An infusion or syringe pump is one of the few battery-powered devices whose failure is measured in patient risk rather than inconvenience. Whether it drives a peristaltic finger mechanism or a syringe lead screw, its internal battery must sustain precise drug delivery through patient transport, mains failure and bedside-to-imaging moves. This first paper in a three-part series dissects the pump's electrical load profile, translates clinical backup-time requirements into battery current budgets, and shows why the load profile — not the nominal capacity on a datasheet — is the correct starting point for pack design. It draws on the IEC 60601-2-24 particular standard for infusion pumps and publicly available device and procurement specifications.
A modern infusion pump contains four functional loads: a microcontroller and sensing block that is always on while therapy runs; a backlit display and keypad; a geared DC or stepper motor that advances the mechanism in discrete increments; and safety elements — occlusion alarm, air-in-line detector, audible annunciator and wireless module. The motor is the dominant and most dynamic load. Rather than turning continuously, most pumps deliver small boluses on a duty cycle set by the flow rate: at 1 ml/h a syringe pump may pulse its motor once every several seconds, while at 1200 ml/h it runs almost continuously. Each pulse carries a start-up inrush several times the running current as the gear train overcomes static friction and syringe stiction.
A defensible battery specification is built by summing measured load blocks, not estimates:
IEC 60601-2-24 governs the particular safety and performance requirements for infusion pumps and controllers (including flow accuracy, commonly specified at plus/minus 2 %), while IEC 60601-1 requires predictable behaviour when internal power runs low. The numeric battery endurance target is set by the manufacturer and reinforced by hospital procurement specifications, which are unusually informative for engineers. Published government tender specifications commonly require an internal rechargeable battery providing at least 5 hours of infusion at 10 ml/h with a 50 ml syringe; commercial syringe-pump datasheets cite, for example, 4 hours at the 25 ml/h "appointed medium rate" referenced under IEC 60601-2-24, while devices with larger lithium main packs quote around 12 hours at 5 ml/h and 25 °C for a new, fully charged pack. These three data points illustrate the rule that quoted runtime is meaningless without the flow rate, temperature, age and charge state attached to it.
The sizing chain is straightforward but must be performed at worst case. First, define the mission: for example, 5 hours of delivery at the declared reference rate plus a safety margin for alarms and wireless, ending above the low-battery threshold rather than at empty. Second, convert each load block to energy using its duty cycle, integrating motor pulses rather than assuming continuous current. Third, derate for the real world: capacity at cool room temperature, after hundreds of cycles (end-of-life is typically defined at 80 % of nominal capacity), and under the voltage sag of motor inrush. Fourth, verify by measurement on the actual pump across the flow-rate range — the non-linear relationship between flow rate and runtime, visible in the animated chart above, cannot be derived from a single datasheet point.
The load profile explains why nickel-metal hydride remains a first-class choice for infusion pumps three decades after introduction: its flat 1.2 V discharge curve holds the rail steadily through millions of small motor pulses, it tolerates the shallow, irregular charge-discharge pattern of a device that is docked whenever it is not in use, and its aqueous chemistry cannot enter thermal runaway beside a patient. Classic NiMH infusion packs are commonly six-cell 7.2 V configurations around 2000 mAh — the chemistry used in widely deployed syringe-pump platforms — with larger ten-cell 12 V packs in higher-power devices. Part B of this series compares NiMH against lithium-ion and primary cells quantitatively, and Part C walks the validation trail through IEC 60601-1, IEC 62133-1 and transport testing.
Weijiang Power designs and manufactures matched NiMH cells and custom battery packs for infusion and syringe pumps — tight internal-resistance binning to survive motor inrush, welded tabs, integrated NTC and protection elements, and documentation to IEC 62133-1 and UN 38.3. Send us your pump's measured current trace, target backup hours at the declared flow rate, and mechanical envelope, and our engineers will size and validate a pack around the real mission profile rather than a nominal capacity figure.
