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Norseman

Strange readings

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Hi folks

A little puzzler to help you take your mind off the current lock-down.

I've been having a spot of battery bother with my '98 523i & have been getting some (to me anyway) strange readings on my multimeter.

When checking the voltage (ignition off) at the battery posts themselves & at the jump-start connections under the hood I get a HIGHER reading at the latter than from the battery !

Surely this should be the other way around ?  There is of course a chance that my meter is telling porkies, but even if it lacks accuracy after all these years the variation in readings remains despite repeated checks.

Is there any other source of power in these cars, apart from the main battery ?  Daft question maybe, but ...

Thanks guys, stay safe.

 

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1 hour ago, Norseman said:

Hi folks

A little puzzler to help you take your mind off the current lock-down.

I've been having a spot of battery bother with my '98 523i & have been getting some (to me anyway) strange readings on my multimeter.

When checking the voltage (ignition off) at the battery posts themselves & at the jump-start connections under the hood I get a HIGHER reading at the latter than from the battery !

Surely this should be the other way around ?  There is of course a chance that my meter is telling porkies, but even if it lacks accuracy after all these years the variation in readings remains despite repeated checks.

Is there any other source of power in these cars, apart from the main battery ?  Daft question maybe, but ...

Thanks guys, stay safe.

 

How much difference are we talking about?

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Thanks to both of you for your replies.

Battery fully charged (though I didn't leave it for more than a few minutes after the charger was disconnected, as I believe I should have) battery posts 12.47v / jump connections 12.70v

After the car has stood unused for 4/5 days 12.06v battery / 12.13v under the hood.

Interestingly the car started first turn today (to my total surprise :o) 4/5 days after the lowest quoted readings, they would probably have been even lower had I bothered to check.

 

I guess you will both suggest a check for a parasitic drain … that is on my list when the weather improves. 'Oh & before you ask, yes the boot lamp(s) are being extinguished when the lid is closed … that was my first check.

nb:  I renewed the battery (90AH) on purchasing the car 18 months ago, but it is the old style removable cell cap(s) variety so I wonder if it was old stock. No good for the BM anyway as 3 of the 6 caps are inaccessible, due the the position of the live leads & the diagonal clamp strap. I'd only just bought the car so didn't realise this … know better next time !

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A possible reason for the discrepancy is that there is some dirt or oxidation at the point where you put the multi meter test leads at the battery terminals. The difference you describe is certainly in that ball park. So I’d say that’s a red herring in terms of the issue it is you are having which I assume is dead battery or car won’t start. Main reasons for battery drain are radio/amp/cd player not switching off, a module not going to sleep when commanded to such, could be a failed hedgehog or fault with IHKA (aircon module). Also could be a fault ignition switch. Failed hedgehog would show up with problems with the heater fan, the ignition switch would show up with other random electrical issues. I personally have had the car stereo being the cause. Essentially it looks like it’s switched off but the amp is still running and kills the battery. You can hear a faint hum from the speakers when it’s off. 

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Your reply is much appreciated.  Your assumption of a dead battery is partially right in that I had a DOK situation about a month ago, but the car has not let me down since giving the battery a good charge. I'm more puzzled at the car's ability to start easily after standing for 4/5 days with a meter reading (back then) of barely over 12v. Following your suggestion I'll be putting my ear (the one that isn't used to listen to 'ere indoors) to a speaker, though it might be worth mentioning that the stereo is not factory fit as it's been replaced with a Xtrons unit before my ownership of the car.

 

I can't really complain about reliability, though I've taken to carrying a compact power pack in the boot 'just in case'   I think I've been spoilt by my 33yr. old Range Rover that always starts, despite being unused for anything from a week to a month  :rolleyes:

Edited by Norseman

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Some very good suggestions already.

 

Whne you replaced/recharged the battery, did to clean down the sides of the battery posts and clamps, and tap the clamps lightly into place before tightening? these things can be maddening as I've posted previously ....

 

One easy check that might be worthwhile - measure battery voltage (1) on the tops of the battery posts and then (2) on the tops of the battery clamps.

 

I'm thinking that measurement 2 should agree well with the under-bonnet terminal post, and what you are seeing is the dirt or oxidation on the posts as @Mashmanu suggests, in measurement 1. (NB This is not unusual, the battery posts will be a lead-antimony alloy, so the exposed tops will be grubby/oxidised - but cleaner under the clamps)

Edited by Huff

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Many thanks Huff, a simple test for me to carry out … I'll do it tomorrow, weather permitting.

Appreciate your input, as I do all others.

 

nb:  just ordered a battery post/terminal cleaning device, will employ when it arrives next week.

Edited by Norseman

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On 06/01/2021 at 21:21, Norseman said:

Many thanks Huff, a simple test for me to carry out … I'll do it tomorrow, weather permitting.

Appreciate your input, as I do all others.

 

nb:  just ordered a battery post/terminal cleaning device, will employ when it arrives next week.

 

@Norseman did you get to the bottom of this? I've had a similar experience recently, turned out that my multimeter's battery was running low, good enough to provide a reading, but inconsistent. It's a Fluke so I wasn't expecting strange behaviour like that but seems it's a bit sensitive when the battery is starting to go.

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Not really Tough, I've given both battery posts & clamps a good clean with the wire brush device I purchased recently, re-connected with smear of the good old petroleum jelly & left it at that. That was last weekend & car hasn't been started since, but I'll need to use it later today (Thurs) to go shopping so we'll see if it starts.

As for the multimeter, I changed the battery fairly recently & am still awaiting my chance to compare the readings with another.

Thank you for your interest.

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Update >

Central locking operated 'crisply' which based on past experience is a good sign & there was no hesitation at all when starting … as the car had been standing for six days in the cold I'm now cautiously optimistic :)

Still like to get the multimeter checked out though.

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