A powered wheelchair is a road-going electric vehicle that carries a person, and its battery is judged by range rather than runtime on a bench. Paper A of this three-part series dissects the traction load profile, explains how the ISO 7176-4 range test converts current draw into a theoretical distance, and shows why gradient, driver weight and start-stop duty dominate the energy budget.
Nearly all powered wheelchairs run on a 24 V system — historically two 12 V sealed lead-acid (SLA) batteries in series, with NiMH and lithium packs using 20 NiMH cells (20 × 1.2 V) or 7-series lithium. Two brush/brushless DC gear motors drive the rear wheels through a dual-channel controller; continuous current is typically 15–25 A on level ground, with hill-climb, kerb climb and acceleration transients of 30–40 A or more. The load profile is therefore a high-current, highly variable traction waveform rather than the steady floor current of a bedside device.
ISO 7176-4 defines a standardised determination of energy consumption and theoretical distance range on a dynamometer or reference course, so that range claims are comparable across manufacturers. Range derives from usable energy divided by Wh per kilometre, and Wh/km is set by driver weight, rolling resistance, tyre pressure, gradient, speed and stop-start frequency. A 24 V 20 Ah pack stores 480 Wh nominal; at 18–25 Wh/km that yields roughly 19–27 km before derating — which is why real-world range is always below catalogue figures quoted on a level dynamometer.
Indoor chairs accelerate, stop and turn constantly; outdoor chairs meet ramps, cambers and kerbs. Every start from standstill draws a current multiple of cruise current, and a sustained climb holds the pack at high discharge for minutes. Regeneration is limited on most chairs, so almost all kinetic and potential energy is dissipated in brakes and motors. The battery must therefore deliver high continuous current without excessive voltage sag (which the controller reads as an empty battery, cutting power early) and tolerate deep daily discharge.
SLA is cheap but heavy and loses usable capacity rapidly under deep cycling (roughly 300–500 cycles). Lithium is light and long-lived but needs a BMS and careful transport handling (airline spare-battery rules cap spare packs around 101–160 Wh, which rules out most large traction packs as carry-on spares). NiMH sits between: mechanically robust, intrinsically safer aqueous chemistry, tolerant of abuse and high-rate discharge, about 500 cycles to 80 %, and substantially lighter than SLA for the same energy — a credible upgrade path for chairs designed in the SLA era. Paper B compares the chemistries and sizes the pack; Paper C covers ISO 7176 testing and battery standards.
Weijiang Power manufactures high-rate NiMH cells and custom 24 V traction packs for mobility OEMs: resistance-matched welded strings for hill-climb current, robust mechanical construction and IEC 62133-1/UN 38.3 documentation. Send your motor current trace, target ISO 7176-4 range and weight budget and we will size a pack around the real driving profile.
