Battery Bank Size

pvoffgrid · June 2013

Using broad generalizations for a 48v, 4.6kW system with a 11kw generator what size battery bank would be appropriate given a desired 3-day autonomy (no generator running) for central New Mexico?
2,000 average daily watt-hours?
3,000 daily watt-hours?
4,000 daily watt-hours?
5,000 daily watt-hours?
6,000 daily watt-hours?

Thanks.

Cariboocoot · June 2013

Re: Battery Bank Size

Always start with the load number for of-grid design.

The generator size is not really relevant (unless used for a gen-support system) and the array size should be based on the battery bank, not the other way around.

Do not fall for the old "3 days autonomy" myth; it is not practical. It is far more sensible to expect 25% DOD daily, knowing you can take the batteries to 50% DOD if the sun fails to shine the next day, and then start the generator on day 3 if it's still bad weather. Since your location is New Mexico you probably will not have much trouble from lack of sunshine. If you went with the "3 day rule" you'd be buying more battery and panel capacity than you actually need.

2 kW hours on a 48 Volt system requires roughly 42 Amp hours of consumption, which equates to a minimum 168 Amp hour bank @ 25% DOD. The basic formula:

kW hours / system Voltage (round up) * 4 (for 25% DOD).

Applied to 3kW hours: 3000 / 48 Volts = 62.5 (round up to 63) * 4 = 252 Amp hours @ 48 Volts.

Fudge factors involved:
A 48 Volt system does not operate at 48 Volts; that is the nominal and minimum Voltage it should function at.
Batteries do not come in every size; it is best to round the capacity up to the next nearest available.
Panel capacity should be calculated from the battery bank capacity obtained from the last process; that size which is actually available not which is calculated.
Some of the Watt hour capacity will be supplemented by power available from panels once batteries are fully recharged; the more of this that can be utilized the more efficient the system will be.
Conversion and other inevitable system losses are ignored for initial calculations because they are unknown at that point; they should be mitigated by the above procedures but need to be checked after equipment has been selected and wire runs calculated.

If you go the other way 'round and calculated how much battery capacity your 4600 Watt array could support it would be something like this:
4600 Watts @ 77% efficiency / 48 Volts = 73 Amps peak current, 10% of 730 Amp hours @ 48 Volt battery bank. At 25% DOD that would be roughly 8.7 kW hours per day. Quite substantial.

Your 11 kW generator may be oversized even for that system. And yes you will have to redo the calculations several times as you fine-tune the design and the variables become precise factors.

pvoffgrid · June 2013

Re: Battery Bank Size

That is a good point about the power being supplemented once the batteries are fully charged. I was following you until the end and then lost it...I think. In this/your example, using my 4600 watt array, a 730 amp hour battery bank could support 8.7kW hours per day, or 8,700 watt-hours, correct?

If I went to 915 amp hour or a 1,125 amp-hour battery bank, the amount of kW hours to be supported would decrease because more of the power would be needed to charge the batteries vs being used to power items in the home, correct?

Cariboocoot · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

That is a good point about the power being supplemented once the batteries are fully charged. I was following you until the end and then lost it...I think. In this/your example, using my 4600 watt array, a 730 amp hour battery bank could support 8.7kW hours per day, or 8,700 watt-hours, correct?

If I went to 915 amp hour or a 1,125 amp-hour battery bank, the amount of kW hours to be supported would decrease because more of the power would be needed to charge the batteries vs being used to power items in the home, correct?

Yes: the 4.6 kW array would support 8.7 kW hours from the battery. That does not include any potential directly from solar. Two ways of looking at it:

730 Amp hours * 0.25 = 182 Amps hours * 48 Volts = 8.7 kW hours

Checked against array size:

4600 Watts * 4 hours minimum good sun * 0.52 over-all system efficiency = 9.5 kW hours (this number should always be higher than the battery-based number).

As you can see what the battery will support at a DOD limited to 25% is less than what the array would support using a basic over-all efficiency rating. Naturally as the hours of good sun go up there is more potential capacity directly from the sun and if you can make better use of that power potential the over-all system efficiency increases (more power used "directly", less loss from putting it in and taking it out of batteries). The downside is that such power is only available during daylight. As such usage patterns and opportunity loads play a large role in determining a particular system's efficiency.

pvoffgrid · June 2013

Re: Battery Bank Size

Ok, my understanding is gaining..slowly..

So, then
915 Amp hours * 0.25 = 228.75 Amp hours * 48 Volts = 10.98 kW hours

and

1,125 Amp hours *0 .25 = 281.25 Amp hours * 48 Volts = 13.5kW hours

Therefore, both the 915 and 1,125 Amp hour battery banks are too big for the 4600 watt PV system given the parameters above.....4 hours sunshine, 52% efficiency, etc.

And, since I am not finding my power usage exceeds 8,700 watt-hours then the 730 Amp hour battery bank should be sufficient as it will support up to 8,700 watt hours daily with 4 hours sun & 52% efficiency.

So, buying the Concorde PVX-7680T or the PVX-6720T are the best choices.
And, the PVX9150T is too big.
And, of the 7680 or 6720 the 6720 is the better choice.
Am I understanding this better so far?

768 Amp hours *0 .25 = 192 Amp hours * 48 Volts = 9.216 kW hours
672 Amp hours *0 .25 = 168 Amp hours * 48 Volts = 8.064 kW hours

What this whole discussion and my review of loads is proving to me is is a battery bank of 1,125 Amp hours is way too big for a 2.1kW or even a 4.6Kw, and hence why a battery bank of 3 strings of 6-volt 375 Ah L-16s Interstate batteries is an incorrect design. This 4600 watt system appears to be best served by a single string of Concorde PVX-6720T or similar batteries. Well, maybe the 7680's would work.... hmmm

Cariboocoot · June 2013

Re: Battery Bank Size

When checking the array size against its ability to recharge a given battery bank the simplest thing to do is look for the peak charge current using this rule-of-thumb formula:

Watts * 0.77 (typical efficiency) / nominal system Voltage = current.

So 4600 Watts * 0.77 = 3542 / 48 = 73 Amps.

That should be about 10% of the battery bank capacity at the "20 hour rate" (standard for RE use).

You can check it as a percentage against other capacities this way:

Current * 100 / Amp hours = charge rate percentage.

So 73 * 100 = 7300 / 915 = 7.9%

This will still work as it is above the recommended 5% minimum, providing the loads running at the same time do not take the net rate below that minimum and provided you get good sun. If the sun is "iffy" where you are it is better to go for a higher peak charge current as less sun = less charging ability.

Keep in mind that the peak charge rate is not the current you will always see. You may never see that much. It is basically a short cut way of getting the array size and battery bank capacity in balance. The actual current will depend on a number of factors including SOC, loads drawing, and insolation.

10% peak charge rate is a nice "middle of the road" number between the minimum requirement and where it gets to be impractical in array size (20%). This will work about 90% of the time in combination with 25% DOD average and good weather. These are just ballpark figures, though; fine tuning to the particular install has to be done to include local parameters like hours of sun and temperature, et cetera.

Yes, you are getting it.

Now we need to note a couple of other things here. One is that the largest single charge controller you can get will only handle 80 Amps output. With the 10% rule-of-thumb that means you are limited to battery banks <800 Amp hours or else you end up buying a second charge controller. In this case it works out fine for 73 Amps peak off the array and a battery bank of either 768 or 672 Amp hours. (Yes 768 Amp hours will work from 73 Amps - it's close enough).

The other thing is that you've chosen AGM batteries. Very good AGM batteries. Very expensive AGM batteries. Very easy-to-ruin-first-time-out AGM batteries. Is there a particular reason why you want AGM's? They can be a mistake if you do not actually need their advantages.

BB. · June 2013

Re: Battery Bank Size

AGM Batteries are expensive and may not last quite as long as "good quality" flooded cell battery bank. Add a "goof" (taking batteries dead, over charging battery bank, etc.) and you have a load of very expensive/toxic scrap metal.

AGMs are nice--They don't need water, are 90%+ efficient vs 80%+ efficient for flooded cell batteries (note that efficiency is highly dependent on actual power usage profile). AGMs can supply very high surge current, much cleaner, no gassing during normal operation, etc...

Besides costs, AGM batteries are "sealed" so you cannot measure specific gravity for state of charge. And I would suggest a Battery Monitor is virtually a requirement for AGM/Sealed batteries.

So--if this is your first bank, you might want to look at the costs and other issues when looking at AGM vs Flooded Cell batteries.

It would be helpful to know more about your loads--Maximum "average" power draw (watts) and peak search power (well pump, etc.). Batteries have limits for maximum surge current, recommended discharge current, and even a rule of thumb for how much charging energy you should connect to the bank...

For example, on a 48 Volt battery bank, it is recommended that you have 100 AH of battery for each 1kW of inverter (and 2kW of surge) and/or Solar array power (24 volt would be 200 AH, and 12 volt would be 400 AH per kW).

It also depends on when you use power vs your solar charging... If you charge during the day and use power at night, then most of the solar array output goes to charging the battery bank. If you use loads during the day, then you have to have enough solar array to both power your loads and properly recharge your battery bank at the recommended rate of charge (5% to 13% to ~25% maximum or so).

Say you have a 730 AH @ 48 volt battery bank, using the 5% to 13% rate of charge recommendation:

365 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,839 Watt array minimum
365 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,678 Watt array nominal
365 AH * 57.6 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 4,782 Watt array "cost effective maximum"

If you use power at night, have lots of long sunny days (say less power is used during the ~3 months of winter), etc.--You can get down towards the 5% minimum rate of charge.

However, if you use power during day and night, and cannot shift power/use backup generator" too much--Then you should be towards the 10-13+% rate of charge.

A 4,600 watt array will produce on an off grid system an average (assuming ~4 hours minimum sun for 9+ months of the year):

4,600 Watt array * 0.52 end to end system efficiency * 4 hours minimum sun per day = 9,568 Watts of 120 VAC power (typical minimum)

Note that AGM batteries are more efficient, so you can get a bit more power per day. But--Like to be conservative with initial design.

-Bill

BB. · June 2013

Re: Battery Bank Size

I forgot to add your AC generator requirements...

An 11 kW generator is "big" and can use upwards of 1 gallon per hour minimum. And are typically most efficient when operated at 50% or more of rated load (i.e., 5.5 kW of AC loads).

For a 730 AH battery bank, you could go down towards (for example) 13% rate of charge pretty easily--On a Wattage basis that would be:

730 AH * 57.6 volts charging * 1/0.80 charger efficiency * 0.13 rate of charge = 6,833 watt load from AC battery charger

730 AH * 0.13 rate of charge = 95 amp charger

So, for such a large bank, the 11 kW genset with an appropriate battery charger is a good match.

But if you go with a 365 AH @ 48 volt battery bank, then the generator would probably be too large (run at less than 5.5kW power for longer periods of time). Many people do have two (or more) generators... A smaller one for efficient battery charging (and powering smaller tools/loads for backup) and a larger genset to power a shop/whole home when needed.

Note that I reworked my previous post--I did it at 24 volt instead of 48 volt battery bank.

-Bill

pvoffgrid · June 2013

Re: Battery Bank Size

I tell you...I have learned more this morning from this discussion than in 12 yrs of being off-grid...to use an analogy of being just a driver of a car vs driving AND understanding how a car works I have been "just a driver". And, I have been unhappy with the battery life and propane usage. As batteries age and charge unequally generator run-time & propane use increase.

Finding that middle of the road number is not difficult but can be easy to miss. Again, using the load evaluation and Kill-A-Watt helps pinpoint inefficiencies and can guide one to unplugging or implementing a power strip as in the case of my DeWalt battery charger...I never would have thought it used that much power. Wow..the equivalent of a 70 watt incandescent light bulb running 24/7. Who wants to do that? And, LCD TVs vs. LED TVs....LED TVs are the better choice.....Kill-A-Watt tells me that.

But, back to the batteries. 77% typical efficiency tells me that even with a 1,125 Amp hour bank I would still be above the minimum 5% charge rate percentage.
4600 * 0.77 = 3542 watts / 48 volts = 73.9167 amps * 100 = 7392 amps / 1125 Amp hours = 6.56% charge rate percentage. Yes, this is still above the 5% minimum BUT simultaneous running loads could drop that below the 5%.

And, going to a 672 Amp hour battery bank gives this:
4600 * 0.77 = 3542 watts / 48 volts = 73.9167 amps * 100 = 7392 amps / 672 Amp hours = 10.98% charge rate percentage
and the 768 Amp hour battery bank:
4600 * 0.77 = 3542 watts / 48 volts = 73.9167 amps * 100 = 7392 amps / 768 Amp hours = 9.61% charge rate percentage

So, there is a range for my system of 6.56% to 10.98% but what is the sweet spot? To find this I would think one has to look at the loads that will be run the majority of the time, and being sure to count those times when there will be heavy usage such as with the well pump (both the well pump and also the submersible pump in the cistern), the radiant floor zone controller, an air compressor, power saw, running a washer, using the central vacuum (oops..still need to get that "Kill-A-Watted") etc., which is normally on the weekends

Given this whole discussion:
1) the charge rate percentage range between 6.56% - 10.98%
2) the loads I have evaluated to be between 6,000 - 9,000 (depending on what I unplugged, decide to sell, use less of, whoever buys this house if we sell, etc.)
3) the battery bank supported load of 8700 watts
4) the array size supported load of 9500 watts

I am dialing in on the 768 Amp hour battery bank. I could also go with the 915 Amp hour battery bank so that if in the future more PV panels are added it will just provide a better charge rate percentage (increasing by (4) 210 w modules and jumping to 5.44 kW = 9.54% and so on). But, since I do not see myself adding more PV panels in the near future I think I would be safe to have a battery bank close to the 768 Amp hours.

And, 2 charge controllers is what I have. And, a 1,125 Amp hour battery bank is what I have had for 10 years and the L-16 batteries do not last much more than 5 years, no where near the 7 - 10 years me & my neighbors were all told they would last, especially given maintenance that borders on poor - non-existent. So, I have figured that historically the batteries were being undercharged and this discussion has confirmed that in spades. And, I now understand why....which is the big take-away here.

And, why Concordes? Reading the different posts on this site have led me to the understanding of the issues about multiple strings vs. single strings. Researching batteries to go with a single string led me to the 2-volt cells and this is where Concordes came up. Interstate has come out with a new 2-volt battery in the past year or so but it still requires maintenance and although 24 cells to check vs. 72 is easier and less time it still requires checking, the maintenance free aspect of the Concordes are appealing. Couple that with a high likelihood of selling this house in the next 24 months also makes the maintenance free idea appealing as we have seen recent buyers of houses in this neighborhood not be of a "do it yourself" kind of person.

But, the price of the Concordes...ugh..... and the Surrettes and the HUP-Solar One batteries..double ugh....we were hoping to sell in the next 6 - 9 months and buy batteries at time of sale and make an accommodation in price to the buyer so they would bear more of the cost of new batteries since they would be getting full benefit of them new batteries but it is not working out that way...So, we may end up being here 24 months more since we need to buy now...we dropped from 3 strings & 1125 aH to 2 strings and 750 aH and now are at 1 string & 375 aH..and 375 aH on old batteries does not cut it.

BB. · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

But, back to the batteries. 77% typical efficiency tells me that even with a 1,125 Amp hour bank I would still be above the minimum 5% charge rate percentage.
4600 * 0.77 = 3542 watts / 48 volts = 73.9167 amps * 100 = 7392 amps / 1125 Amp hours = 6.56% charge rate percentage. Yes, this is still above the 5% minimum BUT simultaneous running loads could drop that below the 5%.

I have to go right now--But there is an "error" here... Batteries when discharging run around 48-50 volts or so... When charging they will be near 58 volts, so:

3,542 Watts / 58 volts charging = 61 amps charging
61 amps / 1,125 AH bank = 0.054 = 5.4% rate of charge

My generic suggestion would be to get the rate of charge closer to 10% or more... Either by doubling the solar array or cutting the battery bank 1/2 in AH rating.

"Over sized" battery bank are generally an expensive pain in the behind. Many times you have to more fuel to "top off the battery bank" because of the "poor" solar array to battery bank capacity.

The 2 days of backup and 50% maximum discharge is still a pretty conservative number--And gives a "well balanced" system that is pretty cost effective. If you have "unusual load patterns", then we should discuss those needs further (vs the "normal" charge during the day and use much/most of the power at night off grid home pattern).

You may need a bit more generator power during bad weather/extra heavy loads (guests coming by)--But overall, you should be using less fuel because the solar array can "keep up" with both battery charging and power usage for 9+ months of the year without any generator use.

Solar panels are "cheap" and batteries are now "expensive" (and fuel is expensive too)--So there is usually not much good reason to "over battery" an off grid power system these days.

-Bill

Cariboocoot · June 2013

Re: Battery Bank Size

Just a note on your old 1125 Amp hour bank.
That had barely a 6% peak charge rate with that array size. Now here's the thing: these "tall case" L16 batteries really want the higher charge rate so that they can be charged quickly and spend a lot of time in Absorb because they need lots of bubbling to remix the electrolyte. Chances are those batteries were undercharged a lot unless they were getting a weekly run on the generator. Hence the shorten lifespan. You should have had about two 4600 Watt arrays with two 80 Amp controllers for that bank.

pvoffgrid · June 2013

Re: Battery Bank Size

ha ha..that was my 1st reaction...LOL.....two 4600 watt arrays..higher charge rate desired/needed....no wonder the life span we had on those batteries.... wow. Well, this system is getting better as the years go on. I guess a lot of us do get better with age, don't we?

And, Bill, your thought about the maximum "average" power draw (watts) and peak search power (well pump, etc.). is a good one but how do you best measure those items since they are direct-wired and also are 220v so the Kill-A-Watt won't work, even they were plug-in type of device. I guess I could watch the Tri-Metric meter and see what happens but.....using the 58 volts for the charging vs. the 48 volts as you did below changes my #'s for the 915 Amp Hour Concordes to a 6.67% charge rate and the 768 Amp hour Concordes to a 7.95% charge rate and the 672 Amp hour Concordes to a 9.09% charge rate.

And, your comment about the getting the rate of charge closer to 10% or more by either doubling the solar array or cutting the battery bank 1/2 in AH rating leads me to this......no more than 768 Amp hours and preferably the 672 amp hours, as the 672 is closer to cutting the battery bank in 1/2 (1125/2 = 562.5). I think the 672 is the sweet spot, although not knowing the max average power draw is of concern.

And, this is kind of stepping out into the unknown...it has "always" been 1,125...we have "always" done it that way...and now you want to change and go to 672 or 768? Uh, I am not sure...I know the #'s say it will work, and I understand it will work...but...a change?

So again, either 672 or 768...choices..choices...choices......without knowing the max average draw, which would occur during time off from work, which if we were to believe in the old stereotype of 9-5 M-F, I think I am leaning towards the 768 Amp-hour because there is more capacity there, but the 672 is closer to the 10% charge rate....

And, if I stick with the L-16's I would have to go with two strings of eight 6-volt batteries ( each which I was trying to avoid, unless I go with the new Interstate 2-volt cells..nope..that would still require 24 of the 2-volt cells..24*2 = 48v which = $$$$)

Cariboocoot · June 2013

Re: Battery Bank Size

For things like 240 VAC pumps you can isolate a wire and use an AC clamp-on Ammeter to see what it draws (and check the Voltage) then multiply out by running time. Getting an accurate number on the time can be tricky because you either have to hang around and count how many times it turns on and for how long or else rig up an electric timer that will advance automatically whenever the pump runs.

I would definitely work on getting the numbers "fixed" for consumption before picking the battery bank size. Could save yourself a few dollars there.

pvoffgrid · June 2013

Re: Battery Bank Size

I think this is a new wrinkle...speaking with the tech dept at US Batteries (Interstate) @ 1-800-695-0945 http://www.usbattery.com/usb_Reneable_REHD2Vxc.html just now about their 2-volt cells ..the guy stated I should use a 75% DOD and he used the analogy of why would you fill your gas tank up in your car after only using 25% of the total gas available in the tank?

And, he also stated the size of the battery bank does not matter because a larger bank would mean less capacity would be used and the batteries would last longer.

I am not sure how his statements tie into what we have discussed this morning...those 2-volt cells are rated A 110 Amp hours over 20 hrs, and I believe the 20-hour rating is the one to use when deciding what size battery to buy, as there are multiple hour ratings on each battery...

Cariboocoot · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

I think this is a new wrinkle...speaking with the tech dept at US Batteries (Interstate) @ 1-800-695-0945 http://www.usbattery.com/usb_Reneable_REHD2Vxc.html just now about their 2-volt cells ..the guy stated I should use a 75% DOD and he used the analogy of why would you fill your gas tank up in your car after only using 25% of the total gas available in the tank?

And, he also stated the size of the battery bank does not matter because a larger bank would mean less capacity would be used and the batteries would last longer.

I am not sure how his statements tie into what we have discussed this morning...those 2-volt cells are rated A 110 Amp hours over 20 hrs, and I believe the 20-hour rating is the one to use when deciding what size battery to buy, as there are multiple hour ratings on each battery...

Were you speaking with a tech rep or a sales rep? Drag them down to 25% SOC and you'll be replacing them pretty fast: these are not forklift-class batteries, are they?

Size does matter if you want the system balanced and working. You know how many unbalanced systems I've corrected in just the past few years? I don't; lost count a long time ago. Ask him how many off-grid solar electric power systems he's designed. How many has he installed? Most importantly how many has he got to work? So far I'm still "batting 1000".

The "20 hour rate" is the standard for RE when it comes to doing the calculations. You may notice the use of 4 hours of "equivalent good sun". Add the two together. This is another fudge factor: if you expect 24 hours of battery power out of the 20 hour rate and 4 hours of sun to charge and run then it all works out nicely about 90% of the time.

pvoffgrid · June 2013

Re: Battery Bank Size

This is the Ammeter to go with, correct? http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P

And, since it appears a battery bank of somewhere between 672 - 915 Amp hours is what this discussion is guiding me towards then getting the numbers fixed for consumption will do what? Further pinpoint the direction..up towards 915 or down towards 672?

Even once the #'s are determined there is a fixed aspect here: the array is not going to grow in size due to $$ limitations. I guess I could add another four Kyocera 210w modules (12v * 4 = 48 volts) and then spend less on batteries but.....I would guess that would out-strip the savings of reducing the # of batteries due to cost of racks, wiring,etc.

pvoffgrid · June 2013

Re: Battery Bank Size

Hmmm..I asked for the tech dept but...who knows who is on the other side. My warnings signs went up as what he said was so 180 degree off what we are discussing today, plus what I did learn over the years.

ok, 20 hrs rating plus 4 hours sun....thanks.

Cariboocoot · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

This is the Ammeter to go with, correct? http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P

Probably. Lack of availability in Canada (aka Outer Canuckistan) means we don't get to buy them here.

And, since it appears a battery bank of somewhere between 672 - 915 Amp hours is what this discussion is guiding me towards then getting the numbers fixed for consumption will do what? Further pinpoint the direction..up towards 915 or down towards 672?

Yes; why buy more battery than you have to?

Even once the #'s are determined there is a fixed aspect here: the array is not going to grow in size due to $$ limitations. I guess I could add another four Kyocera 210w modules (12v * 4 = 48 volts) and then spend less on batteries but.....I would guess that would out-strip the savings of reducing the # of batteries due to cost of racks, wiring,etc.

I wouldn't add panels either. Just see where the consumption best fits with a battery banks size and how that affects the charge rate. It is easier to turn off a load than to add more capacity, and far cheaper.

Steve Higgins · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

I think this is a new wrinkle...speaking with the tech dept at US Batteries (Interstate) @ 1-800-695-0945 http://www.usbattery.com/usb_Reneable_REHD2Vxc.html just now about their 2-volt cells ..the guy stated I should use a 75% DOD and he used the analogy of why would you fill your gas tank up in your car after only using 25% of the total gas available in the tank?

And, he also stated the size of the battery bank does not matter because a larger bank would mean less capacity would be used and the batteries would last longer.

I am not sure how his statements tie into what we have discussed this morning...those 2-volt cells are rated A 110 Amp hours over 20 hrs, and I believe the 20-hour rating is the one to use when deciding what size battery to buy, as there are multiple hour ratings on each battery...

Wow... Batteries in general like to stay charged and they don't like to sit for long periods of time uncharged... AGM's usually need a full charge at least once per week or Sulphation starts to develop.

I personally would rather have a smaller battery bank and make sure you get a full charge other than having to run extra time burning fossil fuels to charge a extra large battery bank.

vtmaps · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

the guy stated I should use a 75% DOD and he used the analogy of why would you fill your gas tank up in your car after only using 25% of the total gas available in the tank?

Probably not good advice for you, but he's not entirely wrong. For some types of batteries and applications you get the most bang for the buck by deeply discharging them. From GB battery's FAQ:

Industrial batteries are typically designed to last at least 1,500 charge cycles, over a five to fifteen year period. Each time you charge a battery, regardless of how long, it constitutes one cycle. Consistently charging a battery twice per day, during lunch breaks for example, is known as Opportunity Charging, and reduces the useful life of a battery by 50%.

Routinely charging the battery before it is 80% discharged is another common form of over charging. For example, if you only use the battery a few hours a day, it’s best to use it until it is truly in need of charging before actually plugging it in. Remember, each charge constitutes one cycle, so try not to charge unnecessarily.

--vtMaps

vtmaps · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

And, he also stated the size of the battery bank does not matter because a larger bank would mean less capacity would be used and the batteries would last longer.

He is correct that a larger bank means less capacity is used (per cycle) and the batteries will last longer (more cycles). The question is "does it make sense for you to have a larger bank?"

Suppose that you could buy a $1,000 battery that would last you five years, but if you buy a $2,000 battery it would last you 10 years... which is a better deal? All other things equal, I would prefer the $1,000 battery. But all things are not equal: the larger battery can handle larger peak loads with less voltage sag. If that is important then the $2,000 battery is a better deal.

--vtMaps

NorthGuy · June 2013

Re: Battery Bank Size

People will recommend different things. Don't pay attention to the recommendations unless they exlain why.

One more consideration. You will have to take your batteries to the full charge at least every week or so. When you do so, batteries bubble. Some of the energy you put into batteries is used for the bubbling and therefore lost. The amount of the wasted energy is directly proportional to the bank size. It is 4-5kWh for my batteries (673AH@48V). If I doubled my bank size, this amount would double.

inetdog · June 2013

Re: Battery Bank Size

pvoffgrid wrote: »

..the guy stated I should use a 75% DOD and he used the analogy of why would you fill your gas tank up in your car after only using 25% of the total gas available in the tank

Guess what, one reason that you did that during the gas shortage was that you could not be sure that you would be able to fill up your tank the next day. Or the day after that.
The analogy holds when allowing for weather. If you have a generator it does not matter as much.

Just as an aside, current Lithium rechargeable batteries deteriorate faster (degradation with loss of capacity, not just self-discharge) when they are stored fully charged than when they are stored at about 1/3 charge. Does not apply to lead-acid though.

pvoffgrid · October 2013

Re: Battery Bank Size

Ok, I need to close this thread out.

Here are the results. We went with 24 Concorde AGM Sun Extender PVX-9150T Solar Batteries. We dropped from 1,125 aH battery bank to a 915 aH. The voltage on the Trimetric meter has dropped to 48.6 one time that I am aware of since mid-June 2012 and only briefly, before climbing back up to 50.x. Maybe once per week I might see 49.8 when we are all getting ready for work & school when some combination of the following is on: toaster, coffee maker, microwave, hair dryer, refrigerator, freezer, or well pump...again, some combination of all those items..not all at once. And, this is only momentarily as when something shuts off, like once the coffee maker is done brewing it is shut off or the bread pops up out of the toaster, or the well pump shuts off, the voltage jumps back into the 50.x range.

Since we went with the new batteries the Kohler 11RMY generator has not run. The battery and inverter room has no smell and is clean.

I love the AGMs 4 months into ownership.

Thanks for all the help.

BB. · October 2013

Re: Battery Bank Size

That is wonderful news. Always nice to hear a happy poster after they have operated their system for a while.

Sounds like the math worked out for you. Do you think you will be adding to your solar array in the future (for winter/monsoon season/"more power")?

-Bill

pvoffgrid · January 2014

Re: Battery Bank Size

Hi,

I came back to ask another question (not written yet) and decided to revisit this thread. Yes, I would like to go to about 5.4kW and I believe that would put me right smack in the middle of the sweet spot for kW vs. aH. I believe the increase to 5.4 kW will not occur for at least 1 - 2 years. Ok, off to start another thread, unless I can find the answer to my question when I do a search 1st.

BB. · January 2014

Re: Battery Bank Size

This is "your thread"--You may continue here (recommended if knowing the history in this thread is helpful)--Or feel free to start another thread for (relatively) unrelated questions.

-Bill

Battery Bank Size

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