Estimating battery runtime realistically

Operating pattern matters

A radio usually draws different currents in transmit and receive. A simple model splits operating time between those two states. With 20% transmit duty cycle, 5 A transmit current and 0.5 A receive current, Iavg = 0.2·5 + 0.8·0.5 = 1.4 A.

Use suitable measurements or specifications for the actual radio and operating point. With a varying signal envelope, average current can differ from peak current. Extra loads such as a hotspotHotspot×A digital-radio hotspot connects a local radio access point to a digital network. Its modem, software and network configuration determine supported modes and features.Example & full explanation →, lighting or a converter also belong in the budget; the two-state calculator includes them only through appropriately adjusted inputs.

Usable capacity, not just the label

A nominal 10 Ah battery with an assumed 80% usable capacity provides 8 Ah in the model. At 1.4 A average current, this gives about 5.71 hours. The 80% value is a planning assumption, not a general recommendation for every cell chemistry or battery type.

Temperature, ageing, discharge current, cutoff voltage and protection circuitry change available capacity. Current dependence is especially relevant for some battery types. A simple Ah/current calculation does not automatically model these effects. Measured runtime under comparable conditions is therefore more useful than extra decimal places.

Plan an explicit reserve

Calculate several scenarios: normal operation, more transmitting and reduced usable capacity. Record the assumptions. If voltage during transmit peaks falls below the radio’s limit, it may shut down before the calculated Ah is used. Check wiring loss and loaded voltage as well as capacity.