Battery runtime calculator
Find out how long your battery bank will last at a given load, and how long it takes to recharge.
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Worked example
A 200 Ah AGM bank powering a 10A average load (fridge, instruments, lights):
- Usable capacity at 50% DoD: 200 × 50% = 100 Ah
- Runtime: 100 Ah ÷ 10A = 10 hours
- With a 40A alternator/charger: recharge time ≈ 100 Ah ÷ 40A = 2.5 hours
Lithium version: 200Ah × 80% = 160 Ah usable → 16 hours runtime.
How it works
The calculator applies your battery type's maximum depth of discharge (DoD) to find usable capacity, then divides by your load current to get safe runtime. Charge time is the usable capacity divided by your charger output — a rough estimate that assumes constant charge rate (real chargers taper as the battery fills).
Runtime (h) = (Bank Ah × DoD%) ÷ Load (A)
Real-world runtime will be slightly shorter due to Peukert effect (heavier loads reduce effective capacity) and battery age.
Depth of discharge by battery type
| Battery type | Safe DoD | Notes |
|---|---|---|
| Flooded lead-acid | 50% | Deeper discharge shortens life rapidly |
| AGM | 50% | Slightly more tolerant than flooded, but 50% is still best practice |
| Gel | 50% | Similar to AGM; don't fast-charge |
| Lithium (LiFePO₄) | 80% | Can go to 90–95% but 80% preserves cycle life |
Frequently asked questions
Why can't I use 100% of my battery?
Discharging lead-acid batteries below 50% state of charge significantly reduces their cycle life — a battery discharged to 80% DoD regularly may last only 200–300 cycles vs 500+ at 50% DoD. Lithium batteries are far more tolerant, but manufacturers still recommend stopping at 80–90% DoD for maximum longevity.
What counts as "total load" in amps?
Add up the current draw of everything running simultaneously: fridge (3–5A average), instruments (2–3A), lights (1–3A), autopilot (1–3A at light load). Use the battery capacity calculator to build a detailed load list. For this tool, enter your typical sustained average, not the peak.
How long does it take to fully recharge?
This calculator gives a rough estimate based on constant charge rate. In reality, most chargers use a 3-stage process: bulk (constant current, fast), absorption (constant voltage, slows as battery fills), float (trickle). The bulk stage covers roughly the first 80% of capacity — that's the useful "time to ready" figure for most purposes.
Why is my actual runtime shorter than calculated?
The Peukert effect means batteries deliver less capacity under heavier loads. A 200Ah battery at 20A discharge may only deliver 160–180 Ah in practice. Battery age matters too — after 3–5 years, a lead-acid battery may hold only 70–80% of its original capacity.