Mysterious 12V battery drain

Get it tested with an electronic load tester. A lot of automotive retailers have such a tester and will test for free; it’s good for business. Dealerships for sure have such testers, but will more’n likely charge. Or DIY with something like Solar BA9.

As a rule, if Cold Cranking Amps has dropped below spec (just a touch over 300) it’s time to replace.

 

Hopefully yes; did you see what they did/used. Still, if they “say it is good”, but it’s constantly losing charge, someone is obviously dropping the ball.

 

The OP needs to determine if there really is a load on the battery when the car is off. To do so the current from the battery must be measured. I use a UT210 clamp ammeter for this sort of thing, because it is easy to clamp it on, no need to unhook any wires. Alternatively, he could also buy a standard ammeter and place it in line with the battery, for instance between the negative post and the ground. Just be sure that the ammeter can handle at least a couple of amps if the suspected leak current is that high. Do not turn the car on during these tests, those currents are much much higher and only a pretty expensive multimeter will be able to handle those without damage! Then close all the doors (else there is half an amp or so triggered by any open door or hatch sensor) and wait a couple of minutes for the car to stop fussing. If all is well the current should drop down to a few tens of milliamps. If it does not, then start narrowing the problem down by pulling out fuses one at a time until the problem circuit is found. Having an assistant would be useful, since the current must be read near the battery, but the fuses are under the hood or dash.

 

The tests are free.
Get another one or two at different shops and this time watch the guy doing the load test.

 

An even easier "quick and dirty" test is:
Disconnect (at least) one of main battery cables from the battery.
Read the battery voltage with a digital voltmeter.
Let it sit untouched for about 12 hours still disconnected.
Measure the voltage again.
If it has gone down AT ALL, meaning a tenth of a volt or more, then the battery is seriously suspect.
If it has gone down 3 tenths or more, you need to shop for a new battery.

A healthy 12 V automotive battery should NOT show any loss of voltage when sitting for 12 hours with no load.
OK......well maybe if you start out with it severely discharged already it might drop some more.
Ideally it should be almost fully charged to start with. That is 12.6 V or more.

 

The disconnected battery test is a good first step. However, having a bad battery does not mean that there is not also a current leak. In fact, the bad battery may have resulted specifically because a current leak dropped its voltage too far.

 

True.
But it is also true that an actual "phantom battery drain" is very rare......IF the vehicle is totally stock.
Given that, trying to test for a phantom drain is likely to be just a big waste of time......IF you do not first be sure your battery is good.

 

If you’re going to the trouble of disconnecting 12v, while it is disconnected, fully charge it, then do a load test on it. If no problem then do the amperage draw test connected.

 

First you did not say how old the battery is? If its over five years then its likely bad given the scenarios you describe such as dropping to 8vdc. Overall I would guess 85% a bad battery based on the low voltages you see. The worst mistake in troubleshooting is to assume one test such as an auto supply load test "passed" and never going back to reconfirm. If that test was flawed you will not determine the problem unless you reconfirm.

Second, if it is a parasitic draw, a new battery won't help. Especially if the draw is big enough to drop the voltage to 8v overnight. Typically you might see 10v with a 100 ma continuous parasitic draw in one day.

The car should draw about 25ma or less after a few minutes IF the circuit is not opened immediately before the test. If you open the circuit improperly, a large draw might exist for thirty minutes after reconnecting due to many computers reinitializing.

Parasitic draws sometimes overlooked are obd2 readers left plugged in, aftermarket radios and amps, and rear hatch lights left on because the hatch is not closing right. It is possible that key fobs left near the car cause extra draw for many hours which might be enough to discharge a marginal battery.

However we have seen the electric power steering draw 900 ma continuously while "off" and still work fine when "on". We have seen windshield wipers draw continuously. Lights such as hatch and map lights can do it. There are many possibilities. Perhaps thousands. Obviously you can pull fuses but that is putting the cart ahead of the horse. Unless you are monitoring a known excessive draw you don't know when to stop pulling fuses.

The point is you don't know about parasitic draws unless you test. Dc clamp meters like the UT210E ($50) are ok at 1000 ma or more but largely useless at lower draws. A series amp clamp, used with knowledge, can tell you for $5 using a low cost meter from Harbor Freight. They have an internal fuse on the amp circuit as well.

The trick to doing a series parasitic draw test is to insert the amp meter in series without ever disconnecting the battery. You can and should test in the batteries negative side. But do another battery load test at a different auto supply first.

 

Great explanation.
Of course once you determine there is a load, then the begins: finding the source of the load.

Chasing down true "phantom loads" is a boring slow methodical procedure.

 

you are making this too complicated. replace the battery, there's nothing wrong with your car.

 

Since you’ll be at the auto parts store anyway, charge the battery for a day and then have them load test it. They will likely show it’s bad and then you can avoid the core charge since you’ll have the battery with you to exchange.

 

Yes. There are several clues in the information that you supplied.

Just for the record, you didn't let the battery get fully charged.....which would be about 12.8.
If after 24 hours it still wasn't fully charged, that would be another BIG clue.

 

I can assure you that most of the input here is more experienced and informed than the typical parts store kid. Double that if it was Autozone. Some of us have been doing this twice as long as the kid has been alive. Plus there is the real possibility that if age is not the culprit, a parasitic draw killed your battery. As they say in carpentry, measure twice and cut once.

 

Somewhere a light was left on and kill the battery. Damaged it. If you put a load across a battery till its 0 volt then hold it shorted with a load on it for hours and hours it will damage an old battery.

Your post is one we have heard a thousand times. test good at the auto parts store. Nope....

Never test the voltage at the battery always test it at the front jump point because that's the only place it matters.And it checks the 8 connections it has to make properly to get to that front jump point.

Read the battery voltage at the front jump point. Then with car OFF turn on the headlights in high beam. While there on what does it measure at the front jump point?

Leave the lights on in high beam with car OFF for 5 minutes. After 5 minutes shut the lights off and with CAR STILL OFF measure the battery again. Yours will be tanked probably 8 volts.

Buy a new battery the dealer battery is best the price on that has come way down. Second choice is Batteries Plus there Duracell looks good.

 

Yes, that's how it's done. What was the final outcome? And don't say you sold the car.

 

did you ever find a solution. I replaced the battery in my second gen prius. Dead within 6 months. Tomato's tested battery and said it's fine. There is clearly something draining it.