Can a bad alternator drain the battery when the car is shut off?

Hellrazer · 06-12-2023 6:24pm

https://www.boards.ie/discussion/comment/121462662#Comment_121462662

Same here @mk7r - I reckon the new battery was faulty when the OP got it - said it earlier - probably a dead cell in it. A BCM only draws 150-200 milliamps so to drain a 12v 350ah battey would take a couple of months.

Hellrazer · 06-12-2023 6:31pm

https://www.boards.ie/discussion/comment/121462692#Comment_121462692

You need to check what the battery is charging at - theres no point in guessing at this stage - multimeter while the car is running. Should be charging at 13.5-14 volts.

If it is charging at 13.5-14 volts then the alternator is ok. Then test the battery after its been fully charged - should be around 12.5 volts - any less or if it doesnt get to 12.5 volts then the battery is fecked.

However a battery that has dropped to between 5.6 volts and 7 is more than likely fecked anyway - its very difficult to bring a battery back to spec if its dropped that low.

Thats where you start - once youre sure the charging system is ok ie battery and alternator and its still dropping over night then you can start eliminating things - but guessing at what could be draining it you may as well be Nostradamus!!!

Hellrazer · 06-12-2023 6:33pm

@Igotadose - Is it a Hybrid by any chance?

gk5000 · 06-12-2023 7:00pm

I had a exact same issue caused by an alternator - draining the battery overnight.

Found an interesting work around.

Turn the car off as normal - it would drain the battery.

Turn the car off, but then turn the key back on until the red light came on (just before starting) and then turn the car off fully - it would NOT drain the battery.

Igotadose · 06-12-2023 7:01pm

https://www.boards.ie/discussion/comment/121462816#Comment_121462816

Nope, conventional 2011 Auris.

I measured 13.9 when running, alternator is likely to be o.k.

with engine off it was 12.6 when I last measured it. I'm waiting for fairer weather to test the voltage, the car's been turned off since yesterday and I disconnected the positive lead on the battery. Hopefully I'll be able to get a reading tomorrow, kinda sucks being out in the blowing wind and rain messing with batteries and a multimeter.

mickdw · 06-12-2023 7:12pm

https://www.boards.ie/discussion/comment/121462953#Comment_121462953

I've seen alternator rectifier issue cause the drain even though alternator would be working normally in terms of keeping battery topped up in use.

I'd get a used alternator from a breaker to rule it out. Would not be mad money.

Could then have faults read again to see did control module fault still exist.

Igotadose · 06-12-2023 7:17pm

https://www.boards.ie/discussion/comment/121463012#Comment_121463012

Bad rectifier causing drain overnight though? I didn't think the rectifier did anything when the engine's off?

My plan of attack is measure again tomorrow, if 12.5 or whatever, it's likely not the battery. Then, back to finding a parasitic drain, I think I am capable of doing that. I'm suspicious of the fobs, it's just weird they stop functioning, then come back randomly, only to stop again. First time it happened I had them reprogrammed, and the guy doing it said, 'they're reprogrammed, but it's like it doesn't take.' But they worked when I got the car from him and worked for some months.

The keys are new and transmitting fine, batteries in the keys are fine - and, sometimes they work!

Herself thinks the car is just plain haunted and telling us it needs to go... She may have something there.

Hellrazer · 06-12-2023 7:24pm

Had a look around recalls on the Auris and found this

https://mag.toyota.co.uk/toyota-auris-owners-12-volt-flat-batteries/comment-page-11/#:~:text=The%20issue%20is%20caused%20when,left%20unused%20for%20several%20days.

It does say 2012 onwards but it could be worth taking out the hazard switch and seeing if you had a drain then.

Hellrazer · 06-12-2023 7:27pm

https://www.boards.ie/discussion/comment/121463012#Comment_121463012

A bad rectifier doesnt usually cause a battery to drain - what a bad rectifier does is prevent the battery from charging so people assume the alternator is causing the drain but in reality the battery isnt receiving a charge in the first place so eventually goes flat.

In the OPs case hes confirmed that the battery is charging correctly so Id rule out the alternator based on that.

Hellrazer · 06-12-2023 7:34pm

https://www.boards.ie/discussion/comment/121463030#Comment_121463030

One thing I did see a few years ago was a door lock that was faulty - basically the motor in the lock was continually trying to open and close all the time. One symptom was the fobs appeared not to work but what was happening was the faulty lock was opening and closing constantly and the fobs didnt get a chance to work. But the noise could be heard from outside the car even with the engine off.

If you still think theres a parasitic drain Id start pulling fuses one at a time - so tomorrow pull the rear screen heater, try it next morning and check voltage - if the same refit the fuse and try another one. Say the heater motor - Id start with the high draw accessories - heaters, rear screen, heated mirrors , heated seats if you have them - anything with heat usually is a higher draw on the battery.

Its going to be time consuming to find it though.

mickdw · 06-12-2023 8:01pm

https://www.boards.ie/discussion/comment/121463065#Comment_121463065

I've seen the vectras do it time and again.

Car starting perfect. Suddenly flat battery. Jump start and car charges fine and drives for a day or a week and then flat battery again.

I've fitted the new rectifiers to alternator myself as it's a cheap repair. No more flat battery.

Hellrazer · 06-12-2023 8:11pm

https://www.boards.ie/discussion/comment/121463195#Comment_121463195

Seems unusual as any rectifier Ive seen fail usually causes zero charging of a battery. Maybe the rectifiers youre replacing are on the way out and still allowing some charging? A good rectifier will always charge at around 14 volts so maybe a borderline one is only charging at say 10? And depending on the cars 12 volt battery can still keep it going for a few days?

mickdw · 06-12-2023 11:17pm

I think it was dry solder joints was the issue so working or not working depending on heating cooling etc.

RiderOnTheStorm · 08-12-2023 11:42am

any update?

Igotadose · 08-12-2023 1:05pm

https://www.boards.ie/discussion/comment/121470185#Comment_121470185

Battery has been hovering around 12.6V overnight when not connected to the car (positive cable detached.)

So, at this point I don't suspect the battery. Unfortunately the weather and various life events I haven't yet been able to check for a parasitic drain per the video that was posted to this thread, but that's the plan in the next day or so. Here is the sequence most recently:

Tuesday 9:30 a.m.: 5.6V after sitting overnight, all cables attached. Noco jump starter didn't detect battery, I attached trickle charger

Tuesday after 3 hours of trickle charger: 9.6V

Jumped (using the Noco jump starter). Car started up right away

Tuesday after jump via & driving for a couple hours, engine on: 13.92

Tuesday sitting idle, battery attached about an hour after running: 12.7

Detached positive cable

3 hours later Tuesday: 12.66

<Wednesday: brutal cold, heavy rain. Not messing with the car in the deluge>

Thursday a.m.: 12.55

Friday 11:40: 12.46

---

Is a small daily loss of .10V indicative of a bad battery? I think not, consensus on the Goog is this is normal.

Thanks for the question. It motivated me to organize my scribbled notes!

listermint · 08-12-2023 1:13pm

Reads like a parasitic drain to me.

Igotadose · 08-12-2023 1:37pm

https://www.boards.ie/discussion/comment/121470598#Comment_121470598

Not when the battery is detached though?

listermint · 08-12-2023 2:16pm

https://www.boards.ie/discussion/comment/121470688#Comment_121470688

Sorry missed that bit in the middle.

I think then the battery is either already snagged itself or was bad to begin with.

That's a very rough 3 percent drop in 2 half days. Not good. Should be maybe 5 percent a month or so.

mk7r · 08-12-2023 5:06pm

12.7 volts is fully charged

12.1 volts is absolutely flat as a pancake

That is your range to work with

RiderOnTheStorm · 11-12-2023 12:55pm

https://www.boards.ie/discussion/comment/121471828#Comment_121471828

wow, did not know the range was so tight. Live n learn ....

Igotadose · 17-08-2024 4:36pm

Finally got this problem resolved today (so, over a year since it started being a problem.)

The hardest part, by far, was finding a mechanic here in West Kerry who could diagnose the problem. Ended up seeing 5 of them, including the dealer in Tralee, who I'll never visit again. Don't want to slag them openly, but feel free to ask about them in PM.

Problem was a faulty BCM module. It was causing a drain of about 1 amp. Everything fine now.

And, once I learned about BCM modules, I found this article which pretty much describes the symptoms I was seeing exactly:

https://www.gmride.com.my/blog/5-common-symptoms-of-a-failing-body-control-module-bcm/

The last mechanic said he'd done 3 on Auris's and wasn't surprised that this was the problem.

Finding an automobile electric mechanic was hard! The closest one besides the one I found in Kerry was in Cork.

jmreire · 19-08-2024 11:26am

https://www.boards.ie/discussion/comment/121453752#Comment_121453752

I caught this tread too late, but whenever I've come across this problem first step, change out the battery, and see if the fault persists, if it does, the problem lies with the vehicle electrics. If not, it's the battery. Begs the question, maybe the old battery didn't need replacing, and was caused by the BCM?

Igotadose · 19-08-2024 11:58am

https://www.boards.ie/discussion/comment/122518838#Comment_122518838

Hiya. Ended up changing out the battery twice, and the problem persisted.

The original battery was pretty much dead when we returned from holiday. Like, I think, under 4 volts and it wasn't chargeable.

Anyway, been a few days now and the fix seems to be working. I have the old BCM unit, trying to figure out how to take it apart to see what's going on inside, whether water had gotten in or rust or who knows. It's mounted alongside the battery in this model of the car, it got moved into the cabin in later models.

jmreire · 19-08-2024 12:12pm

https://www.boards.ie/discussion/comment/122518965#Comment_122518965

Swapping the battery out twice was 100% guarantee that the problem was elsewhere. The problem with modern cars is that they are now too complicated electronically trying to meet emission standards, and it doesn't take much to develop faults. I've seen experienced auto-electricians scratching their heads in puzzlement at some of the problems they face. And you have just seen how attempts to identify the problem went. The theory is, you connect your computer to the OBD socket, run a check on all the electrics, and BINGO, a stored fault code shows, so you insert this code into the computer, and it identifies the actual part, and what's wrong with it. Now this is exactly what should have happened in your case, but as you have seen, what's supposed to happen, and the reality are two different things. Joys of motoring, I guess.

Can a bad alternator drain the battery when the car is shut off?

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