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Pairing lithium batteries of different capacities


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Good day

I need some advice with regards to adding an extra battery to my existing battery setup. Current configuration is two Narada 50Ah Lithium Ion (Model: 48NPFC50) batteries connected in parallel, which in turn is connected to a Axpert MKS II 5KW inverter. SOC is controlled by the ICC software running on a Raspberry PI board. The setup works really well and has done so for the last 8 months now. 

I now want to increase the battery storage capacity by adding an extra lithium ion battery. I already have the extra battery but this battery is a 100Ah battery, also Narada (Model: 48NPFC100), basically the same thing just 100Ah instead of 50Ah. The 100Ah battery is brand new and the existing batteries being around 8 months old (~ 50 cycles up to date, so not heavily used). My question now is whether I can just add the new 100Ah battery to the existing setup with it being the same brand, voltage and chemistry? The rate of charge and discharge between the two might possibly be a problem but I am not sure. I was told this would work but lithium battery storage is an expensive hobby so I do not want to damage any of the batteries.

The way the existing batteries are connected are as follow. The batteries are stacked on top of each other in a custom build frame. Along the side of the frame are two busbars (+ and -, running from top to bottom). From each of the battery terminals (+ and -) I have short cables of equal length connected to the busbars. A single pair of cables then connect from the busbars to the inverter. I was very pedantic about cable lengths having to be the exact same length (~ 5mm) to ensure good load distribution across the batteries. The idea would be to connect the new battery in the same configuration.

Any thoughts or advice will be greatly appreciated. 

Thank you

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@WynandS I'm no expert in batteries, but from my understanding mixing batteries are not always going to work out as you expect. The short answer is that it should not blow up.


You have your 2x 50Ah batteries set up in parallel. In this setup the voltage stays the same, but the amps is half from the one battery and half from the other giving you double the amps for your inverter when you use the batteries, and pushing in half the amps to each battery when you charge them. Thus if each battery is rated at supplying lets say 40amps (check your battery specs) then you can supply 80A to the inverter and if they can be charged at 20A then you can charge them combined at 40A.

As their specs are the same and if they start at the same state of charge (SOC), it means they will be depleted at the same time and be fully charged at the same time (in theory).

Now add the bigger battery. It's capacity is different. Lets assume it's resistance is the same (not sure if this will be the case - I would think it is different if the capacity is different, but I have not check if this is true). It will take longer to be depleted and long to be charged. Because Lithium Ion is always used with a BMS, the BMS of each of the batteries will stop the charge if the battery is full or stop the discharge if the battery is depleted. In the case that this happens the batteries that are left will have to handle the load (charge or discharge).

So if you limit your inverter/charger to stay inside the limits of the smallest values of 1 battery this should be work. There are plenty on assumptions. If all the batteries are exactly the same many of those variables go away and you can set those limits much higher. If your BMS is very smart all this could be a non issue as well. 

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