Can a bad alternator cause parasitic draw?

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.

A bad rectifier/diode can cause a parasitic draw on the battery.

Key Signs to Look For

So, how to know if regulator rectifier is bad? In general, there are two primary ways that the regulator rectifier can fail. First, the diode can burnout and cause the battery to drain. You won’t have any trouble diagnosing the faulty regulator rectifier if the battery is the cause. You’ll note signs right away like poor starts, fluctuating meter readings, and dimmed headlights.


If this were my car, I wouldn’t leave the inverter plugged in. I’d guess that it draws some current even if nothing is plugged into it. That current might be minimal, but over the long term the extra discharging and charging might result in a reduced life for the battery.

My guess is your DC to AC inverter has developed a problem and is now draining the battery. Unplug it, leave it unplugged, charge your battery to a full charge with a battery charger, and see if that solves the problem. If so, replace the inverter.

You might ask how an inverter could fail and then drain the battery? Lots of possibilities, but the most likely: Inverters contain diodes, and diodes can fail with use. When a diode fails it may conduct current in both directions, when it should only conduct in one direction. Likewise alternators also contain diodes, and a common failure mode for an alternator, one of their diodes will fail and the symptom is a drained battery.

If you determine the problem isn’t the inverter, if you know how to use a volt meter safely, here’s the battery/alternator test I use: Before first start of the day the battery should measure about 12.6 volts; then immediately after starting the engine, 13.5 - 15.5 volts. Post your results here for more ideas.

I would use a digital multimeter to verify a parasitic draw overnight . . . or not . . . before possibly condemning the alternator

He did unplug it