We often see people saying “can I connect these together?” and they will show a couple of batteries. And following that are a host of comments ranging from completely dangerous, to correct, and everything in between. So here’s the perspective from PowerPaul about mixing and matching your batteries (image just for attention, we don’t use AA’s in our batteries 😉).
Firstly, DO NOT connect LifePO4 batteries with internal BMS’s (that’s nearly all of them) in series! I don’t know how many times I’ve had to repeat this to people, but I’ll do it again. When you connect these in series, there’s no way for each “battery” to balance with another battery in the string. Even external balancers seem to produce very poor results and ultimately the batteries go out of balance, causing big problems. Just get the correct voltage battery to start with.
Second, the BMS’s are the most important thing with connecting in parallel. It’s our POV that the BMS’s must be identical in specification and settings. So if you have a Scout, a Cub and a Nifty, then you can use them together no problem at all. Similarly if you have a Scout Lite, Cub Lite and Nifty Lite, you can use them together. However you should NOT use a Scout lite and a Cub together.
Third, batter chemistry matters. Basically all LiFePO4 cells are close enough in chemistry and voltage to run together, regardless of brand, capacity, model or physical style. However you must NOT mix LiFePO4 with Sodium Ion, Li-Ion, Li-Po or Lead Acid based batteries.
Myth 1, “you can’t mix old and new LiFePO4 batteries”. This is a latent carry-over from AGM batteries, and does not have the same effect on LiFePO4 chemistry. So if you have a 5yo battery and a brand new battery, with the same BMS (as mentioned above), you can use these together. The caveat to this is that all the batteries must be working ok, a faulty battery should be removed from service immediately.
Myth 2, “parallel battery BMS current capacity add together” e.g. 3 x 100A BMS = 300A. Or “parallel BMS’s only have the current capacity of a single BMS” e.g. 3 x 100A BMS = 100A. The fact is neither of those are correct, and the true answer is “somewhere in between”. The BMS capacity (meaning maximum drain from the parallel battery bank) does go up when adding extra batteries, but not proportionally. It’s our suggestion that if you have 2 x 100A BMS then 150A is a safe maximum. 3 x 100A BMS would be about 200A safe maximum, and so on. In practice, the issues that arise when this is done, normally only occur at the very top or bottom of the SOC range when a single battery takes the entire load, so most people will never experience any problem unless they drain it flat.
Another fact to keep in mind… Most manufacturers will void your warranty when used with another brand or model of battery. Always check the warranty information, FAQ’s, and User Manual for details on using your batteries with others. Generally we suggest to not mix and match anything unless it’s specifically mentioned to be ok, the risk of failure or worse is too high.
