battery charge and discharge

Tradiscantia

We have 4 6v Trojan T 105 flooded batteries. Our system is taking us down to 89% by morning most days. Do we need to discharge the batteries down to 50 % once in a while to prolong their life?

Tradiscantia
490 watt Conergy panels, Tristar MPPT 45, 4 Trojan T 105, (broken inverter right now)

westbranch

Re: battery charge and discharge

how old is the bank?
Assume it has reached float every day.
How long is your absorb cycle going these days?
How fast does it get to Absorb/ how long in Bulk?
Have you EQ'd lately?

What does Trojan say about a 50% DoD?

hth

waynefromnscanada

Re: battery charge and discharge

Welcome Tradiscantia to the forum!
I don't know that there is a real, scientific answer to your question, but there are differing opinions. What I can say, is that my six L-16 flooded batteries have never ever to my knowledge been discharged below the 80% full point, and most likely never below the 90% fully charged point. Furthermore, since I installed the wee hydro turbine about 5 years ago, they haven't been below probably 95% fully charged. Has it hurt them? Well, they're over 10 years old now, and except for slightly raised positive posts, which I'm told was caused by overcharging, and a somewhat increased internal resistance, they are still working very well with no immediate indication of them being ready to quit. A month ago I did resolve the overcharging issue, so now they spend most of their time in float mode. Hey, after 10 years I'm still learning about these batteries! Problem was the setting on the MX-60 had it in absorb for a couple of hours every day even if the battery was already full from the hydro, and it's been doing that for 5 years. I now have the MX-60 set to switch to float by monitoring End Amps, not time. I hope to get a few more years out of them yet, but who can see the future.

ChrisOlson

Re: battery charge and discharge

Tradiscantia wrote: »

We have 4 6v Trojan T 105 flooded batteries. Our system is taking us down to 89% by morning most days. Do we need to discharge the batteries down to 50 % once in a while to prolong their life?

Batteries that get repeatedly shallow cycled tend to lose capacity quite rapidly. Surrette, as an example, recommends deeply discharging the battery at least once every three months. From Rolls Battery Engineering Bulletin 614 (taken out of context and emphasis added to the part about deep cycling):

When a voltage setting is chosen the length of time the bank is being held at constant voltage is to be considered. If only a short absorption time is possible then the voltages settings should be at the higher levels. If a long absorption time is possible then the voltages should be lowered.

For example with a large PV array, small battery bank and minimal loads the lower settings should be chosen if it is apparent the battery bank can be held at the bulk/ absorption voltage for a minimum of fours hours. When the battery bank is put through the first 10 normal cycles the specific gravity (SG) of a pilot cell should check and recorded and if the bank is receiving full charge each cycle the SG should be slightly increasing as the battery gasses and loses water due to overcharge. Please refer to bulletin 609, Voltages, Specific Gravity and State of Charge for further info on determining cycle depth and full charge.

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.
--
Chris

Photowhit

Re: battery charge and discharge

I like Chris's suggestion, though you may find that you have cloudy weather and draw 3 days with out any added to batteries during cloudy times and not need to 'cycle' the batteries. Since you have a pretty marginal array to battery bank, be sure you have sunny days to feed your battery or just let the normal cloudy days help you 'cycle' your battery.

ChrisOlson

Re: battery charge and discharge

Batteries are most efficient when cycled from 50 to 80% SOC. They are least efficient when cycled from 80-100% SOC. So it pays to design the system to cycle the batteries in their most efficient range on the average day and use the exceptional power day, or generator, to bring them to full charge on a maintenance basis (about every week to 10 days).

The other thing is that cycling from 50-80% doesn't really stress the battery because it never heats it up. That final 20% SOC is where all the heat gets generated in battery charging. The caveat, of course, is that the battery will hard sulfate if allowed to run below 80% SOC for too long. So a weekly full charge to 100% is a good thing.
--
Chris

inetdog

Re: battery charge and discharge

ChrisOlson wrote: »

The other thing is that cycling from 50-80% doesn't really stress the battery because it never heats it up. That final 20% SOC is where all the heat gets generated in battery charging. The caveat, of course, is that the battery will hard sulfate if allowed to run below 80% SOC for too long. So a weekly full charge to 100% is a good thing.

It also will not drive the battery up to the gassing voltage and so will allow for electrolyte stratification which will reduce the apparent capacity of the battery and can reduce its life.

NorthGuy

Re: battery charge and discharge

When batteries are cycled that low, they become inefficient, but if your sources and loads are such that they cycle batteries only to 10%, you do not have much choice. If you wanted it differently, you would need to size everything accordingly during the design stage.

People say that it's good to drain batteries from time to time to maintain or increase capacity, but since you're doing only 10% discharges you do no really need extra capacity.

Even if your batteries lose 80% of battery capacity, you still wil be able to use them. Therefore they will last really long time for you.

BB.

Re: battery charge and discharge

NorthGuy wrote: »

When batteries are cycled that low, they become inefficient,...

Just to be clear--NorthGuy is talking about very small/"shallow" cycling of battery bank (i.e., from 100% to 90% state of charge). And not talking about discharging a battery bank below 50% or 20% state of charge (deep cycling).

-Bill

NorthGuy

Re: battery charge and discharge

BB. wrote: »

Just to be clear--NorthGuy is talking about very small/"shallow" cycling of battery bank (i.e., from 100% to 90% state of charge). And not talking about discharging a battery bank below 50% or 20% state of charge (deep cycling).

That's absolutely right. Thanks Bill.

Vic

Re: battery charge and discharge

IMO, Cycling Flooded Deep-Cycle batteries is good for them, and deeper cycles to about 50% SOC at least monthly, or more often might be better.

Cycling to 20-ish% SOC seems extreme to me.

Of course, one can consiously squander some of the energy stored in the batteries to force deeper cycles, or squander available charge energy from PV, Wind Hydro, etc which also will force deeper cycles from the normal loads on the system. This IS horribly inefficient, but is good for the batteries.

It is not uncommon for the DOD to vary from day-to-day, so maintaining additional Capacity can be very useful, and working one's flooded deep-cycle batteries should be good for them.

For the systems that we manage, efficiency obsession seems mostly a waste of time. My obsessions are of a different nature.

However, if one wants to begin cycling one's bank to 50-ish percent SOC, would recommend that for the first deeper cycle, this level of discharge be approached in steps over a period of three of four cycles. Perhaps the first deeper cycle could be to about 80% SOC, then perhaps 70% and so on. It is a good idea during these first few cycles to watch the battery voltages very closely, and be prepared to terminate the discharge, and recharge the bank immediately if it appears that one battery or the bank has reached the limit of its capacity -- this is often evidenced by the bank (or individual battery) voltage decreasing rapidly.

Using measured SG readings for each cell (or at least Pilot cell/s) is the best measure of SOC. And, my opinion is that 89% SOC is an almost impossible level of accuracy using any common device for almost any common battery technology. Seems to be a bit of a false precision. Not to grind on you, Tradiscantia, but +/-- 5% for SOC determination is really quite good accuracy, and is generally as useful as +/-- 1%, IMHO.

Hope that you get the Inverter fixed or replaced. Good question about cycliing, and opinions do differ. Good Luck, Vic

Cariboocoot

Re: battery charge and discharge

You know, there's a good reason for that daily 25% DOD rule-of-thumb you keep hearing me spout.

But hey, don't listen to me: every system I've had a hand in works. :roll:

ChrisOlson

Re: battery charge and discharge

inetdog wrote: »

It also will not drive the battery up to the gassing voltage and so will allow for electrolyte stratification which will reduce the apparent capacity of the battery and can reduce its life.

Ours don't stratify. We cycle our bank from 50-80% (roughly) for days at a time. The batteries still reach 62.0 volts on various days and boil good, just that they don't get absorbed long enough to get to full charge because our loads are too high. The Classics with Waste Not driving our water heating loads let me precisely adjust how I want to cycle the bank on the average day. All I have to do is adjust the offset voltage to adjust how much power goes to water heating vs charging.

They run at a net daily deficit for sometimes up to 10 days, gradually getting to the point where either we get both sun and wind and the RE system catches them up - or the 24 hour timer in the XW-AGS starts the generator and recovers them to full charge. After a week the little yellow light comes on on our MidNite Battery Capacity Meter, and when I see that I start to take notice. But 9.5 times out of 10 the system takes care of itself and the batteries get a full charge before 10 days is up. There's only been a couple times in the last two years since I've learned how to adjust it with the Classics that I've had to intervene and manually the start the generator because of a long stretch of bad weather.

I have deliberately designed our system that way because it dramatically reduces the number of discharge/charge cycles the batteries have to go thru. And yet at the same time prevents them from becoming sulfated. They are now 2 years and 4 months old and all 24 of them load tested at full capacity yet at the 20hr rate the last time I serviced them.

It wasn't actually my idea to do that. I was told how to do it by a guy in Hudson, Wisconsin that has worked with, sold and serviced Surrette traction, marine, and locomotive batteries for over 25 years - and was the guy we got our batteries from. That dealer told me, and I quote, "deep cycle batteries need to be properly maintained, but they don't like to be babied".
--
Chris

Tradiscantia

Re: battery charge and discharge

This has been a very helpful discussion! Thanks to everyone who contributed.
Our new inverter arrives tomorrow, and since it is a 2000/4000 Samlex, to replace our dinky Cobra 800/1600, we'll be tempted to run more things on it. I'm already eyeing the blender (in storage) and a crock pot.

We planned our system to meet the requirements of the fridge, which gobbles up amp hours while we sleep, and we followed online guidelines plus the advice of our NAWS sales rep on the panel strength. I think one reason we've used so little of the battery capacity is that we are in the southwest, and because it was pretty cold when we purchased our equipment in Flagstaff, we headed down to Phoenix, then to S. NM. We we have more abundant sunshine for now. Even on the cloudy days, of which we've had several, the Tristar MPPT brings us up to 100%.

I'm getting those percentages, by the way, from the Bogart engineering Trimetric battery meter we installed. I know it's not rocket science--we set the total amp capacity to 400 when we could actually get more out of our Trojans, on the advice of a friend who thought setting it for cold conditions would keep us from going too low in our evening usage. We're hoping to use a radiator type space heater on its 600 watt setting when the temperatures drop, which they will as we head north and to higher elevations.

It's really good to know we don't have to freak out if we don't get charged up daily to 100%.
thanks again all

Coach Dad

Re: battery charge and discharge

This is what the Deep Cycle Battery FAQ says
http://www.solar-electric.com/deep-cycle-battery-faq.html#Cycles vs Life

Cycles vs Lifespan

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. You have to be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged. For example, one of the widely advertised telephone type (float service) batteries have been advertised as having a 20-year life. If you look at the fine print, it has that rating only at 5% DOD - it is much less when used in an application where they are cycled deeper on a regular basis. Those same batteries are rated at less than 5 years if cycled to 50%. For example, most golf cart batteries are rated for about 550 cycles to 50% discharge - which equates to about 2 years.

Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the the positive plates rather in an even film. The graph above shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary.

So who is right?

Cariboocoot

Re: battery charge and discharge

Coach Dad wrote: »

So who is right?

Everyone ... and no one.
As usually we are all (including the FAQ) making statements about deep cycle batteries in general rather than a specific battery. Also consider that, as I mentioned before, your daily DOD will never be precisely 'X' percent so over the lifespan of the battery you are looking at an average DOD to balance not only battery performance but cost as well. Remember too that what you are actually using is Amp hours, and that capacity diminishes with age but the Amp hours will not. Thus 'Y' Amp hours at the start of a battery's life is 'Z' percent of capacity and at the end of its life it may be '2Z' percent.

Simple? No; nothing ever really is.

Coach Dad

Re: battery charge and discharge

Cariboocoot wrote: »

Simple? No; nothing ever really is.

Yeah... Nothing is Black and White... I hate Gray.

I've actually taken my battery bank down to 60% by turning off the Charge controller for the weekend and while I was there and then turned it back on when I was leaving... But I've only done that 2 times in the past year...
After reading thise thread I'm considering doing it more often... But after referring the FAQ's I hesitate...

The battery manual has a chart that matches the FAQ chart but doesn't mention that bringing the SOC down is good for the battery.

waynefromnscanada

Re: battery charge and discharge

Coach Dad wrote: »

The battery manual has a chart that matches the FAQ chart but doesn't mention that bringing the SOC down is good for the battery.

Thus the conundrum. Is there a "right" answer? Can there be a "right" answer that covers all installations?
With the cool, cloudy weather we've been having here the last while, I've been from time to time running my mini-split heat pump off my 10 year old bank for the last couple of days, and although I don't have a battery monitor to tell what has been taken out, the load when it's running, varies between 40 and 100 amps, although I like to adjust the thermostat to keep it in the 40 mp range, and so far, the SG hasn't dropped that much, nor has the voltage once the load is removed. I know that without an actual record of what has been taken out it means little, but to me it means my bank is in better shape than I had expected. Up until now I hadn't run anything big unless the sun was shining because I didn't want to drain the batteries. My education on my system continues, but the questions remain. When I finally do replace the battery bank, will the new one act like the ones I have now? Or will I kill them by treating them the same way I've treated these old ones?

Blackcherry04

Re: battery charge and discharge

I'd say there is a big difference in the Term " Cycle " used by different battery manufacturers. The Forklift people guarantee their batteries for a specific number of " full cycles " and claim that " Opportunity Charging " shortens the life of a battery and warranty. ( Opportunity Charging = Like charging at Lunch time instead of the end of the day after a full discharge ).

Coach Dad

Re: battery charge and discharge

Attachment not found.
This is the chart that is in the manual for my batteries.

If I look at this chart, it looks to me like:
I get 5000 cycles if I go to 90% SOC
I get 1000 cycles if I go to 50% SOC

I see no evidence that I would purposely want to discharge my batteries any lower than I had to.

ChrisOlson

Re: battery charge and discharge

Coach Dad wrote: »

Attachment not found.
I see no evidence that I would purposely want to discharge my batteries any lower than I had to.

The evidence is in experience, I think. Sandia Labs did a long term study on Surrette batteries many years ago and determined that the amount of energy stored by a battery over its lifetime is the same whether you shallow cycle it or deep cycle it. The old guy we got our Surrettes from has loose leaf binders with every bulletin and study that has ever been done on Surrettes, collected over 25 years of being a Surrette dealer. But the basic premise was that you buy the minimum amount of battery capacity you need and work the livin' snot out of 'em for best efficiency.
--
Chris

Blackcherry04

Re: battery charge and discharge

Coach Dad wrote: »

Attachment not found.
This is the chart that is in the manual for my batteries.

If I look at this chart, it looks to me like:
I get 5000 cycles if I go to 90% SOC
I get 1000 cycles if I go to 50% SOC

I see no evidence that I would purposely want to discharge my batteries any lower than I had to.

For that math to work you have to assume that your 90% cycled battery would last 13 years and it won't. Cut the life to half ( 6 years ) and then subtract the number of AMP Hrs removed. Take the 50 % cycled battery ( 3 years ) and take the Amp hrs removed and see how they come out on a Amps hr consumed and cost per Amp Hr.

The problem with batteries, it's a use them or lose them, everyones math is different. I just replaced a Optima from 2004, the only problem is I only cycled it 20 times.

ChrisOlson

Re: battery charge and discharge

Blackcherry04 wrote: »

The problem with batteries, it's a use them or lose them, everyones math is different. I just replaced a Optima from 2004, the only problem is I only cycled it 20 times.

That's what I was told too. Deep cycle batteries that never get worked go bad and lose capacity quite rapidly after a certain time. As long as you only need 20% of their original capacity they'll "last" more years. But the charging efficiency goes way down and they start to create more heat during charging with age when they deteriorate. Might as well work 'em and work 'em hard. That white paper that Sandia Labs put out (I don't have a copy of it but our battery dealer does) concluded that discharging to rated DoD provides best efficiency for amp-hours in vs amp-hours out and creates the least amount of heat (and heat is bad for lead-acid batteries).
--
Chris

NorthGuy

Re: battery charge and discharge

Blackcherry04 wrote: »

For that math to work you have to assume that your 90% cycled battery would last 13 years and it won't.

According to Wayne from Nova Scotia, they will.

It is more efficient to cycle deeper, but if you already have a bank that only requires shallow discharges, it doesn't make sense to cycle them deeper when you do not need it (may be once a year to see how it goes). It's the same as if you bought a truck which is too big for your loads, and you decide to put some rocks in it, so that you could fully use its capacity.

Blackcherry04

Re: battery charge and discharge

NorthGuy wrote: »

According to Wayne from Nova Scotia, they will.

It is more efficient to cycle deeper, but if you already have a bank that only requires shallow discharges, it doesn't make sense to cycle them deeper when you do not need it (may be once a year to see how it goes). It's the same as if you bought a truck which is too big for your loads, and you decide to put some rocks in it, so that you could fully use its capacity.

According to a lot of people they will, According to a lot of others they won't, believe whoever you want. If you want to rely on a set batteries that have no capacity, go for it. I just removed a set from 2001, The guy was telling me how great they were, I asked why are you replacing them, he said the bank would only carry the load for 30 Minutes and he was spending $8.00 a hour in fuel charging them instead of them lasting 9 hrs. I am like why did you wait so long, he's like they were still good, I guess that meant that they hadn't shorted out yet. Thats the math, you can't change it, batteries lose capacity over time.

If someone buys 2 days of capacity and after 5 years they only have 1 day and they are happy, so be it, most don't want to shell out the money after they figure out it's sitting there wasting away everyday.

Here is a PDF that if you find time has a lot of facts.

http://www.idea2ic.com/FUN_DOCUMENTS/Battery%20Life%20(and%20Death).pdf
.

ChrisOlson

Re: battery charge and discharge

Blackcherry04 wrote: »

I am like why did you wait so long, he's like they were still good, I guess that meant that they hadn't shorted out yet. Thats the math, you can't change it, batteries lose capacity over time.
.

Same here. Our boat batteries were 7 years old and technically they were still "good". But on a load test they only had about 60% capacity at the 20hr rate and got to over 100 degrees during absorb when trying to recharge them. They were like a bottomless pit - dump 1,000 amp-hours in to only get 400 amp-hours out.

So they were basically pooched.

At that point do I want to waste a bunch of power from the generator and shore charger to charge DOA batteries to only get out part of what they're capable of if they were new? We put eight new Interstate UL-16's in 'er. Got $336 scrap value for the old 4D's that were in it, which paid for one new battery.

OTOH, if I felt like living with that 60% of original capacity I could probably get another 2-3 years out of them and they'd still be "good".
--
Chris

Cariboocoot

Re: battery charge and discharge

You could look at it this way:

You plan your system to have a battery capacity of 'X' Amp hours and to use 25% of that capacity daily. Then there's 25% in reserve for two bad days in a row.
When the real capacity reaches 0.75X you're down a day and have no reserve. By this point previous experience should tell you if you can continue operating with one day's capacity or if you'd better stick a crowbar in your wallet and buy new batteries.

Blackcherry04

Re: battery charge and discharge

Sometimes it's slow and sometimes it isn't. Let a big Dendrite growth short it out and it'll be sudden impact. Most old batteries I see all show Positive plate growth and some kind of Positive post protrusion.

stephendv

Re: battery charge and discharge

Blackcherry04 wrote: »

Sometimes it's slow and sometimes it isn't. Let a big Dendrite growth short it out and it'll be sudden impact. Most old batteries I see all show Positive plate growth and some kind of Positive post protrusion.

What are the causes of positive plate growth and post protrusion (which I guess are the same thing)?

Blackcherry04

Re: battery charge and discharge

Here is the best explanation I found. I am sure that someone will come up with a better answer. Basically the charge cycle, that is hard to run from.

http://batteryuniversity.com/learn/article/corrosion_shedding_and_internal_short/

stephendv

Re: battery charge and discharge

Blackcherry04 wrote: »

Here is the best explanation I found. I am sure that someone will come up with a better answer. Basically the charge cycle, that is hard to run from.
http://batteryuniversity.com/learn/article/corrosion_shedding_and_internal_short/

Pure gold! The interesting conclusion was that high voltage absorbs (2.58V) resulted in longer battery life than normal absorbs (2.4V). But it should be noted that the experiments were done on traction applications at ambient temperature, so the batts would be subject to deep daily discharges and recharges. I wonder if the results would still hold for RE applications, where batteries can sit at > 80% SoC for many months in summer?
Maybe then the corrosion from overcharge would outweigh the anti-sulfation benefits of a high voltage charge?