Dump Trucks and Dead Batteries | Tech Tip

My buddy asked me to take a quick look at his new dump truck the other day. It seems the batteries—there were three of them—were going dead whenever the truck sat over night and he was at his wits end as to what was causing it. He'd had the truck back to the dealership, where they replaced the alternator and all three batteries under warranty. He'd driven the truck back from the shop on Friday night and parked it in a warm garage over the weekend, but come Monday morning, the thing wouldn't even turn over. Obviously, something was killing the batteries, and my suspicion was that it was parasitic draw, but where was it coming from, and what does it have to do with motorcycles?

We performed a few preliminary checks, and oddly enough, found no draw on any of the batteries. This particular truck has a large insulated lug mounted externally to the battery box. The lug is connected to one of the batteries' positive terminals, and allows you to conveniently jumpstart the truck without removing the battery box lid. In concept, it's similar to the type of charger connector used on motorcycle batteries for maintenance charging.

The lug looked a little green and was filthy with road dirt and corrosion, and so was everything else, including the tops of the new batteries. On a whim, I connected the positive probe of my voltmeter to the lug and touched the ground probe to the plastic battery case; sure enough, 12 volts were flowing from the terminal across the dirty batteries to the ground connections. Cleaning the lug and the battery cases cured the problem.

Here's the deal: Road grime often contains metals and salt (especially where the latter is used to deal with ice on roads). Salt is especially conductive when it's damp, so under the right conditions the dirt can provide a current path that'll drain your battery as quickly as leaving a light on will. Like they say, an ounce of prevention is worth a pound of cure, so wipe down your battery top from time to time, and if you've got any external connectors check them for signs of corrosion or shorting, particularly if you've got a battery that goes dead without any apparent cause.

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