Most “battery dies too fast” complaints trace back to the same mistake: picking a cell capacity that matches nameplate runtime, not real-world runtime. The right method is uncomfortable because it forces you to give up capacity on paper to gain it in the field.
Start with the current trace, not the datasheet
You need four inputs before you can size a cell:
- Average current (Iavg) across a representative use session.
- Peak current (Ipk) and its duration — for radio bursts, display ramp, motor kick-in.
- Ambient temperature range the device operates in.
- Target lifetime in charge cycles, or in years for calendar-limited designs.
If you don’t have a current trace, capture one. A cheap shunt + logger in series with the battery terminal gets you a usable profile in an afternoon.
Compute ideal capacity
The arithmetic is trivial:
Qideal = Iavg × Ttarget
For a wearable that draws 18 mA average over an 18-hour session: Qideal = 18 × 18 = 324 mAh. That’s your starting point, and you should never stop there.
Apply the derating cascade
Four factors chip away at the headline number. Multiply them in order:
| Factor | Typical value | Why |
|---|---|---|
| Usable SOC window | × 1 / 0.90 | You can’t drain a Li-ion cell to 0% without harming it — most BMSes cut at 3.0 V. |
| End-of-life capacity | × 1 / 0.80 | A cell rated for 500 cycles has 80% SOH at end of life. Design for that, not day-1 capacity. |
| Temperature derating | × 1 / 0.90 (if used below 10 °C) | Low-temp capacity falls off a cliff. Ignorable for indoor wearables, critical for outdoor IoT. |
| Peak-current headroom | × 1.05–1.15 | Voltage sag during Ipk cuts into usable capacity. Bigger pulses = bigger factor. |
Chained together, the 324 mAh wearable needs roughly 324 × 1.11 × 1.25 × 1.0 × 1.05 ≈ 470 mAh to deliver 18-hour runtime through its end of life.
A worked IoT example
Consider a LoRa asset tracker with this duty cycle:
- Sleep: 20 µA, 59 minutes per hour
- GPS fix: 25 mA for 45 seconds, once per hour
- LoRa TX: 120 mA for 1 second, once per hour
Hourly charge consumed: (20 µA × 59/60 h) + (25 mA × 45/3600 h) + (120 mA × 1/3600 h) ≈ 0.345 mAh/hour.
Target: 3 years without battery replacement = 26,280 hours → 9,067 mAh.
Apply the derating: for an outdoor device operating at −10 °C to +50 °C, the cold-temperature factor alone pushes this to ∼11.3 Ah. The design converges on either a large Li-SOCl⊂2 cell or a Li-SOCl⊂2 + high-pulse-capacitor hybrid to handle the TX bursts.
Three shortcuts that ship real products
- When you don’t know, over-size by 30%. Design in more capacity than you think you need. Shrinking later is cheap; fielded returns are not.
- Target end-of-life runtime in the marketing copy. If you say “7 days” and the user gets 7.8 days out of the box and 6.1 after a year, they feel cheated. Say “6 days” and over-deliver on day one.
- Measure, don’t estimate. One honest lab current trace beats a spreadsheet full of datasheet numbers.