What is the difference in batteries? I am working with some rc units etc and they eat batteries. for example is a componet calls for 6 volts could I use an old cell phone battery instead of 4 AA 1.5?

Why are rechargeable AA only 1.25 volts instead of 1.5 as alkaline batteries?

 

Rather than try to go over all the material, let me give you a link to a site that has it all - http://batteryuniversity.com/.

For your sub of a cell phone battery for 4 AA's, that may not do very well. The cell phone doesn't pull much current, so the battery pack may not work as well as the AA's. The cell pack voltage may be different, too.

Rechargeables have different voltages than throw-aways because of different internal chemistry. The Battery University link can explain.