Runtime is energy divided by consumption

Calculating a lithium battery's autonomy rests on a simple idea: divide the available energy by the rate at which it's consumed. The whole difficulty is expressing each term in the right units.

The key quantity is energy, in watt-hours (Wh). It comes from the capacity in amp-hours (Ah) and the nominal voltage (V):

Energy (Wh) = Capacity (Ah) × Voltage (V)

Example: a 10 Ah scooter battery at 36 V stores 10 × 36 = 360 Wh. A 48 V, 20 Ah battery stores 960 Wh — nearly triple — even though the "Ah" only double, which is why you should always reason in Wh, not Ah.

Autonomy as distance (mobility)

For a light electric vehicle, divide energy by consumption per kilometre:

Distance (km) = Energy (Wh) ÷ Consumption (Wh/km)

A scooter roughly consumes 10 to 20 Wh/km depending on weight, speed, terrain and wind. With 360 Wh and 15 Wh/km: 360 ÷ 15 = 24 km in theory. In practice you subtract a margin (10 to 20%) because the BMS cuts off before full discharge and cold reduces available capacity.

Autonomy as duration (storage / appliance)

To power a load at constant power, divide usable energy by power:

Duration (h) = Usable energy (Wh) ÷ Power (W)

Stationary example: a pack of 2,000 Wh usable powering a fridge averaging 100 W will last 2,000 ÷ 100 = 20 hours. Be sure to reason in average power: many appliances have peaks (motor start-up, heating elements) above their average draw.

What lowers real-world autonomy

  • SOH: a battery at 80% SOH has only 80% of its original capacity left. Our theoretical 360 Wh drop to ~288 Wh usable.
  • Depth of discharge (DoD): you don't use 100% of capacity. Multiply by the allowable DoD (e.g. 0.85) to get the truly usable energy.
  • Temperature: in the cold, chemistry slows and internal resistance rises; a battery can lose 20 to 40% of apparent autonomy below 0 °C.
  • Current drawn: at high power, voltage sags more due to internal resistance, and the energy actually delivered falls. A slow, steady discharge (typical of stationary use) returns more energy than a harsh one.

A full step-by-step calculation

Take a 48 V, 20 Ah battery again, but at 80% SOH and with 85% DoD:

  1. Nominal energy: 48 × 20 = 960 Wh.
  2. Corrected for SOH: 960 × 0.80 = 768 Wh.
  3. Actually usable energy (DoD): 768 × 0.85 ≈ 653 Wh.
  4. For a 100 W load: 653 ÷ 100 ≈ 6.5 hours. For a scooter at 25 Wh/km: 653 ÷ 25 ≈ 26 km.

This logic — real measured energy, weighted by SOH and DoD — is exactly what cirBATT applies to characterise every pack. The capacity stated on our product sheets isn't theoretical: it comes from a real discharge test, cell by cell.