Ah – what´s that?
The stored energy is measured in ampere hours (Ah).
Simply put, Ah can correspond to the surface of the plates in the battery.
The measure is based on a determined discharge profile. There are several different profiles depending on the area of use. Each profile is based on discharging a fully charged battery with a constant current for a certain number of hours.
The discharge is normally interrupted when the voltage has dropped to 1.8V / cell, ie 10.8V for a 12V battery.
The C/20 profile means that a constant current can be extracted for 20 hours.
Example: A 70Ah battery should leave 3.5A for 20 hours (70/20 = 3.5)
The C/10 profile means that 7A can be removed from a 70Ah battery for 10 hours.
Also profiles for C/5 is in use.
Note that a 70Ah C/20 battery only leaves about 60Ah at C/10.
A 70Ah C/10 battery, on the other hand, leaves close to 80Ah at C/20.
It is therefore crucial for battery comparisons to carefully check for which profile the manufacturer has specified the size of the battery.
The C/20 profile is usually used as a measurement method for applications in vehicles, boats, aids, alarm systems, etc.
The C/10 and C/5 profiles are usually used for fork lift applications, but can also be found in aids calculation.
Why these different C/20 C/10 C/5 profiles one might wonder?
The background is to provide a good and fair guide when choosing correct battery size.
The origin requirement is usually that a certain operating time is reached with the selected battery.
A fork lift draws momentous high currents when lifting heavy goods.
It is therefore not meaningful to dimension after C/20, here C/5 is used instead.