If you will charge your battery for 5 hours, will you also be able to use it for 5 hours? NO.
In this article, I am going to discuss how long your battery could last based on its discharge rate. I will also try to tackle how to compute the hours through Peukert’s Law.
Knowing the Battery Capacity
Battery capacity refers to the amount of energy it can store. Manufacturers typically provide the Amp Hours or the Reserve Capacity to indicate the figure. However, the battery capacity depends on the discharge rate. If the discharge rate is high, then its capacity will be low.
In 1897, German scientist W. Peukert presented an equation for calculating the capacity of the battery.
The Peukert’s Law Equation
- t – Time in hours. The actual hour/s the battery will last when the discharge rate is provided.
- H – Typically provided on the battery label, this indicates the “nominal” battery capacity. As an example, if you have a battery that has a 150 Amp Hour at a 30 hour discharge rate, then your H should be 30.
- C – The battery capacity in Amp Hours.
- I – The discharged current.
- k – The Peukert Exponent. Every battery has its own Peukert exponent. Sometimes the manufacturer will provide it and other times we may need to figure it out.
The Peukert Exponent
The Peukert exponent depends on the type of battery:
- AGM batteries range between 1.05 and 1.15
- Gel batteries range between 1.1 and 1.25
- Flooded batteries range between 1.2 and 1.6
Computing the Discharged Rate through Peukert’s Law
Let’s say for example you are using a 100 Amp Hour flooded battery with a 20 hour discharge rate. Provided with the Amp Hour and discharge rate, you can conclude that it will give you a consistent 5 amp (100/20). But what if we will increase the amp to 20? You might automatically conclude that the battery could last for 5 hours. Unfortunately, no. Let me help you compute the discharge rate using the Peukert’s Law. Basing on the Peukert exponent for flooded batteries as provided above, let us use the average 1.4.
So, based on the calculation, you can use the battery for as long as 2.8 hours if it continuously provides 20 amps. Not really 5 hours as what you may have expected, because again, the higher the discharge rate, the lower the battery capacity is.
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