The capacity of the two LiPos must match, and they must have the same number of cycles on them. They must also be the same brand, series etc.
IE: They must be essentially identical cells to each other.
If one LiPo has a lesser capacity to another, and are run together, they are discharged at the same rate, therefore, the pack with the lower capacity will be discharged first, and will probably drop below the 2.8V threshold before the LVC kicks in. This will damage the pack, and could cause a battery fire.
This is the exact reason that I wrote that no 4mm connectors can be used in my draft set of rules - to stop anyone from being tempted to use two 1/10 packs. Obviously, you asked first, which is the sensible thing to do, but I fear that others may try.
Over time, (cycles), the overall capacity of a cell will change, as can the IR. This means that even if the two cells are of the same brand, capacity etc, but are on different cycles (obviously, one or two can't hurt, but if they are 5 or more apart, then there will be effects), then essentially they ARE different cells as they will have slightly different capacities, and yes, it does make a difference.
Different (brand's) cells have different cell chemistries, so the IR (internal resistance) is different, as are other factors.
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11-11-2009
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