Even though batteries have been around for a while, it is still not clear as to why a lead acid battery connected to a 5 amp appliance last 20 hours, but when connected to a 10 amp appliance, the time drops by more than half, to around 7.5 hours.
In 1897 a German scientist by the name of Wilhelm Peukert came up with a formula to calculate how long batteries would last under different loads. To use the calculator below you will need to know:
- The discharge rating – what did the manufacturer use to determine the battery capacity. The industry standard is the “20 hour rate”, but do not assume this, as less reputable manufacturers will use a higher rating (see Which deep cycle Ah battery? for more on this).
- The rated Ah capacity on the battery – e.g. 100 Ah.
- Current – the current of the appliance to be powered, usually written somewhere on the casing of the appliance – e.g. 5 amps.
- The Peukert Exponent – this is different for every manufacturer, but should be made readily available by the manufacturer and is often included in their technical specification sheets on their websites. As a guide:
- Flooded lead acid batteries – between 1.2 and 1.6
- AGM batteries (most sealed lead acid batteries) – between 1.05 and 1.15
- Gel batteries – between 1.1 and 1.25
|Discharge rating:||hour rate|
|Hours of operation:|
Limitations of this calculator
It should be noted that the formula does not take into account temperature (batteries perform worse when it is very cold or very hot) or battery age , but it is useful as a comparison tool.