Paper B turns the wearable pump's micro-load into a cell decision: comparing primary cells, rechargeable lithium and NiMH for a body-worn device, building a week-scale milliamp-hour energy budget from basal and bolus pulses, and designing for self-discharge, pulse sag and skin-safe safety.
Primary AA/AAA (alkaline or lithium-iron-disulphide) maximise shelf life and avoid recharging — the classic tubed-pump choice, at recurring consumable cost and variable internal resistance as the cell drains. Integrated rechargeable lithium enables slim sealed patch pumps with daily charging, at the price of a BMS, swelling/thermal management against the skin and IEC 62133-2 evidence. Rechargeable NiMH AAA/AA sits between: a flat 1.2 V curve that holds actuator pulse voltage far better than an ageing alkaline cell, high pulse current, intrinsic aqueous safety (no thermal runaway against a patient's body), tolerance of shallow top-ups, and low-self-discharge grades that retain most of their charge for months — well matched to a reusable, dock-recharged ambulatory pump or controller.
The budget is a sum of small quanta. Suppose sleep current is 30 µA, the radio runs 1 % of the time at 3 mA average, and the pump executes 288 basal pulses (every five minutes) plus 12 boluses per day at 100 mA for 0.8 s: daily charge is roughly 0.72 mAh from sleep, 0.72 mAh from radio and under 7 mAh from actuation — about 8–9 mAh/day, so a single 750 mAh AAA NiMH cell carries the load for weeks before efficiency, converter losses and derating are applied. The arithmetic shows why sleep-current engineering and radio duty matter more than motor optimisation, and why self-discharge (a non-trivial fraction of such a small daily draw) must be specified, which is precisely where low-self-discharge NiMH earns its place.
Even a small motor pulse sags a high-resistance cell, and a sag that trips a brown-out reset mid-bolus is a therapy interruption. Cells are therefore selected on pulse internal resistance as much as capacity: NiMH's low, stable impedance delivers clean bolus pulses to near end of life, and single-cell or two-cell rails are chosen from the actuator's minimum operating voltage at end of discharge, not nominal voltage. Protection is kept minimal but robust — resettable rather than latched, so a transient never locks out therapy.
A worn battery faces 30–37 °C constant temperature (which raises self-discharge and accelerates aging), strict size and mass limits, IP28-class immersion sealing of the enclosure, and abuse from drops and tight clothing. NiMH tolerates warmth and mechanical abuse gracefully, fades gradually, and its aqueous chemistry simplifies the safety case for skin contact; charge is supervised by thermistor with -ΔV termination in the dock.
Weijiang Power supplies low-self-discharge, low-impedance NiMH AAA/AA cells and miniature custom packs for wearable medical OEMs, with matched cells, thermistor integration and IEC 62133-1/UN 38.3 documentation. Send your basal/bolus trace, BLE schedule and wear-time target and we will size a cell strategy that sustains therapy to the last dose.
