Battery Life Calculator
Estimate how long a battery runs a load, with a simple capacity model or a Peukert correction for lead-acid batteries.
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What this tool does
Estimate how long a battery runs a load, with a simple capacity model or a Peukert correction for lead-acid batteries. Load current is assumed constant. Peukert's exponent and the capacity's rating period come from the battery datasheet; lithium batteries have k close to 1.
How to use the Battery Life Calculator
- Enter or select runtime model.
- Enter or select battery capacity (ah).
- Enter or select load current (a).
- Enter or select usable efficiency (%).
- Enter or select peukert exponent k (peukert mode).
- Enter or select capacity rating period (hours, peukert mode).
- Read the calculated result; change any measurement to compare alternatives.
Formula
simple: hours = Ah ÷ A × efficiency/100; Peukert: hours = ratingHours × (Ah ÷ (A × ratingHours))^k × efficiency/100
- mode
- Runtime model
- capacity
- Battery capacity (Ah)
- load
- Load current (A)
- efficiency
- Usable efficiency (%)
- peukert
- Peukert exponent k (Peukert mode)
- ratedHours
- Capacity rating period (hours, Peukert mode)
Load current is assumed constant. Peukert's exponent and the capacity's rating period come from the battery datasheet; lithium batteries have k close to 1.
Worked example
For battery life calculator, the following measurements illustrate the exact method: Runtime model: simple; Battery capacity (Ah): 100; Load current (A): 5; Usable efficiency (%): 85; Peukert exponent k (Peukert mode): 1.2; Capacity rating period (hours, Peukert mode): 20.
Inputs
- Runtime modelSimple (capacity ÷ load)
- Battery capacity (Ah)100
- Load current (A)5
- Usable efficiency (%)85
- Peukert exponent k (Peukert mode)1.2
- Capacity rating period (hours, Peukert mode)20
Result
- Estimated runtime (hours)17
- Estimated runtime (days)0.71
- Usable capacity after efficiency (Ah)85
Results explained
- Estimated runtime (hours)
- Estimated runtime (hours) from the formula above. Load current is assumed constant. Peukert's exponent and the capacity's rating period come from the battery datasheet; lithium batteries have k close to 1.
- Estimated runtime (days)
- Estimated runtime (days) from the formula above. Load current is assumed constant. Peukert's exponent and the capacity's rating period come from the battery datasheet; lithium batteries have k close to 1.
- Usable capacity after efficiency (Ah)
- Usable capacity after efficiency (Ah) from the formula above. Load current is assumed constant. Peukert's exponent and the capacity's rating period come from the battery datasheet; lithium batteries have k close to 1.
Frequently asked questions
In the simple model, runtime = capacity (Ah) ÷ load current (A) × efficiency. A 100 Ah battery feeding a 5 A load at 85% efficiency lasts 100 ÷ 5 × 0.85 = 17 hours.
Lead-acid batteries deliver less usable capacity at high discharge rates. Peukert mode corrects with t = rated hours × (capacity ÷ (load × rated hours))^k × efficiency, where k (the Peukert exponent) is about 1.1–1.3 for lead-acid and near 1.0 for lithium.
Inverter losses, wiring, and the fact that lead-acid batteries should not be fully discharged all eat into nameplate capacity. 80–90% is a common planning figure; use 100% only for a direct DC load you will fully discharge.
Yes. Capacity fades with cycles and age, and cold temperatures temporarily reduce it further. Treat the result as an upper-bound estimate for an ageing or cold battery.
That tool works backwards from a daily energy demand to the bank size you need; this one works forwards from a battery you already have to how long a given load will run.