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Small Cabin Forum / Off-Grid Living / Why do Lead Acid Bats Not Have Expected Power
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# Posted: 25 Feb 2022 20:58 - Edited by: gcrank1

Or, maybe better, why dont they have the expected amp hours we thought we we paying for?
Ive been messing around with stuff using fla bats for over 50years so Ive had more than a few. At some point we turn the key and engine doesnt spin, or spins slow to a grinding stop.
It doesnt have 'beans' any more.
For this lets say this isnt a 'good battery' with a charging system problem, it is a battery with a problem.
More to the forum, my thoughts are to battery storage for the cabin solar-elec system.
At some point we have a balanced system that recharges pretty much daily for the amps/watts used from the bat-bank. Then it doesnt, and it is not in the winter or usage hasnt spiked. It just aint keepin up anymore.....
Lets say it isnt a charging system problem, its the bat(s).
For this example Im going to use the common 12v fla.
We have a 'deep cycle' bat with 100ah rating.
So we figure, correctly, that we shouldnt be using more than 50%, or to 50ah State of Charge (soc) if we want long life.
At first it works pretty well, the hours of use for the amps/watts we draw match up well.
Then it falls off. Well, it didnt likely happen suddenly, it was suddenly we notice. We only get a few of the ah's we expected by calc and experience (or even with a fancy monitor). And we know that fla loses capacity over time or rather cycles of use. 80% is not unusual to hear mentioned.
Here is my thinkin (and I havent seen this postulated elsewhere in this way).
100ah battery @ 50 usable (50%) then it deteriorates to 80% (80 ah).
We figure its now an 80ah so we can go 40 usable (50%).
But NO, it doesnt run stuff 'all that long' before it 'runs out of beans' (depletes sooner than expected by a good bit).
I believe its because the 'base' is still the 50ah (of the orig rating 100ah) and the now 80% capacity has Only 30ah of 'good power' left usable. We go 'deeper' and the battery dies sooner from load abuse.....
This correlates to my experience with the 4 old, failing bats in my system. I knew they were dying so I was monitoring a lot; ie, at first I didnt know quite what to expect, then I did. Lots of practice watching them slowly die.
One of my friends here is a fisherman and he has experienced the same with his trolling motor batteries.
This is probably something to do with the Peukert Effect, which has complicated formula's, but the above may be a useful rule of thumb to anticipate what is going on.

# Posted: 26 Feb 2022 13:39

There is a saying in the solar industry "Batteries don't die, they are murdered"!

Sure FLA batts do loose capacity over time, but it can be managed so as to not happen that fast. This is also why we usually oversize a system.

However, I have seen so many systems that just weren't properly monitored and maintained. Drawing down to what they think is %50 SOC by monitoring voltage (usually using a cheap Chinese volt meter). Then not charging back up properly taking into account the Peukert Effect. Nor running an equalization charge on a regular interval to remove sulphates from the plates and rebalance the cells.

Simply put.. A proper battery monitor is a requirement for a FLA system (all systems really). All the good ones have algorithms that take the Peukert Effect into consideration and have high quality shunts.

Now with all that said.... 5 years is about all you can expect from a properly sized and maintained FLA system that is cycled heavily every day. One or more cells will fail rendering the bank "kaputski".

# Posted: 26 Feb 2022 15:57

I was just talking with another fisher-friend, he has a nice rig and is pretty particular. Rig has 4 12v bats, 1 starter for the big outboard, 1 dc for everything 'command' and 2S dc for the 24v troller. He doesnt buy cheap bats either, but has only got 2yrs out of the newest agm command bat ($200), now it isnt lasting very long. He does faithfully charge up every night with a quality marine charger. When I mentioned to only use normally to 50% soc he laughed, said he goes way deeper than that; as if it was no big deal. Well, yeah, it worked for 2 yrs! Murder can be fast or slow......
Reviewing the Peukert Law there is a formula for the true ah's, and mention that the better bat-monitors want you to input the Peukert rated capacity for them to work correctly. I bet a lot of folks dont do that from the many queries Ive read about 'my meter isnt reading right'.
It was said that a typical 100 ah dc Peukert's out at 75-80. Guess if we want 100ah in fla we need to get 120 ah bats; and we still only get 50 usable

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