An electric battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Connecting a battery's negative to another's positive is a way to increase voltage, useful when a battery outputs low voltage.
An electric battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. Each battery consists of a negative electrode (anode) that holds charged ions, a positive electrode (cathode) that holds discharged ions, an electrolyte that allows ions to move from anode to cathode during discharge (and return during recharge) and terminals that allow current to flow out of the battery to perform work.
There are primary cells, which are not rechargeable, and secondary cells, which are. Common types of batteries include lead-acid, alkaline, NiCd, NiMH, and lithium-ion/lithium-polymer (LiPo).
Making batteries can be as simple as making a lemon/potato battery.
Further reading
An Accurate Electrical Battery Model Capable of Predicting Runtime and I–V Performance (2005).