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Please help me understand the problem of using batteries of the same chemistry and voltage but differing AHr sizes in parellel.

1holaguy

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Oct 24, 2020
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I currently have a battery bank in my RV consisting of 2- 100 AHr 12v SOK batteries in parallel connected to a 12v/120v inverter/charger. I want to add capacity to it and the best option (size/price) would be to add a EG4 400 AHr 12v Server rack battery. Based on Will's EG4 tear-down videos it appear the EG4 12v, 24v and 48v server rack batteries are all made using the same number of same size cells in different configurations (12v/ 400 AH= 16 100 AH cells in parallel, 24/200 Ah = 8P2S, etc.) If this is the case it would seem placing a 12v/400 AH SR battery in parallel with the the two existing SOKs should create a 600 Ahr 12V bank. BUT - I was told by Singular Solar CS that I should instead replace the SOKs with 2 of the EG4s or add 4 SS 100AHr 12v batteries . Aside of increased capacity (800 vs 600 AH) why should mixing battery sizes as noted above make a difference as all are in parallel?
 
You can mix different amp-hour sized batteries if they are in parallel without a problem. Don't do it in series.

The only time the capacity difference will be a problem is if you have one very large and one very small. For example, it's fine to put a 400ah bank in parallel with a 500ah bank, but it probably wouldn't be so good if you put a 500ah bank in parallel with a 50ah bank. So long as the amp hour capacity is reasonably close, you'll be fine.

You can even mix different chemistries in parallel so long as your charge/discharge profiles fall within the voltage and current limits of both.
 
You can mix different amp-hour sized batteries if they are in parallel without a problem. Don't do it in series.

The only time the capacity difference will be a problem is if you have one very large and one very small. For example, it's fine to put a 400ah bank in parallel with a 500ah bank, but it probably wouldn't be so good if you put a 500ah bank in parallel with a 50ah bank. So long as the amp hour capacity is reasonably close, you'll be fine.

You can even mix different chemistries in parallel so long as your charge/discharge profiles fall within the voltage and current limits of both.
Ok, but based on above am I ok adding a 400 AHr SR battery in parallel with 2 - 100 AHr batteries (all LiFePO4)? If so, does it matter if they are configured 100>100>400> I/C, 400>100>1000>I/C or 100>400>100>I/C?
 
The issue is that the internal resistance is so low the LFP does not self equalize based on capacity nearly as well as lead-acid did. Can have those different sizes and they may all contribute the same amps at the start so in effect the smaller batteries may discharge a bit faster until there is some voltage difference. A slow deep cycle will not matter so much but a fast short cycle will have more effect. Keep a watch on the amp measurements the first couple of cycles to know what is happening vs what is expected. Probably not a huge deal in most circumstances.

Post the results.
 
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