3S 2200mah and 3S 1300mah In Parallel

Batteries for model aircraft usually come in the form of "battery packs" made up of several "cells". The cells are joined together by soldered metal strips, and then the whole pack is usually heat shrink wrapped in tough plastic material. The cells can be arranged in different patterns in the pack, to give many possible pack shapes, to fit neatly into the battery bay of whatever plane you have.

I have some 2200 and 1300 mah 3s packs I would like to use in parallel as lipo battery 3S pack in my new airplane. Will they discharge evenly or can you only use it as a 2600 mah pack? I cannot buy new packs before the festive season really takes hold so it would make my life easy if it works.

The internal resistance of the 1300 must be 1.69 times the resistance of the 2200mAh 2s lipo batteries. Otherwise the current will not divide accordingly and the theory that proved me wrong does not hold anymore. What does this mean?

The 1.69 is simply 2200/1300, if the IR of the 1300 is not 1.69 as high as the 2200, e.g. the same as the 2200mAh, the current will be the same, and you'll draw more from the 1300mAh than the 2200mAh. Of course, the IR is never perfectly matched, and with a slight mismatch the 2200mAh will just charge the 1300mAh pack in the mean time as well, but the bigger the mismatch the worse it gets, and at one point you're going to exceed the charging C rating of the 1300, which is when it gets bad for the whole situation. Of course you don't discharge it for 500mAh and then top it up again with the bigger lipo, it's a continuous process. Let's say we have two packs with the same internal resistance and voltage, but one pack has a capacity of 1Ah while the other has a capacity of 4Ah. Now you start drawing 50A from it, which would be perfectly fine if it was just a 4Ah pack, but in this case the current is distributed according to Ohm's law (same IR, same voltage, same current), so you draw 25A from both packs. If the 1Ah pack has a C rating of below 25, this is already a bad thing.

Well, both packs must have the same properties in order to make it work. Otherwise the packs are not going to like it (the lower capacity pack, that is). The closer the capacities are together, the less the effects. The better matched the internal resistance, the better. If they're not matched one will draw a higher current than the other, which is obviously a bad thing.

Of course, the other rules still apply, such as no connecting an empty pack to a full one.

So how do you get two packs with similar properties? Well, they have to be the same brand, that's a given. But they should also be made in the same batch for the best performance. This also means you're not going to get matched properties out of a 1300mAh and a 2200mAh, since those are made in different batches.

Methinks you are thinking of effects in isolation rather than how they apply with other effects in a global system... as Paintz said it is preferable if the impedance is similar, but it isn't make or break. It's just not optimal.

Also not sure what you mean IR is inverse to capacity... afaik it is only so in regards to the relationship between current and capacity, which is to say that available capacity reduces as current increases due to the IR of the pack. A low IR pack will suffer less from this effect than a High IR pack, I'm not sure what leap of logic you made to reach the 1.69 figure - can you explain what you mean?

The overall capacity is not the issue here however, as neither pack will allow the other to exist at a lower voltage state. The overall discharge will end up even between them as though they were a single larger pack.