Batteries produce electrical energy by means of chemical changes inside. Batteries have an anode (+ve connection), cathode (-ve connection) and an electrolyte. The electrolyte changes as the battery is used. Some batteries can be re-charged, but ordinary batteries undergo permanent chemical changes and eventually go "flat".
Batteries are available in different sizes -e.g. AAA, AA, C, D sizes. All these batteries are 1.5 volts; however, it is important to remember that the larger ones provide more electrical energy than smaller ones. It is always better to use larger batteries for powering electric motors if they are not too heavy.
If batteries are connected in series(+ve to -ve to +ve to -ve) the total voltage is the sum of all the 1.5 volt batteries used. Two batteries connected in series give 3 ../60/volts, 3 batteries give 4.5 volts and so on.
Some batteries such as PP3 give 9 volts because they consist of 6 smaller 1.5 volt batteries (or cells) connected in series. It is a mistake to think that a small 9 volt battery will power a motor. The motor may run quickly at first, but the battery will go flat very rapidly.
BATTERY SAFETY
1) Never mix re-chargeable batteries with ordinary ones.
2) Take great care not to short-circuit batteries especially re-chargeable ones.
3) Never put batteries in a pocket where they may touch coins, keys or other metal objects.
4) Never mix used batteries with new ones. (The new ones try to charge up the old ones and may cause them to explode)