Battery voltage?

[AlanD]

When I started it today, I had a TomTom plugged in and my son's smartphone charging in the back. It dropped momentarily to low 9s voltage as it started and displayed the usual Convenience Features Disabled message for about a minute. Watched the alternator bring it back up to 14V within a couple of mins.

It basically hovers around 13.9-14.1V while driving.

You're right about the "resting" voltage; it's not the same as measuring across the terminals when the car is fully off. However it's readily benchmarkable to other owners. Just turning the ignition on 1 click starts about 20 different little motors all over the place (seats, steering, COMAND, CD changer, etc. etc. etc. etc.) so it does drop down a little bit and normally recovers up to about 12.3V or similar. 12.7V would only be seen just after you stop the car from a full recharge .. not the next day. Again, with the ignition on.

To be clear, all I'm doing is dabbing the dash dimmer stalk x 3 while the dash displays outside temp (or kph depending on your settings) while on ignition setting 1. If you leave it alone, it will keep displaying it while driving too, though you have to do it each time you sit in the car.

FWIW, when I used my battery conditioner to charge it up, the last step is a 3 min battery drain test and it passes with a green light, so at the very least it's not a complete dud ...

 

[AlanD]

So, recently, my battery has been even more baffling!

2 weekends ago, I decide to recharge AND recondition the battery on Saturday so it would be ok on Sunday for a 30 mins each way drive. 6 days later on the following Saturday morning, it's stone dead! I charge it again, and at the end of the charge, I get a red light on my charger on the test stage (indicates battery cannot hold charge). Never had that before! Car is completely dead after this charge.

I disconnect and reconnect the charge and let it start charging again. Didn't have time to reach full charge, but the car worked fine that Sunday and again this Sunday (7 days after a 3/4 charge). It is low today, with batter convenience message, but it still worked!

Any explanations!? Is it possible for something to sometimes stay on and cause a drain, but not at other times? What would you make of my red error light?

#puzzled ...

 

[AlanD]

Latest update - got a new MB rear battery a week before a 1 week driving trip to Ireland. Lasted the 2 weeks since perfectly without any issues at all. Fingers crossed that's it for another few years!!

 

[David404]

Few years ago I got a Ctec charger to re-charge 110Ahr caravan leisure batteries, older caravan battery chargers only bring a caravan's leisure battery up to around 80% charge and lead acid batteries benefit from being brought up to 100% charge.

The Ctek came with fly leads that can be fitted to a car/bike/RV to allow quick connection. These I fitted to the car and as I have the Ctek, the fly leads fitted and ready access to power I make a habit of using the Ctek to bring the car battery up to 100% charge 5 or 6 times a year.

Given the reputation all MBs have for power consumption and appetite for volts, think in your position I'd be looking for a trickle charge solution. Fit Ctek fly leads, extend the mains power cable on the Ctek so it doesn't need extension lead, can you close the boot with charger still connected.

Oh and if you have the secondary SBS battery in engine bay I'd be attaching a second Ctek fly lead to that too.

 

[AlanD]

Would be nice and easy with a garage. Not having convenient driveway power however makes it a royal pain to charge! Might give it the odd bump however a few times a year if it's not getting long use to top up ..