I'm thinking about running a 5s setup in my stock summit (I've read it's a little risky, but I'm a fairly conservative driver) and I've read a lot of "you need to use the same mah and same discharge rating for each battery".

Well, that doesn't make a ton of sense to me. If I have a 2s 5000mah 50c lipo paired with a 3s 5000 mah 50c lipo, isn't the 3s going to run down faster than the 2s considering it's delivering more voltage for every given amp? Wouldn't I want a 7500 mah 3s lipo on the other side (Basically each cell installed in the car is 2500 mah)?

Now, given that the 2s lipo is going to be running the BEC side, I guess I could probably get away with 6200 ~ 7000 mah on the 3s battery and have the two batteries hit 3.3v at roughly the same time since the BEC side always drains a bit faster.

Anyway, can somebody tell me why my logic is either correct or flawed?