Battery Storage

Depth of Discharge (DoD)

Depth of discharge (DoD) is the percentage of a battery's total capacity that has been used. If a 100Ah battery has delivered 60Ah of energy, it is at 60% DoD (or equivalently, 40% state of charge). DoD is the inverse of state of charge (SoC) — when DoD is 60%, SoC is 40%.

DoD is one of the most critical factors determining battery lifespan. Every battery chemistry has a recommended maximum DoD for daily cycling. Exceeding this threshold accelerates degradation and dramatically shortens the number of available cycles.

For lead-acid batteries (flooded, AGM, gel), the recommended maximum daily DoD is 50%. Regularly discharging below 50% causes accelerated sulfation of the lead plates, which is the primary failure mechanism. At 50% DoD, a quality lead-acid battery might deliver 500-700 cycles. At 80% DoD, that same battery might last only 200-300 cycles.

LiFePO4 batteries can safely cycle to 80-100% DoD without significant impact on cycle life. Most manufacturers rate their LiFePO4 batteries at 80% DoD and guarantee 2,500-5,000 cycles at that depth. This deeper usable capacity is one of the major advantages of lithium over lead-acid.

When sizing a battery bank, DoD directly determines how much rated capacity you need. To store 5 kWh of usable energy at 50% DoD with lead-acid, you need 10 kWh of total capacity. With LiFePO4 at 80% DoD, you need only 6.25 kWh of total capacity — a significant difference in size, weight, and cost.

Frequently Asked Questions

What depth of discharge should I use for daily cycling?

For lead-acid (flooded/AGM/gel): stay above 50% SoC (50% DoD maximum). For LiFePO4: 80% DoD is standard and safe for daily cycling. Some LiFePO4 manufacturers allow 100% DoD, but staying at 80% extends cycle life further. Always check the manufacturer's specifications for your specific battery.

How does DoD affect battery sizing?

Deeper allowable DoD means you need less total battery capacity. For 5 kWh of usable energy: lead-acid at 50% DoD requires a 10 kWh bank, while LiFePO4 at 80% DoD requires only a 6.25 kWh bank. This is why LiFePO4 systems are physically smaller and lighter despite having the same usable capacity.