries are designed to be cycled and a deeper discharge forces electrolyte deeper into the active plate material and helps open up fresh reaction sites. With large battery to PV systems, it is imperative that the battery bank is returned to 100% SOC once every 30 days. Full charge can be determined by charge acceptance, which is ~2% of capacity at 100% SOC.

this seems contrary to the common advise that if you discharge little your bank will last longer. man, this stuff is art as much as science!!

Polychrest

Re: Battery bank minimum charge current

lamplight wrote: »

this seems contrary to the common advise that if you discharge little your bank will last longer. man, this stuff is art as much as science!!

Sometimes it seems to enter the realm of religion, with competing orthodoxies. What for some is an article of faith is heresy for others. Found this on the Trojan battery maintenance page:

Many experts recommend that batteries be equalized periodically, ranging anywhere from once a month to once or twice per year. However, Trojan only recommends equalizing when low or wide ranging specific gravity (+/- .015) are detected after fully charging a battery.

This is a great thread, really helpful stuff.

Cariboocoot

Re: Battery bank minimum charge current

The quote from Trojan is something I've always followed and encouraged others to. This is possibly because of my penchant for Trojan batteries. It works well, along with their charge rate recommendations.

That's why it's good to get charge & maintenance info from the specific battery manufacturer. That way if anything goes wrong, it's their fault and they can't say you didn't follow instructions!

bryanl

Re: Battery bank minimum charge current

re: "There were not old just crappy batteries. The original batteries were the interstate GC2 from Sam’s club that were poor performers from the beginning."

The message in this thread should make it clear: it isn't the batteries but rather how you use them.

You do get pretty much what you pay for but most batteries that are not providing satisfactory performance are suffering abuse. Trying to blame the battery doesn't serve any useful purpose.

Good to see the Trojan equalize benchmark. I think too many just go through the motions hoping for a panacea to their battery woes. If you have good, full charging with effective maintenance, you should not need to go through a proper equalization process very often. The process is hard on batteries and really needs full hazmat precautions to be followed when doing it.

Lead acid batteries, especially wet cells, need a proper bit of exercise for best life from what I can tell. That means discharge to 80% to 50% SoC (pull no more than half) followed by a prompt, vigorous, and full charge. A full charge will take 8 to 12 hours. (most solar systems, by themselves, can't do this)

With many solar systems sitting near float service with marginal charging, I wonder whether adding a pulse desulfator (e.g. BatteryMINDer type device) might help battery life.