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Must batteries be identical?


Scubadude

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Quick question ... Must batteries in a solar system be identical? Or is it good enough to have the same type and capacity? Why I'm asking ... If I install a Shoto 5.12kWh battery today and want to add another a year or two from now, the charge and discharge characteristics will be similar rather than identical to a battery that's done 1000 cycles. Or not?

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Strictly speaking the battery BMS should take care of any differences in the battery capacity/charge/discharge characteristics. However, one must make sure that the batteries are balanced before adding them to the system.

You charge each battery to 100% independently, then connect them in parallel, without connecting them to the inverter, and allow them to balance overnight. Then you connect them to the inverter. I then discharge the batteries down to 20% SoC nd then let them charge again with the inverter/MS managing the process.. It takes a few charge/discharge cycles for the batteries to settle...

That's why its important to buy decent batteries that can talk to your inverter...

 

Edited by FixAMess
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So @Scubadude,

My understanding is that the batteries do not need to be the same capacities, age, they do however need to be the same chemistry so that the start and finish voltages are the same.  The voltage will stay constant across all the batteries wired. You can connect Pylontech batteries of different capacities in the same bank for example. 

This however does not address the issue of BMS interaction with the inverter. The batteries all need to be working in sync with one of the BMS's being a "master" and the rest slaves to as to ensure that the batteries are all disconnecting at the same time, otherwise you will end up with cascading disconnects which could damage the batteries. 

I doubt that having batteries of different ages would be an issue at all as long as the BMS's are speaking.

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