What’s the difference between Amp Hour and Amperage?

In Short:

  • Batteries rated with Amperage are stating the highest level of energy they are designed to give at a given moment in time for short periods (such as starting an engine).
  • Batteries rated with Ampere Hour are stating how many amps they can supply over a period of time, the industry standard being 20 hours. As such 100Ah battery will last 20 hours when powering a 5 amp appliance (100 Ah / 20 hours = 5) but it should be noted that it would not last 1 hour when powering a 100 amp appliance (see Understanding Ampere Hour ratings below). Sealed lead acid batteries are examples of ampere hour rated batteries.

Why the difference?

Different batteries are created for different tasks. One of the most common defining features is whether the battery should be able to deliver a high rate of energy for a short time (such as starting a motorcycle) or stable energy over a long period (such as powering an emergency light).

As such a battery with amperage is often referred to as a starting battery or cranking battery while a battery with ampere hours is often called a deep cycle battery.

They are built differently and using them in ways they were not designed for will cause damage to the battery and shorten their lifespan.

A starting battery has thin lead plates in its construction which help it to release a lot of energy quickly and then recharge fast. As a general rule of thumb these batteries should always be at least 70% charged. If they are  discharged below this on a regular basis (say to power a light) then the plates warp reducing their ability to charge and release energy.

Deep cycle batteries have thicker plates so they don’t warp but it means they are not as good at providing large surges of energy fast.

Understanding Ampere Hours (Ah)

The capacity of deep cycle batteries is often misunderstood. Many believe that if a 100Ah unit powers a 5 amp appliance for 20 hours then it would power a 10 amp appliance for 10 hours. However this is not true … much will depend on the rate of discharge because, as mentioned above, deep cycle batteries are not as good at delivering high capacity over short periods.

A 100Ah battery that will power a 5 amp appliance for 20 hours will actually last only about 1 hour when powering a 50 amp device. Working out how long your battery will last can be done using a formula known as Peukert’s Law which uses:

  • The discharge rate used to determine the batteries stated capacity (usually 20 hours)
  • The battery capacity in Ampere Hours
  • The current the appliance(s) will draw
  • The Peukert Exponent (a figure supplied by the manufacturer)

You can use our Peukart’s Law calculator to work this out and see the full formula.

Labeling, temperature and depth of cycle

The regulations surrounding deep cycle Ah batteries are limited and enforcement is rare which has lead to many manufacturers mislabeling their batteries as being more powerful than they are or using underhand methods in order to claim what appear to be much higher Ah ratings compared to other batteries.

To what level the battery is discharged on a regular basis and the temperature it operates in are also important factors. For full details on understanding the way deep cycle batteries are labeled and how they operate see Which deep cycle Ah battery?


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