SOC vs. additional series battery

rdanneskjold

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Jun 13, 2021
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I read somewhere that charging the last 10-15% of a battery causes it to expire earlier than they otherwise would. IOW, charging a 3.8v battery to 3.4v greatly extendes the battery's number of charge cycles.

First, is this true? yes, you give up significant capacity, but your battery will last a lot longer.

As i understand it, you can control state-of-charge settings in some controllers to do this, i.e., tell it to what voltage to charge to. my battery is going to be 12 x 4 12v AGM deep-cycle. what if I put 5 batteries in series instead? then each would only ever see 9.8v.

background: planning to go off-grid with solar, battery bank, and backup generator. NOT for economy, but for security and peace of mind. and because I am an irredeemable geek. :)

Thoughts?
 

wiseacre

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Take my post with a grain of salt. I don't know much more than you.

Not fully charging to extend life is a LiFePo4 characteristic - not lead acid.

AGM lead acid batteries need to be fully recharged or they will lose life.
Lead acid also need the DoD limited to about 50% to extend life
12 x 4 12v AGM deep-cycle
No idea what you mean. 12 x 4? what is the 12, what is the 4?
what if I put 5 batteries in series instead?
Voltage adds in series, put 5 ~ 12vbatteries in series and you end up with a 60v (same Ah as a single one) battery bank.
Going back to the 12 x 4 it seems like you mean (4) 12v batteries.

nomenclature when using series and parallel connections
4 in series is 4s
2 in series then paralleled to another pair in series is 2s2p
4 all in parallel would be 4p

each of those examples would need a different charging/discharging profile
For a very basic example:
I have (2) 12v, 100 Ah GEL (lead acid) in series - 2s - that makes a 24v, 100Ah battery bank. I discharge to 24.4v and recharge to 28.8. There's more to just recharging to 28.8. Proper settings of the Solar Charge Controller for different charging stages need to be set.

Hope that helps some. If I'm incorrect I'm sure someone will correct me. This is after all an on-line forum and the best way to get corrected is to post inaccurate info.
 

rdanneskjold

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Jun 13, 2021
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sorry, I wrote that post in a fit of insomnia. I meant 4S4P. 100Ah deep cycle AGM 12v each for 48V.

I was asking about charging 60v of battery capacity to only 48V. apparently there is no benefit with lead-acid batteries by doing this.

My understanding is that deep cycle batteries can be discharged more than you are indicating. are your batteries deep cycle? I gather that DoD is where my focus ought to be.

Thanks for the info!


RD
 

wiseacre

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4S4P. 100Ah deep cycle AGM 12v each
I get 48v, 400Ah
4s x 12v = 48v
4p x 100Ah = 400Ah
I could be wrong but my understanding is, Series adds volts / Parallel adds amps

Yes, keep them fully charged. Lead acid is different from LiFePo4. While battery life for LiFePo4 is extended by keeping them at a lower state of charge it will do more harm than good to keep any lead acid under charged.

Yes, focus on DoD
for longer life ~ 50% DoD at most is recommended for deep cycle AGM or Gel batteries
 

MisterSandals

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4s x 12v = 48v
Ah, sorry. I was thinking cells. I missed quite a few notes where you mentioned AGMs too!
I could be wrong but my understanding is, Series adds volts / Parallel adds amps
Nope, you're spot on.
Yes, focus on DoD
for longer life ~ 50% DoD at most is recommended for deep cycle AGM or Gel batteries
Yes, below 50% will soften them up when the batteries are newer. When they get near end of battery life, my experience is that it often kills the battery so it won't hold a charge over 12V.
 
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