Reading a battery warranty: years, cycles and throughput

A battery warranty isn't one promise. It's a set of limits, and coverage may end when you hit the first one. The terms are short, but each one has a meaning.

Years

The years figure is the length of cover from a start date. Check what starts the clock: the purchase, the installation or the date the system was switched on. The warranty document states it. Also check who the warranty covers if you sell the home.

Cycles

A battery wears as it's used. Batteries suffer from cycle ageing¹. When batteries are recharged, additional side reactions reduce capacity for subsequent discharges².

For a rechargeable battery, lifespan can mean the number of charge and discharge cycles possible before the cells fail to operate satisfactorily². A cycle limit in a warranty draws on that idea.

How a cycle is counted differs between documents. Does a half discharge and recharge count as half a cycle or a full one? Read the definition in the warranty. It also links to depth of discharge, since cycle life and how deeply you drain the battery are linked³.

Throughput

Throughput is the total energy a battery moves in or out over its life, usually in kWh or MWh. Warranties for these systems typically include lifetime limits on energy throughput¹. They can be expressed as a number of charge and discharge cycles¹.

A throughput limit and a cycle limit are two ways of saying a similar thing. If a sheet gives total throughput, divide it by the usable capacity to get an idea of how many full cycles it allows. That's a rough estimate only. The usable figure is the one to divide by, as the capacity note explains.

End-of-warranty capacity

Batteries lose capacity with age, so makers often state what the battery should still hold when the warranty ends, usually as a percentage of the original. The percentage is in the warranty document. I don't quote one here, because it varies by product.

Conditions that can change cover

Read the fine print for:

• Operating limits, such as temperature, charge settings or the way the battery is used.
• Installation by a licensed installer, which a warranty often requires.
• Whether a throughput limit applies to some uses and not others.
• What the maker does if the limit is reached: repair, replace or refund.

Warranty and consumer-protection rules differ by country and state. For your rights, check your government's consumer agency, and for your battery's terms, the maker's own warranty document.

Don't open, repair or modify a battery to try to restore capacity. That can void the warranty and it isn't work for a home owner.