Undiagnosed Battery Drain

I have a 1999 Volvo S70 which has developed a battery drain in the last year. After going through three batteries in less an a year, I was told the problem was the alternator. After replacing the alternator, a month later, I had a dead battery. My Volvo shop fully charged it, did a complete check of all the items in the vehicle using the battery, and reported that they could not find a single source of the drain. A week after they did this check, the battery was dead again. I now have the car on a Battery Tender but prefer to actually solve the problem. Any thoughts?

Unless someone has a better idea (and I hope someone does) you may have to look into temporarily adding an ammeter. Ammeters were a standard dashboard instrument until the 1950s. Back in the 1950s, they would let you know when the generator (functionally equivalent to your alternator) started to fail and would tell you a bit about some other problems.

The problem is that modern vehicle electrical systems are very complex. You need a lot of skills you don’t have (yet) to analyze problems there. There is also a small risk that a faulty ammeter installation will start a fire, do some impromptu arc welding or even destroy something expensive. My understanding is that everything in a Volvo is expensive.

If you want to check for static drains, you might look into clamp-on style ammeters. These things measure current flow in a single wire. Since they don’t involve rewiring the car, they are less risky than a panel type ammeter. But they require isolating wires so you can clamp the meter onto (around) it. Not especially easy. And you need to understand what it is telling you. The low end ones are not terribly expensive. I have one. It has never, however, been useful in actually solving any problem – for me. Probably that’s just me.

Like I said, I hope someone has a better idea.

You must choose a mechanic who is “committed” to finding the problem. It may take a good afternoon of troubleshooting with a voltmeter to find the source of the drain. A good mechanic will start by removing one fuse/circuit breaker at a time until the drain is removed. That will give you the area to start troubleshooting in. Simply replacing a battery or alternator is not enough. Throwing parts at a problem will just drain your bank account.

did a complete check of all the items in the vehicle using the battery

I kind of doubt that, since the problem remains.

You need a mechanic with a Digital Multi-Meter that can read volts and amps, and who knows how to use it, as Missileman described.

Here’s a trick that may or may not help. But it’s easy and risk free.

Connect an ammeter to the battery and measuer the parasitic drain. Pull the fuses one by one for the unneceeary circuits until the drain drops off. The last fuse pulled will tell you what circuitry to further analyze.

In new cars it gets complicated because so much including lights are controlled by the Body Control Module, but I suspect the '99 Volvo wasn’t “there” yet.