Real Loads Are Never Constant

Very few devices draw a flat, continuous current. Power tools burst, radios transmit in pulses, pumps cycle, cameras flash. Designing a NiMH pack around a continuous worst-case current usually means oversizing the battery — adding cost, weight and size. Understanding how NiMH responds to pulse loads lets engineers size packs correctly for the duty cycles that real products actually run. This article covers pulse discharge behaviour, voltage sag and recovery, and practical sizing rules.

How NiMH Behaves Under Pulse Load

When a load is applied, the cell voltage immediately drops because of internal resistance, then continues to relax as electrode reactions and concentration gradients settle. When the load is removed, the voltage partially recovers as the cell redistributes charge and the interface relaxes. The key insight: a cell can deliver a much higher current in short pulses than it can sustain continuously, because the average heating and charge-depletion are lower.

• Voltage sag — proportional to current and internal resistance. At the very start of a pulse the sag is dominated by ohmic resistance; over the pulse it deepens as mass-transport limits take over.
• Recovery — between pulses the cell recovers some usable voltage and capacity, which is why duty cycle (on-time vs off-time) matters enormously.
• Heating — pulse operation spreads current into short, high bursts; the average heat generation is what drives pack temperature, not the pulse peak.

Why High-Rate Cells Help

A cell engineered for high rate (thinner electrodes, higher-surface-area active material, low internal resistance) shows less sag and recovers faster, so it can carry aggressive pulse profiles without premature cutoff. For a power tool that demands 10C-20C in short bursts, a high-rate NiMH cell is far smaller than a standard cell sized for the same peak pulsed current.

Duty-Cycle Sizing: A Practical Method

A robust sizing approach for pulsed loads:

1. Map the load profile — record the pulse current, duration and rest time across the operating cycle, including the worst realistic case.
2. Compute the RMS (root-mean-square) current — this approximates the average heating the pack must dissipate and is the right basis for thermal sizing.
3. Size against the RMS current for continuous rating — the pack must sustain the RMS average without overheating.
4. Check the peak pulse against the cell's pulse rating — the peak current must stay within the cell's short-duration capability with margin for voltage sag above the equipment cutoff.
5. Verify cutoff margin at end of discharge — the deepest sag usually appears when the pack is nearly empty; confirm the terminal voltage stays above the equipment's cutoff under the worst pulse at low state of charge.

Thermal and Management Considerations

Pulse-heavy duty still generates heat over time. Monitor pack temperature and ensure the charging circuit understands the load pattern — a pack that is discharged in aggressive pulses should be charged conservatively and allowed to cool before recharging. In multi-cell packs, matched cells (consistent capacity and internal resistance) keep the pack balanced under the stress of pulsing, which is why Weijiang Power grades and matches cells before assembly.

Getting the Sizing Right the First Time

Sizing NiMH packs for pulsed loads is an engineering exercise, not a guess. The right answer balances the cell's pulse capability, the pack's thermal budget and the equipment's cutoff voltage. Weijiang Power helps OEMs work through exactly these trade-offs — we match high-rate cells, design the pack topology and verify performance to your real duty cycle. Share your load profile and we will help you build a pack that delivers the power you need without paying for capacity you do not.