Voltage question

[norseman]

There has been a lot of talk about batteries lately, hardly surprising at this time of year. My query relates to voltage readings on a multimeter taken after a full charge from a (new) battery charger. The car in question is a '98 5 series BMW that has charging/jump-starting facilities at both ends of the car ie the battery itself in the boot & live/neutral connections under the hood. The strange thing is that the reading taken from the latter was 12.7v whereas direct from the battery posts themselves was only 12.47v. Given the length of cables between the two I would have thought the readings would be the other way around.
Any thoughts guys ?

Happy New year to all.

 

[jj4091]

Are you sure your meter probes are OK, perhaps you are not getting accurate readings on different terminal materials.

 

[Harry Bloomfield]

The car in question is a '98 5 series BMW that has charging/jump-starting facilities at both ends of the car ie the battery itself in the boot & live/neutral connections under the hood. The strange thing is that the reading taken from the latter was 12.7v whereas direct from the battery posts themselves was only 12.47v. Given the length of cables between the two I would have thought the readings would be the other way around.

Something is not right with those measurements, or the measurement technique - that is too high a reading at the far end from the battery. Did you perhaps have the engine running or a charger connected at the front, when taking the readings?

You can get what is called voltage drop, the further away from the source, the more the drop in voltage, but how much depends upon the cable size and the load on the cable. With the minimal load of a meter, there would likely be almost no drop. Perhaps you got the two readings mixed up and you had some load somewhere? Perhaps you had a poor connection to the battery at the back of the car?

12.7v is an OK reading for a car battery which has no load upon it, is not on charge and has not been on charge for over an hour.

BTW - live and neutral are the terms used for the ac mains in your home. For dc as used on a car we refer to them as red/positive (+ve) and black/negative (-ve).

 

[norseman]

If I can answer both replies (Harry & JJ) together.
It's a small pocket multimeter I've had for years, the probes are good & I scratched away at the battery posts to get a good contact.
The engine was not running, nor the charger connected. The only thing I didn't do was to leave it for an hour after disconnecting the charger, probably only a few minutes.
There was some load at the time, but that would have been the 2 boot lights.
I may be silly bugger some times, but did not mix up the readings

Update > just checked the readings again, after the car has been unused for 4 days & there is still a variation in the readings. 12.13v under the hood & 12.06 at the battery itself. With a reading that low I wouldn't expect the car to start, but strangely the operation of the central locking is still 'crisp' whereas a sluggish reaction to the fob is normally the first sign of a battery problem. Maybe my meter is telling porkies, I'll try to get a comparative reading at my local garage by way of a test.
We have no plans to use the car until Tuesday (medication run) so will see if the cat starts or not. Fortunately I have a powerpack on standby.
The car's battery is only 18 months old, but somehow I don't have confidence in it … on the other hand there could be a parasitic drain at work.

Thank you both for your input, much appreciated.
nb: yes Harry I do know the difference between Pos/Neg & Live/Neutral … put it down to another senior moment

 

[Burnerman]

Check the voltage as you crank the engine, Norse.....it will drop dramatically but shouldn’t drop below 10v.
John

 

[EFLImpudence]

BTW - live and neutral are the terms used for the ac mains in your home. For dc as used on a car we refer to them as red/positive (+ve) and black/negative (-ve).

...and the hood is a name used in some weird foreign place; we call it the bonnet.

 

[Harry Bloomfield]

It's a small pocket multimeter I've had for years, the probes are good & I scratched away at the battery posts to get a good contact.
The engine was not running, nor the charger connected. The only thing I didn't do was to leave it for an hour after disconnecting the charger, probably only a few minutes.
There was some load at the time, but that would have been the 2 boot lights.
I may be silly bugger some times, but did not mix up the readings

If it is not a mistake, then something very strange is happening and I have absolutely no explanation as to why there might be more volts at a distance from the battery.

 

[norseman]

Check the voltage as you crank the engine, Norse.....it will drop dramatically but shouldn’t drop below 10v.
John

I will try that John, have to be quick though as (assuming the battery is good at the time of the test) the motor will start first turn.

 

[norseman]

...and the hood is a name used in some weird foreign place; we call it the bonnet.

Another brownie point lost
In my defence (a) I've owned American vehicles in the past & got into the habit (b) BMW often refer to this terminology, ie sedan as opposed to saloon, in their owner handbooks … perhaps it translates better from German to English, bearing in mind that a large number of these cars are sold in the 'States.
Must try harder

If it is not a mistake, then something very strange is happening and I have absolutely no explanation as to why there might be more volts at a distance from the battery.

Thanks Harry, let's see what develops in time. Fortunately with both of us retired I can cut the car a bit of slack … for a while at least.

 

[Harry Bloomfield]

I will try that John, have to be quick though as (assuming the battery is good at the time of the test) the motor will start first turn.

Inhibit the starting! Sometimes wrapping cooking foil around the plastic part of the key, will inhibit the starting, but allow cranking.