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Amp Hour (AH) spec - LiFePO4 vs Lead Acid

markaubin

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Aug 4, 2021
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I have a 48v lead acid battery bank in my off grid cabin that I installed in 2010, composed of eight 6V Rolls S-530 batteries. The label on the battery has three amp hour numbers as follows:
320 AH 8Hr
400 AH 20Hr
532 AH 100Hr

My question is when sizing a replacement LiFePO4 battery bank, which AH number number do I use for comparison? This is just to give me a starting point as I realize there are a lot of other variables. Back when I first installed the lead acid batteries, the capacity seemed to be adequate.

For all the LiFePO4 battery banks, I'm seeing just a single AH number, not a chart like the lead acid.

Thanks!
 

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The practical answer is all the LFP AH's you can afford or place within the space you have.

As to matching your use case battery life that you had with lead-acid, it depends on your typical use case current draw from lead acid batteries.

The twenty-hour number is likely more closely matching your use case. Also realize lead-acid should be limited to about 50% depth of discharge so not to degrade plates' lead to support grid structure. 'Deep discharge' lead acid batteries just have thicker lead plates so it is a bit longer before lead to grid degradation occurs. The Roll's '4000' series plates are thicker than auto or marine battery but not super thick like the nearly half inch thick plates of the 2v individual Roll's cells.

You can use more of the AH rating, deeper discharge, on LFP batteries so you effectively get more useable AH's for same battery AH capacity.

If you are just looking to match the capacity you presently have, sixteen 280AH LFP cells will give you as much useable capacity as you presently have with 400 AH Roll's batteries.
 
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Generally amp hour rating is for 20 hours unless specified as different. Most lead acid deep cycle batteries will only give one number and not a range like your Rolls.

As to sizing replacement. Loads. Size for your loads.
 
Lead acid batteries are subject to Peukert's law, which accounts for the fact that at higher currents the battery will provide fewer Ah than at lower currents. For LiFePO4, the Peukert coefficient is nearly 1.00, meaning that it will nearly deliver the same Ah at any current. So that is why lead-acid batteries have different Ah ratings at different currents, but LiFePO4 does not.

So that explains why there are different ratings for your Rolls batteries. @RCinFLA and @Mattb4 gave you where to start to think about going to LiFePO4.

Many folks here try to stay between 10% and 90% state-of-charge for their LiFePO4 banks (the specific limits are subject to opinion), so it is safe to assume you can utilize 80% of the Ah capacity compared to 50% of the 20 hour rating of your lead acid bank.
 
Thanks everybody this is very helpful. I'm going to start with a "25kWh SOK 48v Rack Battery Kit" from Current Connected.

Based on @Horsefly's numbers above:
old - 400AH lead acid * 50% = 200 effective capacity
new - 5 * 100AH LiFePO4 = 500AH * 80% = 400 effective capacity

This is roughly double the capacity of my 12 year old system when it was new. It will be really hard to compare since my old system's performance has been degrading for a while and more recently has gotten really poor. So anything new obviously will be way better.
 
Generally amp hour rating is for 20 hours unless specified as different. Most lead acid deep cycle batteries will only give one number and not a range like your Rolls.

As to sizing replacement. Loads. Size for your loads.
As an example, the WallyWorld marine deep cycle batteries are specced at 1a draw, pretty misleading unless you can find the fine print.
 
The wonderful thing about standards is that there are so many to choose from. WallyWorld does a 1a test for its capacity testing. 20 is the standard, but double check the sticker.

I have found reserve capacity, RC (minutes discharging @ 25A above 10.5V), to be more common on "consumer" batteries. One can ratio out the C20 capacity based on comparison to another similar battery with more thorough specs. It's not foolproof, but its a good approximation.
 
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