Combined in Parallel When imbalanced batteries are connected in parallel, the voltages of the batteries should match, but the capacities can be different. When lithium-ion batteries are connected in parallel, their capacities are effectively combined, resulting in a higher overall capacity.
Yes, you can mix different capacity lithium batteries, whether a normal 12V 100Ah battery or a Lithium server rack battery. You can combine different capacity batteries in parallel. You cannot combine different capacity batteries in series. There are a few points you need to consider when wiring in parallel. Let’s explore these three points.
There are at least several different kind of "lithium" batteries out there. If all you know about a battery is that it is "lithium" then you don't know anything about the battery. There's Lithium ion, lithium polymer, lithium iron phosphate, and whatever crappy non-rechargeable AA and AAA batteries labelled Lithium are. They're all different.
Let’s suppose you have 3 different 12V batteries, wired in parallel to supply 12V power to your RV. They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren’t.
The batteries must have the same chemistry as well. It is not possible to combine lithium-ion with LiFePO4. This is because lithium-ion has a slightly higher voltage than LiFePO4. The batteries get charged all at the same time. A smaller battery will be at the same voltage as a larger battery.
They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren’t. Charge will flow from one battery to the other two until they’re balanced.