Batteries, UPS, and Power Conditioning
Unit 2: Fundamentals of Electricity and Electronics. Lesson 12 of 60 in CBET Academy.
Stored power keeps infusion pumps running during transport and keeps monitors alive during the ten seconds before the generator picks up. Battery problems are among the most common repairs a biomed makes.
Battery basics
A battery converts chemical energy to electrical. Primary cells are single use; secondary cells recharge. Capacity is rated in amp-hours: a 2 Ah battery delivers 2 A for one hour or 0.5 A for four hours, less as it ages. Cells are stacked in series to reach the working voltage.
Chemistries and cell voltages
- Lead-acid (sealed lead-acid in most medical carts and older devices): 2.0 V per cell, heavy, tolerant of trickle charging, but it sulfates and dies if stored discharged.
- Nickel-cadmium: 1.2 V per cell, rugged, suffers the memory effect where repeated partial discharge shrinks usable capacity; recover with full discharge and charge cycles.
- Nickel-metal hydride: 1.2 V per cell, more capacity than NiCd, less memory effect, self-discharges faster.
- Lithium-ion: 3.6 to 3.7 V per cell, light, high capacity, no memory effect, but must not be over-discharged or overcharged; protection circuits guard against thermal runaway, and damaged or swollen packs are a fire hazard.
- Alkaline primary cells are 1.5 V.
Battery management
Batteries wear out on a schedule whether or not they fail visibly. A program includes dated labels, runtime tests at PM (run the device on battery to its low-battery alarm and compare to specification), conditioning cycles for NiCd, replacement at the vendor's interval, and correct disposal. A device that runs on AC but dies immediately on battery has a dead battery, a charger fault, or a bad battery connector.
Uninterruptible power supplies
A UPS carries a load through short outages and the generator's start-up delay.
- Standby: passes utility power through, switches to the inverter on loss; brief transfer time.
- Line-interactive: also corrects sags and surges with a regulating transformer.
- Online double-conversion: always rectifies to DC and inverts back to AC, so the load never sees the utility directly; zero transfer time and full conditioning; the choice for critical equipment.
Runtime depends on load; size the UPS for the load and the time needed to shut down or reach generator power.
Power quality
- Sag or brownout: voltage too low; surge: too high; spike or transient: a brief high-energy pulse from lightning or switching; noise: high-frequency garbage; blackout: none at all.
- Surge protectors clamp spikes; power conditioners regulate voltage and filter noise; a UPS does all of it plus outage bridging.
Devices with unexplained resets, corrupted data, or damaged supplies often live on a bad circuit.
Batteries, UPS, and Power Conditioning, explained slowly
Stored power is patient safety
An infusion pump in an elevator, a ventilator during a transfer, a monitor during the ten seconds before the generator starts: batteries carry them. A UPS carries the systems that cannot ride a battery. And power quality decides whether devices reset at 3 a.m. for no reason anyone can see.
Battery basics
primary use once (alkaline, lithium coin cells) 1.5 V alkaline secondary rechargeable; the ones you maintain capacity amp-hours: 2 Ah = 2 A for 1 hour, or 0.5 A for 4 hours; shrinks with age and cold cells stacked in series for voltage: six 2 V lead-acid cells = 12 V; three Li-ion cells = 11.1 V
Chemistries
Lead-acid (sealed, SLA) 2.0 V/cell heavy; cheap; tolerates float charging; SULFATES if stored discharged;
carts, older defibrillators, UPS units
Nickel-cadmium (NiCd) 1.2 V/cell rugged; MEMORY EFFECT from repeated partial discharge shrinks capacity;
fix with full discharge-charge conditioning cycles; cadmium is toxic waste
Nickel-metal hydride 1.2 V/cell more capacity than NiCd, milder memory effect, self-discharges faster
Lithium-ion 3.6-3.7 V/cell light, high capacity, NO memory effect; but over-charge, over-discharge,
puncture, or heat can trigger THERMAL RUNAWAY (fire); packs carry protection
circuits; swollen packs are removed from service immediatelyThe symptoms and what they mean
runs on AC, dies within seconds on battery battery at end of life, or not charging (charger circuit, connector) never reaches full charge charger fault, or a battery that cannot accept charge runtime shrinking month by month normal aging; replace at the vendor's interval NiCd device dies early but shows full charge memory effect; condition it pack is warm, swollen, or smells lithium hazard; isolate it, do not charge, dispose per policy
Runtime is the test that matters: at PM, run the device on battery to the low-battery alarm and compare against specification. A battery that passes a voltage check can still fail a runtime test, because voltage says little about capacity.
A battery program
Date every battery on installation. Track replacement intervals in the CMMS. Condition NiCd packs on a schedule. Keep spares charged and rotated. Dispose through the recycling stream; lead, cadmium, and lithium all have rules.
UPS: the bridge
Standby utility passes straight through; on loss, switches to battery and inverter
cheap; a few milliseconds of transfer gap; fine for a PC, not for a lab analyzer mid-run
Line-interactive adds a regulating transformer that corrects sags and surges without switching
mid-priced; short transfer gap
Online double- rectifies utility to DC, charges the battery, inverts back to AC continuously
conversion the load never sees the utility; zero transfer time; full conditioning; the choice for
critical equipment and anything with a computer that must not glitchSize it by load in volt-amps or watts and by the minutes needed: enough to shut down cleanly or to reach generator power (which arrives within 10 seconds, but the UPS should cover far more for safety). Batteries in a UPS age like any other; runtime testing applies.
Power quality: the invisible faults
sag / brownout voltage dips motors starting, overloaded circuits; devices reset or run hot surge voltage rises heavy loads switching off; stresses supplies spike/transient brief high-energy pulse lightning, switching; kills semiconductors noise high-frequency garbage motors, dimmers, switching supplies; artifacts on signals blackout nothing the generator's job, after 10 seconds
Fixes: surge protectors clamp spikes; power conditioners regulate voltage and filter noise; an online UPS does all of it and bridges outages. A device that resets, corrupts data, or eats power supplies on one outlet and behaves elsewhere has a power quality problem, not a device problem, and the fix is the circuit.
How the exam asks it
- "Nominal voltage of a lithium-ion cell?" About 3.6 to 3.7 V.
- "Which chemistry is prone to the memory effect?" Nickel-cadmium.
- "Which UPS type provides no transfer gap?" Online double-conversion.
- "A device runs on AC but not on battery; first suspects?" Battery, charger, connector.
- "A short high-voltage pulse from lightning is called a..." Transient or spike.
What to memorize
- Lead-acid 2 V and sulfates; NiCd 1.2 V and memory; NiMH 1.2 V; Li-ion 3.6 V, no memory, thermal runaway.
- Runtime test beats voltage check. Standby, line-interactive, online.
- Sag, surge, spike, noise, blackout; conditioners, surge protectors, UPS.