Does adding a new string of batteries to a bank of older strings of LiFePo4 batteries cause a problem? As in adding two more strings of 16 to a year or two old bank with two strings of 16. Expanding every year or so until desired size is met? Or do they all need to be put into service at the same time?
Adding new batteries to older batteries
- Thread starter Knownot
- Start date
wiseacre
Solar Addict
Adding new LiFePo4 to an older bank is fine and the reason I went from AGM (which can't/shouldn't) to lithium
remember to charge all batteries to the same SOC before adding them
I'll soon be adding a couple more DIY batteries to the ones I built last year
Capacity is not a concern either if wired in parallel
remember to charge all batteries to the same SOC before adding them
I'll soon be adding a couple more DIY batteries to the ones I built last year
Capacity is not a concern either if wired in parallel
So, you can add 280ah strings in parallel with 230ah strings?
Yes in parallel.
Got it
Yeah, but the lower capacity bank will determine the overall battery capacity if you are treating all the strings combined as one battery. You could have a bms on each string and have the 230 ah string cut out when it's SoC approaches zero, but that opens up a whole can of worms. IMO, don't do it. Not that big of difference.
Having said that, even a two year old 280ah string may have lost some capacity so adding even a identical new 280 ah string will have some of the same issues. There is an OffGrid Garage youtube video on capacity drop in 4 year old cells.
If they start at 100 SOC, when the 230AH battery is at 20% SoC (.20*230=46AH remaining), the 280 AH will have (280-(230*.80))=96 AH remaining and will be at 96/280=34% SoC. If you are running the 230AH to 0% SoC, then at that point there will be (280-230)=50AH left in the 280AH battery. So 50 AH and 50/280=17.8% of your 280 AH battery goes unused.
On edit: That's a rough estimate, but IIRC the effect is muted by the larger bank feeding the smaller bank...which throws some complexity into just how much capacity is waster/unused. IMO, just don't do it on that scale. 272 vs 280 is fine. I'm all for keeping it simple.
On edit: That's a rough estimate, but IIRC the effect is muted by the larger bank feeding the smaller bank...which throws some complexity into just how much capacity is waster/unused. IMO, just don't do it on that scale. 272 vs 280 is fine. I'm all for keeping it simple.
Last edited:
I intend to keep them all the same size.
