Most of us use sealed lead-acid batteries without sparing a thought for what is going on inside them. We may have a vague idea the casings contain lead and sulfuric acid, but that is where it ends. If the shop assistant says, “Will that be a 20 Amp Hour for your mobile home” we mostly say, “Yup, you got it.” That could be right depending on your circumstances.
Go On, Hit Me. What Does 20 Amp Hour Mean?
An amp hour specification defines the capacity of a battery or how much energy it can store.
This is in turn dependent of the battery’s design, but that is another matter.
Let’s take as example a lead acid battery marked 100 AH @ 20HR. The label is telling us the battery will provide 5 amps of current at a usable voltage continuously for 20 hours.
The formula we use is dividing the 100 AH by the 20HR. Similarly, a battery marked 150AH @15 hours will provide 10 amps for 15 hours.
That Sounds Quite Complicated. What is the Catch?
The devil is in the detail of a thing called Peukert’s Law. This law defines how rechargeable lead batteries lose capacity at different rates of discharge.
For now, just know the time a battery takes to discharge depends on the rate thereof. This is an uneven struggle with capacity being the loser.
In reality, a 100 AH battery will go flat in 40 minutes if we expect it to deliver the full 100 amps. So the relationship between battery capacity and rate of discharge is not a linear one. If you left the boat lights on all night and found the battery flat, this is why.
So What Kind of Battery Should I Look For?
We have difficulty knowing where to start because there are so many different types of sealed lead acid battery applications. Requirements vary between say alarm batteries, power packs for gate motors, and UPS devices. Your equipment supplier can provide a detailed specification and the chances are excellent we can supply. So now you know what a 20 Amp Hour label a battery says.
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