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All these words basically describe the strength of a battery, but they’re all specifically different.

Voltage = force at which the reaction driving the battery pushes electrons through the cell. This is also known as electrical potential, and depends on the difference in potential between the reactions that occur at each of the electrodes. The higher the voltage, the more work the same number of electrons can do.

Current = the number of electrons that happen to be passing through any one point of a circuit at a given time. The higher the current, the more work it can do at the same voltage.

Power = voltage x current. The higher the power, the quicker the rate at which a battery can do work—this relationship shows how voltage and current are both important for working out what a battery is suitable for.

Capacity = the power of the battery as a function of time, which is used to describe the length of time a battery will be able to power a device. A high-capacity battery will be able to keep going for a longer period before going flat/running out of current. Some batteries have a sad little quirk—if you try and draw too much from them too quickly, the chemical reactions involved can’t keep up and the capacity is less! So, we always have to be careful when we talk about battery capacity and remember what the battery is going to be used for.

Another popular term is ‘energy density’. This is the amount of energy a device can hold per unit volume, in other words, how much bang you get for your buck in terms of power vs. size. With a battery, generally the higher the energy density the better, as it means the battery can be smaller and more compact, which is always a plus when you need it to power something you want to keep in your pocket. It’s also a plus for electric cars—the batteries have to fit in the car somehow!

For some applications, such as storing electricity at a renewable power plant like a wind or solar farm, high energy density isn’t so much of a requirement as there will most likely be ample space to store the batteries. The main goal for this use would be to simply store as much electricity as possible, as safely and cheaply as possible.