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Discussion Starter #21
Personally, I would take a reading at the charge point under the bonnet. You would be looking at 12.6V or above. The batteries are wired in parallel so if you measure at the batteries and they are different you've got an even more interesting problem ;-)
I took a reading as you suggested, and it shows 12.1V.

Is this too low and is it this that is causing my first key turn "donk" first thing in the morning.

I am going a long run tomorrow (300 miles or so) so presumably that will charge the battery back to 12.6V.

What do you think ?
Thanks.
Hayden
 

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I took a reading as you suggested, and it shows 12.1V.

Is this too low and is it this that is causing my first key turn "donk" first thing in the morning.

I am going a long run tomorrow (300 miles or so) so presumably that will charge the battery back to 12.6V.

What do you think ?
Thanks.
Hayden
A resting voltage of 12.1v is not good, you may well have found the culprit for all your problems.
There are tables for calculating battery amps v voltage, see Basic to Advanced Battery Information from Battery University
I went exactly the same pathway as yourself with BMW techs, changing allegedly defunct starter motor etc before I identified the battery issue.
I have little faith in BMW franchise operations anymore than the current Mercedes ones.
We always take battery function for granted.!
 

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Discussion Starter #23
A resting voltage of 12.1v is not good, you may well have found the culprit for all your problems.
There are tables for calculating battery amps v voltage, see Basic to Advanced Battery Information from Battery University
I went exactly the same pathway as yourself with BMW techs, changing allegedly defunct starter motor etc before I identified the battery issue.
I have little faith in BMW franchise operations anymore than the current Mercedes ones.
We always take battery function for granted.!
That is brill thank you.

What do you suggest....charging via posts under bonnet ??
Do you this is normal run-down due to not doing enough miles or are batteries dying ??
 

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You should preferably use the engine posts for both charging and jumpstarting.
Because I work abroad, I have a battery isolator in the boot.This avoids most of the battery discharge on standing, although there is some loss.
To facilitate starting, I use a cheap 12v battery as a surrogate top-up, and the cars always start with no battery associated failsafe issues, and use then charges the batteries.
You should also test what voltage the alternator is providing.
Strangely, my 95 SL500 is also battery voltage sensitive and will go into "failsafe" mode.
If your battery (combined) voltage is 12.1v, then they are down to 25% charge and probably require changing.
See what the position is after a slow extended charge
 

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All you can really do is to give them a proper charge and see what happens. Remember to give them a couple of hours after charging to settle down. If you don't use the car every day I would strongly recommend investing in a CTEK charger that you just leave hooked up when you aren't using the car. It's a lot cheaper than new batteries.

Normal car batteries don't like being "deep cycled" - they won't last long.
 

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my 650i when i press start. sometimes it sort like a wine sound then turns then starts.. my battery is 12.2v with my multimeter.. and a drop test doesmt show a drop.. wheen the starter does connect and turn normally it fires up lovely. but after a charge it says EXCESSIVE BATTERY DISCHARGE
 
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