Depth of Discharge and Battery Life Explained

Every battery loses a little life each time it is charged and discharged. How deep each discharge is has a large effect on how many cycles the battery gives. Understanding this helps you choose, size and use a battery to get the most from it.

Key takeaways

• Depth of discharge (DoD) is the share of the battery's capacity that you use in a cycle.
• A shallower cycle usually gives many more cycles than a deep one.
• Lead-acid batteries are generally cycled to about half; lithium (LiFePO4) batteries can be used much deeper.
• A cycle-life figure is meaningful only with the DoD, the temperature and the end-of-life point.

1. State of charge and depth of discharge

State of charge (SoC) is how full the battery is. Depth of discharge is how much you take out. If you start at 100 per cent and stop at 40 per cent, the DoD is 60 per cent.

2. Why depth matters

Charging and discharging cause slow changes inside the battery. The deeper the discharge, the greater the stress. For many batteries, going from a 50 to a 100 per cent DoD can cut the cycle life by a large factor. The exact curve is in the datasheet.

3. Lead-acid and lithium

Lead-acid batteries, such as VRLA and tubular types, are normally limited to about 50 per cent DoD for regular cycling and suffer if left discharged. Lithium iron phosphate (LiFePO4) batteries tolerate deep cycling and often allow 80 to 100 per cent DoD, which is one reason for their popularity in storage. See lithium vs lead-acid and LiFePO4 battery datasheet guide.

4. Reading cycle life

A figure such as "6,000 cycles" means little without conditions. Ask: at what DoD, at what temperature, at what charge and discharge rate, and until what remaining capacity (for example 70 or 80 per cent)? Compare batteries on the same conditions.

5. Calendar life

Even if you never cycle a battery, it ages with time, faster when hot. A battery rated for thousands of cycles is not guaranteed to last as long as its cycle count suggests if the years pass first. Read temperature and battery life.

6. Using it well

1. Size the bank so that normal use stays within the recommended DoD; see how to size a battery bank.
2. Do not leave lead-acid batteries discharged for long.
3. Use the charge settings recommended by the maker.
4. Keep the batteries cool.

Next step

If you are unsure how your battery is being used, we can check it and read its settings; contact us through the contact page.