I use Li Po batteries alot. In RC helicopters. Not the same battery but the basics still apply. Safety has been the major factor in my learning about electrical safety. Specifically in reference to multiple cell battery packs.
Batteries should be matched when used in multiples. True mAh and internal resistance having been measured. The labels aren't the best reference when matching as they are mass produced. And I haven't seen any batteries with IR listed on them, as it increases with use / age. So you have to measure this. And the discharge rate (C rating) should be the same.
What you call stacking is really placing the batteries in Series. Meaning the positive on one is connected to negative on the other. This basically doubles the voltage output. The capacity, measured in mAh in this case, will remain the same.
Running them in Parallel is when the negatives are connected to each other, and the positives are also connected to each other. This doubles the mAh, with the voltage output remaining the same.
Doubling voltage will basically drop amperage by half. But the mod or coil may not be able to handle the extra power.
"C rating" is the discharge rate. It's important, as you don't want one battery discharging faster than the other. IR will have an impact here. So you want batteries with similar IR
Like wise you don't want two different True capacities (mAh).
As the capacity is used up the voltage will drop. This is a predictable ratio based on battery chemistry.
Differing C ratings or true capacities can easily result in one battery becoming over discharging and becoming unstable. And batteries catching fire and or exploding near your face usually falls into the "Not a good day" category.
So when measuring voltage levels on multi battery (cell) devices you'll need to measure each individual cell.
You'll have to research battery chemistry, and the minimum safe voltage and minimum safe capacity of the every battery type you use.
Most LiPo batteries are made at the same few factories. The quality of batteries vary depending on what each company wants to pay. So just because they are made at the same factory, the name on the label can make a difference in quality.
Higher quality batteries will almost always cost more. But they will almost always have a high number of cycles over the course of their life (number of times it can be charged and discharged). Along with lower IR and more consistent mAh rating. You'll end up saving money in the long run.
If you plan to use unmatched batteries in series or parallel I recommend you test them before use. And research the basic info on the battery chemistry you will use. You'll also have research the equipment you use it in.
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