Lithium Battery for Elevator Rescue Device: Safe Passenger Evacuation
Lithium Battery for Elevator Rescue Device: Safe Passenger Evacuation
When building power fails, an automatic rescue device (ARD) has one job: drive the elevator car to the nearest landing and open the doors before anyone panics. That job lasts under two minutes but demands a violent burst of current, and it may be the only time the battery works all year. Buildings across Asia and the Middle East are now specifying lithium ARD packs instead of sealed lead-acid, and a competent lithium battery manufacturer will size the pack around the traction motor’s inrush rather than around nameplate kWh.
What the Duty Cycle Actually Looks Like
An ARD sits on float charge for months, then delivers 15–40 kW for 30–90 seconds through the drive’s DC bus. This is the worst possible profile for lead-acid: the plates sulfate during the long idle, and Peukert losses cut usable capacity exactly when the surge arrives. Site engineers routinely find a three-year-old VRLA string that tests fine on a trickle load but collapses below the drive’s undervoltage trip the moment the motor engages. Lithium iron phosphate has none of that behaviour — its internal resistance stays flat across state of charge, so the 90th rescue performs like the first.
Sizing for Inrush, Not for Energy
The controlling number is peak discharge current, not amp-hours. A 630 kg passenger lift with a 5.5 kW geared machine may only need 0.3 kWh of energy per rescue, but it will pull 60–80 A from a 96 V bus during acceleration. Specify a pack rated for at least 3C continuous and 5C for ten seconds, then confirm the BMS discharge MOSFETs or contactor are rated above that figure with margin. Undersizing the BMS — not the cells — is the most common failure we see in retrofit projects.
Machine-Room-Less Installations
MRL lifts hide the controller in the shaft headroom or a slim landing cabinet, where ventilation is poor and ambient can reach 45 °C in summer. Lead-acid gasses hydrogen in that space and needs clearance most MRL cabinets do not have. LiFePO4 is sealed, non-gassing, and roughly one third the mass, which matters when a technician has to lift the pack into a headroom niche on a ladder. Cell-level fusing and a UL 1973 or IEC 62619 certificate should be part of the tender documents.
ARD Battery Technology Comparison
| Attribute | LiFePO4 | VRLA (AGM) | Supercapacitor |
|---|---|---|---|
| Service life in ARD duty | 8–10 years | 2–3 years | 10+ years |
| Peak current capability | High (5C) | Medium, voltage sags | Very high |
| Rescues per charge | 20–40 | 3–6 | 1–2 |
| Mass for 96 V / 20 Ah | ~22 kg | ~65 kg | ~30 kg |
| Hydrogen gassing | None | Yes | None |
| Cabinet temperature tolerance | −10 to +55 °C | Derates above 30 °C | Wide |
Retrofit Checklist for Existing Lifts
Three checks decide whether a swap is straightforward. First, the ARD charger: most legacy units float at 2.27 V per lead cell and must be replaced or reprogrammed for a lithium CC/CV profile, otherwise the pack floats permanently at 100% and ages faster. Second, the low-voltage trip in the drive must be re-set to match the flatter lithium discharge curve. Third, the annual inspection procedure changes — instead of measuring specific gravity, the technician reads pack health from the BMS over RS485 and logs cell delta. Building the reporting output into the lift monitoring system turns a manual visit into a remote check.
Total Cost Over a Ten-Year Lease
A lithium ARD pack typically costs 1.8–2.4 times a VRLA string at purchase. Over ten years the lead-acid string is replaced three or four times, each replacement carrying a service call, disposal fee, and a period of non-compliance while the lift runs without a working rescue device. Once labour is included, lithium is usually 30–45% cheaper across the lease term, and the building avoids the reputational cost of trapped-passenger callouts.
People Also Ask
How many rescues should an ARD battery support without recharging? Codes commonly require at least three consecutive rescue operations. A correctly sized LiFePO4 pack delivers 20 or more, which covers a rolling blackout rather than a single outage.
Can I keep the original ARD charger when switching to lithium? Usually not. Lead-acid float voltages overcharge lithium cells over time. Either fit a lithium-profile charger or use a pack with an integrated charge regulator.
Is lithium allowed in elevator shafts? Yes, where the pack is certified to IEC 62619 or UL 1973 and installed per local fire code. LiFePO4 is the preferred chemistry because it does not vent flammable gas under normal operation.
Written by Karl at China Battery Technology. Request a quote.
