The transition to 48V and 80V lithium-ion batteries has eliminated the heavy lead-acid watering and changing routines. But it has introduced a violent, sudden failure mode: the BMS blackout caused by voltage sag.
A lithium battery's nominal voltage (say, 80V) is a resting measurement. When a heavy-capacity forklift attempts to lift a maximum load at full mast extension, the traction and lift motors can demand massive current spikes-sometimes 600 to 800 amps for a split second. Lead-acid batteries are heavy and sluggish; their voltage drops slowly under load. Lithium-ion batteries respond instantly.
If the battery's internal resistance is slightly elevated due to age, or if the pack is below a 30% state of charge, that sudden 800-amp draw causes a severe, instantaneous voltage sag. The pack voltage can plummet from 80V down to 55V in a millisecond. The forklift's main controller and the Battery Management System (BMS) are designed to shut down instantly if voltage drops below a critical threshold to prevent cell damage. The contactor pops open, the truck goes completely dead, and the 10,000-pound load drops two inches before the hydraulic brakes lock up. The operator has to sit and wait for the BMS to reset before they can lower the load safely. Facilities are finding they must oversize their Li-ion battery packs by 20% just to provide enough voltage "headroom" to survive these heavy-lift current spikes.
