Most people compare solar batteries on two things: kWh and price. Those matter, but two other numbers decide how many years you’ll actually get. They’re usually in small print on the datasheet: depth of discharge and cycle life.
- DoD is how much of the battery you use before recharging.
- Cycle life is how many charge-discharge rounds it lasts, at a stated DoD.
- Always compare cycle life at the same DoD and temperature.
- Using a battery a little less deeply usually makes it last longer.
Depth of discharge (DoD)
DoD is how much of the battery’s capacity you use before charging it again. Use 4kWh from a 5kWh battery and you’ve done 80% DoD.
Lithium iron phosphate (LiFePO4) batteries are comfortable with deep discharge, commonly 80 to 90% daily. Lead-acid and tubular batteries age fast if you take them deep regularly, so around 50% is the usual guidance.
Cycle life
One cycle is roughly one full charge and discharge. A solar battery used every night does about 300 to 365 cycles a year. Cycle life tells you how many of those the battery can do before it drops to a stated level, usually 70 or 80% of its original capacity.
So a battery rated “6,000 cycles at 80% DoD” should, in theory, give about 16 years of daily cycling before it’s down to around 70 to 80% capacity. In practice, heat and charging habits shorten that, but it tells you the battery’s quality class.
The trap: comparing different conditions
One brand says 6,000 cycles, another says 8,000. Before you pick the second, check the conditions. Is it at 80% DoD or 50%? At 25°C or 35°C? At a gentle charge rate? A battery tested at 50% DoD will always show more cycles than one tested at 90%. If the datasheet doesn’t state conditions, treat the number with caution.
Heat is the hidden factor
Batteries age faster when they’re hot. A battery in a closed store room that hits 40°C in June won’t match its lab rating. Keep it in a shaded, ventilated place, off direct sunlight and away from the inverter’s hot air outlet.
Simple habits that extend battery life
- Set the inverter’s low-battery cutoff so you’re not running to 0% every night.
- Use the correct battery type setting in the inverter (lithium, not lead-acid).
- Let solar fill the battery during the day before heavy evening use.
- Keep firmware and BMS communication working if your inverter supports it.
We’ve covered day-to-day care in more detail in our LiFePO4 maintenance tips. And if you’re deciding on size, see panels only or with a battery.
Comparing two battery datasheets?
Send both to us on WhatsApp. We’ll point out which numbers actually compare.