Can a bad alternator cause parasitic draw?

squeamishossifrage
Tue, 31 May 2022 15:58:31 +0000
Maintenance/Repairs

I bought a used 2007 Dodge Ram 1500 a few years ago, and for the first 15 months I had no trouble with parasitic draw or dead batteries or anything - I use one of those 150-Watt AC-to-DC power inverter strip in my center console, plugged into the auxiliary power outlet, so that I can charge various devices in the cab while I’m driving. I’ve used the same one for years in multiple different vehicles and never had any issue with parasitic draw, including in the first 15 months with the truck. I left it plugged it into the outlet at all times, even while the truck was off, and always the battery would hold its charge, starting just like normal even after multiple days without driving.

In January the truck battery died over a particularly cold weekend, and I assumed it was just at the end of its lifespan (it was already fairly old), so I replaced it with a brand-new battery. Two months later the battery died overnight again, but this time I was unsure why. I eventually noticed a correlation between leaving the power inverter plugged in overnight and discovering the battery had no charge when starting the truck in the morning, so I figured the issue was parasitic draw - but again, I’ve never had that issue before, despite using the same exact power inverter.

I figured maybe I had a bad alternator, so I had it tested for free at a few auto parts stores - both battery and alternator passed the tests every time. I’m aware those tests are fairly narrow, however, and can miss a lot of problems.

Is it possible there are sub-components of my alternator (e.g. the regulator) that are bad, which can cause apparent parasitic draw from devices that previously never caused parasitic draw? In other words, is there some single cause that can explain all of my observations over time, and is it likely to be related to the alternator?

After the battery died overnight again last week, I’ve had continuous trouble re-building or maintaining a charge, even though I never plug in the power inverter anymore. Does this point to a faulty alternator component? What might cause an alternator to pass a voltmeter test, but still fail to actually do its job? Is it possible there’s a connection between a bad alternator and the sudden trouble with my power inverter draining the battery overnight?

Thanks so much for any help you can provide.

9 posts - 9 participants