Lithium-ion battery addition to existing battery bank

[Marcel Beaulac]

Good day!

I'm in the buying market for LiFePo4 lithium-ion batteries. I am planning to buy two 100Ah batteries in the next month and two more next year. I'm told that I can't mix old and new batteries. Mixing them will affect the life expectancy of the 4 batteries. Have you heard this before?

[Rob Beckers]

Hi Marcel,

Mixing lead-acid batteries of various ages doesn't work well. New lead-acid batteries added to old ones quickly become just like the old ones and you don't get the benefit or longevity of those new batteries.

You mention lithium-ion batteries though, and things are a little different (and still somewhat unclear) for those.

Adding new lithium-ion batteries to a set of older ones should work fine. For the time being my advice is to stick with the same brand and type lithium-ion batteries as the old ones (when adding). While in theory they likely all use LiFePO4 as their chemistry and should all charge at the same Voltages there may be differences between brands that would cause batteries to get out-of-sync when charging/discharging. For series connections you have to use batteries with the same Ah rating anyway (and as mentioned, I would advise the same brand). How old the batteries are matters less, since for most people we expect lithium-ion batteries to last 20 years (or thereabouts).

-RoB-