Battery & Solar ReferenceUpdated 2026-08-15
Battery Engineering Fundamentals: Ah, Wh, C-Rate & Chemistry
Essential technical reference covering battery capacity metrics (Ah vs. Wh), Depth of Discharge (DoD), State of Charge (SoC), C-rates, cycle life, and Lead-Acid vs. Lithium comparison.
Key Formulas: Wh = V_nom × Ah | C_rate = I / Ah | Usable Wh = Wh × DoD × η
Understanding Amp-Hours (Ah) vs. Watt-Hours (Wh)
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Amp-Hour (Ah): Measures electric charge capacity (). It indicates how many amperes a battery can deliver for 1 hour at nominal conditions.
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Watt-Hour (Wh): Measures total electrical energy (). Watt-hours allow direct comparisons between batteries of different voltages (e.g., a 12V 100Ah battery and a 48V 25Ah battery both store 1,200 Wh of energy).
Depth of Discharge (DoD) & Cycle Life
Depth of Discharge (DoD) represents the percentage of battery capacity removed relative to total rated capacity:
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Flooded & AGM Lead-Acid: Recommended maximum 50% DoD. Regular discharges to 80-100% reduce lifespan from 500 cycles to under 150 cycles.
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Lithium Iron Phosphate (LiFePO4): Can safely be discharged to 80-90% DoD daily, achieving 2,000 to 5,000+ full charge cycles.
C-Rate Discharge Rating
The C-rate normalizes discharge or charge current against battery capacity:
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for a 100Ah battery = 100 Amperes (discharges in 1 hour).
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(or ) for a 100Ah battery = 20 Amperes (discharges in 5 hours).
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for a 100Ah battery = 200 Amperes (discharges in 30 minutes).
Frequently Asked Questions
Why does lead-acid deliver less capacity at high discharge rates?
Due to Peukert's Law: high discharge currents increase internal resistance losses and deplete active electrolyte ions near the lead plate surfaces faster than diffusion can replenish them.