I believe that battery voltage is additive when cells are connected in series, ie a 3.7v cell is 3.7v; two 3.7v cells in series yields 7.4v. This seems to be confirmed by voltages in multi-cell battery packs.

But I also though that max amps or amp-hours are additive when the cells are connected in parallel. For example, if a single 3.7v cell is capable of 10 amp max rate and 1000 maH, connecting two of these in parallel would yield 3.7v-20 amp max rate-2000 maH total current.

Why would this be of interest? I am thinking of increasing the runtime of a Losi Micro DesertTruck. It has an optional battery carrier/wheelie bar. If a second battery could be carried in this carrier, and connected in parallel (not series) to the battery in the chassis, then the same voltage would be available to the motor, but twice the run time should result.

Twice the max current would also be available so I'd have to be careful I don't burn the motor out by being heavy footed (fingered?)

Is this correct?

I am going to try LiPo on this car; and if I get adequate run time I'll drop the 2-battery idea as it just adds a lot of weight

Regards

Yes, when in series, voltage adds and current capacity stays the same; when in parallel, voltage stays the same and current capacity adds.

But past a certain point, the parallel thing doesn't really make sense, unless you are doing something like endurance racing where you are prohibited from swapping the battery...

If the battery is perfectly cool after the run, let's say a 5000mAH LiPo, just swap it out and keep running. Running them both in parallel is only going to add weight.

One other thing to be REAL careful of is if you are going to connect two LiPo in parallel on-the-fly, make real certain that they are at the same charge level. If one is fully charged and the other is somewhat discharged, one is going to charge the other real fast and you could end up with a bad situation on your hands.