series: add voltges. A battery is a series of cells.
parallel: add currents — sort of. This is not a good way to store batteries, by the way.
V = ℰ − Ir
|V =||terminal voltage, the voltage that you actually get out of the battery under operating conditions, the voltage that you'd measure across the terminals|
|ℰ =||electromotive force (emf), the ideal voltage of the battery, the electic potential difference caused by the chemical reaction taking place inthe battery, a number that chemists learn to compute|
|I =||current, the load that the battery is delivering to the circuit|
|r =||internal resistance, every part in a circuit has some resistance — even batteries, which are normally thought of as pure voltage elements|