What is cordless power tool battery capacity and voltage?

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Different batteries sizes and weights are affected by battery capacity and voltage. Along with choosing a cordless power tool battery chemistry, the voltage of the tool (and therefore the battery) and the battery capacity also need to be decided upon before you buy.

Battery Capacity

Battery capacity is the measurement of how much energy the battery can store and release. The battery capacity is a measure of how much electrical energy the battery can store and release.
Battery capacity is measures in how many amps can be produced over an hour (amp-hours). It is measured in ‘amp-hours’ (also known as ‘ampere-hours’ and abbreviated to ‘Ah’). It is the theoretical maximum number of amps (there is always some variation) that can be supplied by the battery over an hour.
Amps used vary depending on the size of the load on the cordless power tool battery and affects how long they last. For example, if a fully charged battery has a capacity of 10Ah then it can supply 10A of power over an hour period or 5A of power over two hours or 20A of power over half an hour and so on.
harder materials require more current in amps from the cordless power tool battery to carry out a task. The number of amps drawn from a battery at any given time will depend on how much energy the cordless power tool requires before the motor starts (known as ‘no load’) and the effort required to complete the task (called ‘load’).
Working on metal or wood uses different loads and uses different number of amps. For example, drilling through wood takes less effort than drilling through steel and so will draw less power from the battery. The number of amps that can be drawn at any one time is limited by the circuits in the battery that prevent overheating.
Wonkee Donkee says: Imagine the battery is a car: the voltage is the engine size and the battery capacity is the size of the fuel tank.
Check your technical specifications for which batteries fit you cordless power tool. You can usually choose your own battery capacity so long as the voltage is correct (although always check your cordless power tool technical specifications).
There is more electrode material in cordless power tool batteries with larger battery capacities so they are heavier. Battery capacity is linked to how much electrode material is in the cell which means that batteries with larger capacities are bigger, heavier and more expensive than smaller ones.
Number of amps input into charging battery by charger. The larger the battery capacity, the longer it will need to recharge. You can work out an approximation of how long it will take by dividing the battery capacity by the number of amps the charger is putting into the battery (this can be found in the technical specifications or after ‘Sec.’ on the base of the charger, see What are the parts of a cordless power tool battery charger? Section Printed Information). This will give you an overestimation of the charging time in hours.
Charging a battery takes different lengths of time depending on the size of tha battery capacity. For example, a 4Ah Li-ion battery charged in a charger that has an output of 9A charges in about:

4 / 9 = 0.44 hours = 44 minutes

This estimation is about 2% longer than the actual charge time.

Two smaller cordless power tool batteries are sometimes better than one larger one. It is sometimes better to purchase two batteries with smaller capacities, particularly if you need to use the tool constantly because you can charge one while using the other.

Battery voltage

Voltage provides power and strength to the cordless power tool. The voltage indicates how powerful the battery is. The higher the voltage, the more powerful the battery.
Harder materials and tougher jobs can be carried out with higher voltage batteries for cordless power tools. Higher voltages allow larger, tougher components (such as large screws and big drills) to be used, or harder materials to be worked on.
A battery that is too heavy is pointless if you can't lift it or just need it for light work. On the other hand, increasing the voltage tends to increase battery size and weight which is pointless if you only need to use the cordless power tool for light work.
Voltage and battery capacity information on the side of a cordless power tool. Most cordless power tools have the correct battery voltage printed on the side of the tool. This information will be in the product specification too.