Let’s talk about ‘energy throughput’: Should it replace battery ‘cycle life’?

Australians households are fairly solar-savvy lot. With a solar panel for every human being in the country, Australia is a world leader in solar uptake, and there’s tons of information online about how to shop for solar (Solar Choice is one case in point). It wouldn’t take anyone too long to learn to talk meaningfully in terms of watts, kilowatts and kilowatt-hours, inverters and modules, etc.

What is battery cycle life and why is it important?

When it comes to the burgeoning field of battery storage, however, there is even more jargon to keep up with for those who want to ensure they have a full understanding of the product they’re looking at. One of the trickiest terms you’ll hear is ‘cycle life’ – which refers to the number of times a battery can be fully charged and discharged before the reach the end of their functional life. (Depth of Discharge is another term everyone should know.)

Cycle life specifications vary significantly between different battery chemistries and brands, but any decent battery bank for household use should bear a figure of at least around 1,000 (and should be clearly detailed on the product’s spec sheet). A typical lithium-ion battery, for example, will typically have a cycle life of 4000-8000 cycles, while low-end lead acid batteries could have cycle lives as short as 800-1,000 cycles. Generally speaking, the more you cycle a battery, the more its ability to hold a charge is diminished (the exception if flow batteries like those from Redflow.)

Why is ‘cycle life’ sometimes not a helpful term?

Where things get complicated with cycle life as a term is the fact that it doesn’t reflect that the capacity of (most) batteries degrade over time. Let’s say we have a lithium battery bank with a capacity of 10 kilowatt-hours (kWh) with a cycle life of 5,000 cycles. While the first thousand cycles of a battery’s life may each effectively store and deliver 10kWh of energy to your home (minus inefficiencies), the last thousand will probably not. In fact, by that point the battery may only be able to store 60% of what it did at the beginning of its life – translating into only 6kWh. The question of what happens during the other 3,000 cycles in the middle is up in the air (unless it’s clearly spelled out in the product’s warranty).

The point is that it’s very difficult to determine just how much energy the battery will deliver per cycle at any time other than the beginning of its life (when you’re pretty sure to get close to the nominal capacity – 10kWh, in this example). Batteries offer value mainly in the form of the energy they store – if they store less energy over their life, then their value is diminished. Don’t you want to know for sure (or be as certain as possible) how much your batteries will be worth over their operational life?

In favour of ‘energy throughput’

One potential solution is to do away with use of the term ‘cycle life’ or to relegate it to a less important metric and replace it with another, arguably more useful term – ‘energy throughput’. Energy throughput is the total amount of energy a battery can be expected to store and deliver over its lifetime. This term would be especially useful written into the warranties of all battery products.

Let’s say the example 10kWh battery bank mentioned above has a warranty on its throughput instead of its cycle life. Then you would be able to rest with confidence that it would deliver at least (for example) 36,000kWh (or 36 megawatt-hours, MWh) of stored energy over the course of its life. This figure would be relevant and useful regardless of how the battery degrades over time (although it would still probably be useful to know what to expect from the battery on a daily basis in the early years vs the later years).

Flow battery manufacturer Redflow already offers a warranty that eschews ‘cycle life’ and replaces it with ‘energy throughput’. It would be encouraging to see more manufacturers follow this example – even if they did so alongside their cycle life figures. In fact, many battery manufacturers do detail throughput figures in their warranties – if you look for it.

Check out Solar Choice’s battery throughput comparison tool

Where unavailable from manufacturers, we here at Solar Choice have worked out a way to estimate total battery lifetime energy throughput based on cycle life, warranty life and end of life retained storage capacity. You can check out the results in our Battery Storage Product Performance Comparison Tool.

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© 2016 Solar Choice Pty Ltd

Jeff Sykes