What is battery cycle and depth of discharge on a solar battery?

When looking at solar batteries for your solar systems, there’s a lot to pick through on the specs to understand what it all means and which battery is right for your system. Here, we’ll unpick the main issues of contention by explaining what battery charge cycles are and depth of discharge.

Elsewhere, we’ve covered whether batteries are worth buying for a solar system; here, we’re diving into the specs to get a better understanding of what they mean.

Capacity

Before explaining anything else, it’s worth looking at battery capacities and how they’re defined. With batteries for solar systems, capacity is listed in kilowatt-hours (kWh), where one kilowatt is one thousand Watts. A kWh is the energy you’d use if you used 1,000 Watts of power for an hour. For example, a mid-power electric heater would draw about 1kW, so over an hour, it would use 1kWh of electricity.

A battery with a 10kWh capacity could run that heater for ten hours. Or, if you had multiple appliances turned on simultaneously, drawing 2kW of power, that 10kWh battery would last for five hours. In all cases, the larger the figure in kWh, the bigger the battery. All batteries lose capacity over time and only ever store their maximum quoted capacity when they’re brand new.

Charge cycle

A charge cycle is exactly as it sounds: power goes in to top up the battery, and then power goes out, draining it. Each complete charge/discharge session is known as a cycle. This becomes important when looking at longevity, which

Depth of discharge

A standard specification for batteries is the depth of discharge (DoD). This is quoted as a percentage and states the safe amount that a battery can be drained by before it should be recharged. Take our 10kWh battery for example. If this had a 90% DoD, then it would mean that only 90% of the battery’s capacity should be used before the battery should be topped up. In our example, it would mean that the battery can only be drained by 9kWh before the battery should be recharged. A battery with a 100% DoD, such as the Tesla Powerwall 2, can be fully drained in each cycle, meaning that the full capacity of the battery can be used each time.

Batteries that have a DoD below 100% often quote the maximum capacity and the useable capacity, with the latter the total amount of power available on each cycle after the DoD has been taken into account.

Number of charge cycles

There are multiple ways of quoting how long a battery will last, but most manufacturers use some version that quotes the number of charge cycles. If a total is quoted, such as 10,000, then this is the battery’s life: how many times it can be drained to the DoD level and then recharged. Some manufacturers will quote the number of charge cycles until the battery hits 80% capacity. Beyond this, the battery will keep working, but capacity will continue to reduce until it no longer holds a charge or there’s another failure. In both cases, the higher the number, the better, as it demonstrates that a battery will last for longer.