using different size batteries.

[skeetr]

What kind of problems could one run into by using different size batteries.
Say an 18650 and a 18350 run in series for a regulated 5v mod. Just trying to maximize mah without going too big and yet providing a chunk of space for electronics or a juice bottle.

 

[WillyB]

It's not recommended. Even the same batteries, one new and one older and used is not a good idea.

 

[Rocketman]

Draw out your circuit with two batteries in series with an atty and a switch.
Put them in any order you want, they are in series, doesn't matter. Also draw out a circuit with a battery a switch and a charger all in series. Notice the similarities? Notice one MAJOR difference.
If one battery voltage is lower than the other it will work like a battery dis-charger for the weak battery.

R