Car Battery drains

The drop in voltage is not the drain but it is an indicator of the effects of the drain. Think of the water in a municipal water tower. The drain would be the water leaving the tank. That is current as in current in a river, and current in a wire. The RESULTS of that drain is the pressure in the tower would go down because there's less weight in the remaining water. Voltage is electrical pressure. As current flows through circuitry, the battery becomes discharged and the voltage goes down, so the voltage readings do have merit but there's more to the story.

Generally speaking we consider a battery to be fully charged when we find it at 12.6 volts, and fully discharged at 12.0 volts. You will often find higher than 12.6 volts right after charging it with a battery charger or by driving the car because some of the electrons haven't been absorbed into the plates. That's called the "surface charge" and must be removed for proper battery testing. That is done by turning on the head lights or anything similar for about ten seconds.

Even when just parking the car after driving it, that surface charge exists so in the case of the 12.69 volts, you can expect the voltage to drop a little as the electrons leave the acid and go into the plates. Typically the voltage will stay near 12.6 volts. 12.42 volts appears to be one third of the way to discharged and that is too fast over three days but those readings can be misleading, or at least inaccurate. Unless specified differently, the industry standard is a good battery will supply all the computer memory circuits and still be able to start the engine after sitting for three weeks. Going just by your voltage readings that means the battery will be too run down to crank the engine after nine days. That implies a drain of three times the allowable.035 amps, or roughly 0.1 amp. Most amplifiers draw more than that when they're on but not amplifying any signal so it should kill the battery in a few days. If you find the battery holds up considerably longer with the amp unplugged, that proves my assertion that the voltage readings do not accurately represent the drain. Just because it's down to 12.42 today, it may not drop another.27 volts in another three days.

Regardless of all my assumptions and observations, I'm hoping you found the cause. You originally said the battery would be dead in three or four days and that would fit with the typical amplifier staying on. The heat is a good clue too although if it does not get warm, it still could be staying on.

You might try disconnecting one battery cable, then monitoring the voltage periodically. I've never done that myself but I would expect that if the voltage still drops off, it's not really getting fully charged and you're measuring the surface charge. I built a lot of "bugged cars" for my students to diagnose and one was a single bad diode in the diode trio of an older GM generator. One of the clues they needed to find was under a load test, it could only produce 20 amps. That was one third of normal and enough to run the car but not much left over to recharge the battery. A harder-to-spot observation was that the system voltage actually was a little higher than normal. Even though voltage was higher, that didn't equate to fully charging the battery. Easy to explain; not so easy to diagnose without performing a load test.

A computer is drawing heavy current in an attempt to go to "sleep mode". Current flow through the light bulb results in a voltage drop so the computer is actually getting much less than 12 volts. That equates with power being switched off, the computer turns off, current flow stops, no voltage is dropped across the bulb so supply voltage to the computer goes back up, the computer wakes up and prepares to go to sleep mode which can take up to 20 minutes, draws high current, and the process starts all over.

A bulb is not accurate for this test. They worked well many years ago when cars didn't have computers. A drain was typically caused by a trunk or glove box light not turning off. Those were significant drains that killed a battery overnight, and the test bulb would glow about half brightness.

With all the computers on cars today, it is common to have a drain that will kill the battery in a few days but is way too small to make the bulb glow. You need a digital amp meter instead but using that has its own problems from the computers timing out and going to sleep mode. Check out this page for a better explanation:

http://randysrepairshop.net/testing-for-battery-drain.html

You can have a car with no battery drain problem and the test bulb will act exactly the same as what you're seeing, or you can have a drain that is too small to show up with a bulb.

In your third reply, I assume the engine is running to get over 13 and 14 volts, so any current measurements are meaningless. There will be current flowing into the battery to recharge it but that should be closer to 5 - 15 amps. With all the things you turned on, current should be up to 20 - 30 amps but that would be coming from the generator, not the battery. Since the voltage dropped very little with those things turned on, that just means the generator is not having a hard time keeping up with demand.

Thanks for the info. You would think this would a common enough problem that the dealer's mechanics would be aware of it and not have to waste so much time diagnosing the drain.