An older electric stand-up reach truck was brought into the shop with a terrifying electrical failure. The operator turned the key switch off, pulled the steering handle out to the run position, and the truck immediately took off backward on its own, smashing into a rack. The only way to stop it was to physically disconnect the main battery cables with a wrench.
When an electric forklift runs without a key, the immediate suspect is a shorted key switch. But when we bypassed the key switch entirely, the truck still had full power the moment the batteries were connected. We traced the power flow and found that the main DC contactor-the giant heavy-duty electrical relay that connects the batteries to the motor controller-had its contacts physically welded shut. The copper contacts had melted and fused together.
Replacing a welded contactor is easy, but if you don't figure out why it welded, the new one will just explode on the first day. Main contactors weld for one reason: extreme heat caused by high electrical resistance. We looked at the battery pack, which consisted of a series of heavy lead-acid batteries. We checked the cable connections, and they were tight. We then load-tested the individual batteries and found a massive discrepancy. Three of the batteries in the middle of the string were heavily sulfated with a lot of internal resistance.
When the forklift went to drive up a ramp or lift a heavy load, it pulls hundreds of amps from the batteries. Because those three middle batteries were sulfated, they couldn't pass the current efficiently, causing their voltage to instantly plummet. When voltage drops, amperage spikes dramatically. The main contactor was being forced to carry massive, sustained electrical surges far beyond its rated capacity. The heat from those surges literally melted the contact points together. The root cause of the runaway forklift wasn't an electrical gremlin; it was a fleet manager who wasn't properly equalizing and watering the lead-acid batteries, allowing sulfate crystals to build up on the plates. We replaced the contactor and the ruined batteries, but it was a harsh reminder that poor battery maintenance on high-voltage industrial trucks is a major safety hazard.
