So, if I used the 1500mAh cells, that would give a total of about 12000mAh. Wouldn't that take way too long to charge though, even if it did last longer?

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Your math is not quite right. A mAh is not energy, so you don't just multiply the number of cells by the mAh rating. It is the product of current (milliamperes) and time (hours). If I have two cells in parallel, then I get double the current, and thus double the mAh. If the two cells are in series, the mAh rating does not change because the current does not change; the current is simply delivered at twice the voltage.

The original pack consists of 3500mAh or 4000mAh cells. So, using 1500mAh cells in a purely series combination would give you just 1500mAh, and hence shorter, not longer, runtime.

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