Amp-hours, watt-hours and C-rate, and why the label rarely matches the runtime.
Amp-hours, watt-hours and depth of discharge
Compare usable watt-hours, not nominal amp-hours: lithium gives you most of its rating, lead-acid about half.
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Estimating runtime under real load
Divide usable watt-hours by the real load, then discount 10–15% for inverter and conversion losses.
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Voltage & sizing questions
The questions that come before an order.
What voltage should I choose for a lithium battery?
It is set by the equipment: 12V for small loads and vehicles, 24V for light traction and vans, 48V (51.2V) for storage and most traction, higher for industrial systems. Match the device first, then choose amp-hours.
How do I work out runtime from amp-hours?
Multiply amp-hours by the nominal voltage for watt-hours, then divide by the actual load in watts. A 48V 100Ah pack is about 4.8kWh, which at 500W is roughly nine hours.
Is a higher voltage always better?
No. Higher voltage means lower current and thinner cable, but it also changes the charger, the BMS and often the whole system. Step up only when the equipment or the cable run demands it.
What does C-rate mean?
The current expressed relative to capacity: 1C on a 100Ah battery is 100A. Discharging faster than the cell rating shortens life and usually trips the BMS.
Why is a 12V lithium battery labelled 12.8V?
Because LiFePO4 cells are 3.2V nominal, so four in series give 12.8V. Equipment designed for 12V lead-acid accepts that range without modification.
