How far do you discharge your AGM batteries?

[Texas-Mark]

I am pretty conservative with my FLAs and AGMs and have a discharge cutoff of 12.1V on both. I have four 140Ah AGM batteries that are about 6 years old. I just did a capacity test down to 12.1v and got 51Ah each on all four. Obviously that is not 50% of the 140Ah rating. Since they are somewhat old and I will eventually replace them with Lifepo4, what would be a good voltage cutoff that will not kill them in the next year or so, but give me more than the 51Ah?

 

[jberger]

What was the C rate and temps during the test?
AGM's use a 20 hour (0.20C) rate at 77 degrees F for standardized testing.
Pull the manufacturer's spec sheet for the model battery you have and validate the discharge curve and 50% voltage level.

 

[Texas-Mark]

What was the C rate and temps during the test?

I did the test at 5 amps @ about 78 degrees, which is generally the the most I would be pulling from it.

Pull the manufacturer's spec sheet for the model battery you have and validate the discharge curve and 50% voltage level.

I often see 50% listed. But for light loads, I have read going as low as 11.8V (80% discharge) is fine. I'm just trying to see what people are actually using, vice what it recommended.

 

[jberger]

I'd stick with 50% discharge on AGM, anything more is really asking for trouble, espectally on 6 year old units.

Given your very low discharge rate and test results, I'd say you are looking to replace them in the next year or so.

What does the spec sheet for those batteries list as the 50% mark?

 

[Texas-Mark]

What does the spec sheet for those batteries list as the 50% mark?

Well the spec sheet does not really list that as these are UPS12-490 batteries. Yes I know these are not the best batteries for continuous cycling, but they are 100lbs each, I got them for next to nothing, and they have been working well all this time. What it does say is that they can go down to 10.5 volts, but that is most likely only for intermittent discharges. In any case, I do plan to replace them withing a year, and figured I might as well get the most out of them that I can.

 

[wholybee]

Discharging them to 10.5 volts at a 20hr rate for a capacity test once per year or so will not harm them. Just bring them to 100% right afterward. During that test you can write down the voltage every 10 minutes. Then when it is done and you know the actual capacity you will also know what voltage corresponds to 50% under a 20Ah load. That is really the only way to know the condition of the battery. Trying to guess it by the voltage at some point in the middle of the discharge cycle doesn't really help.

 

[pollenface]

Mine are anywhere between 12.4v and 12.7v just before sunrise. Mainly just running a couple of led lights and a 260L fridge/freezer.

You will almost never get 50% of the advertised capacity at 50% depth of discharge with an AGM despite popular consensus. I think 30-40% is a more realistic.

Also the weight is important to take note of, a 140ah (@ 20hr rate) AGM should way at least 34kg.

 

[Substrate]

You may have walked them down in capacity if you haven't left them in the absorb stage until current falls to at least C/100 before going to float. Soo... they should stay in the absorb stage until you reach about 1.4A.

Sulfation not only reduces capacity obviously, but internally the cells can become unbalanced too.

If they are totally sulfated, then reaching C/100 in absorb may never be achieved - watch for a "stall" where there is no change in current for an hour as your trigger to float or stop.

But I'm talking non-interruptable ac charging. Are you doing solar only? That, and the word "pretty conservative" kind of caught my attention.

I guess what I'm saying is that when doing your capacity tests, are you really fully charged in the first place?

And if this is a solar concern, I have some techniques that might help mitigate this conservative undercharge.

 

[Texas-Mark]

I guess what I'm saying is that when doing your capacity tests, are you really fully charged in the first place?

To clarify, all I wanted to know was what the lowest voltage people were taking them down to. I originally had them set for a 12.1V cut-off like my FLAs. I wanted to know if those using them go lower. I did another capacity test down to 11.8v and got 83AH out of them. I think that is pretty good for 6 year old batteries. Since they will all be replaced within a year, I wanted to know if they would likely last that long if they occasionally were taken down that far.

And yes they are fully charged in the first place.

 

[Substrate]

That's fine and recommended. I never try to take mine below 12.1v and my LVD is set like that. My loads are light and I'm not series nor parallel connected however.

12.1v should be just fine as long as you say your batts are getting fully charged and then they will also be in balance. If they are actually out of balance, or go that way with age when you put them back into series/parallel service, then maybe think about raising the lvd to 12.5v. Give a little more headroom for the weakest one in the bunch not to be driven too low.

Sorry - I know, it's TMI time. It's a lead-acid thing and we can't help it.