For the Dually mod:
It's important that you run the same kinds of
batteries. It's not AS important that you run a married pair, like in a series device. Running two batteries in parallel is nowhere near as sensation as two in series, so you don't really have to do anything special. Running the same kind is mostly so they fit and the tops are as close to the same height as possible, not so much due to the chemistry of the cells and keeping them balanced etc, your typical charger cycle should be just fine.
In a series mod, I run the Kalamazoo for example, I have three matched pairs of cells that I always run together, I alternate them from first to second etc. and make sure they are always balanced. Because they are stacked, in series, there's some procedures to follow.
For the reason of testing, I have been pretty bad about rotating cells and I'll I've done with the Kalamazoo with either dna200 or sx350j boards installed, is to keep the pairs of two cells consistently together and try n of yo over discharge them and I've never had any kind of trouble. The fact is that 18650 cells are made to run together, in series or parallel, so it's not too much of an issue anyways.
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