Hi, Can anyone offer any suggestions why the battery on my 2011 Monte Carlo is intermittently (but frequently) going flat.

This started last winter, I took it to my main dealer who tested it and said it must be due to the low mileage I do and the cold weather. They charged up the battery and said it would be fine. However, between 27th May and 1st June this year, it failed to start on at least 4 occasions (turned over, but would not start) despite the battery being charged overnight on 3 occasions, and having driven about 150 miles in one journey.

I booked it in to the main dealer who has had it for 2 whole days; they say they have tested the battery, checked for loose connections etc. but are unable to diagnose the problem. This means I will now have to carry around a battery pack in the boot as it is inevitably going to happen again.

Any help would be much appreciated.

Failed cell in the battery, a short or some device on the CAN bus not turning off properly, so chattering until the battery is flat.

You've not installed any aftermarket bits such as a radio or phone kit have you? Also do you leave anything in the cigarette lighter?

Boot light or glove box light not switching off?

Thanks for the suggestions: I have had no extras fitted – radio was already there. My sat nav/ phone charger (which are charged in the cigarette lighter) are NEVER left in the car. I don’t actually switch off the radio when I leave the car, obviously it goes off when I switch off the ignition. Not sure about lights in glove box/boot. Will have to think of a way I can investigate this (without hiding in the boot : )

On at least 2 occasions when the car failed to start, it had only been stopped for 5 mins max. On another, when driving after it had been jump started, the headlights were quite dim and didn’t brighten when the engine was revved

(I don’t know what a CAN bus is!)

Ok, I'd go with the CANbus issue or a failed cell in the accumulator.

So question - Has the dealer done a proper load test and SG tests or just put a voltmeter across the terminals? You need the proper tests to diagnose failed cells.

CANbus - http://en.wikipedia.org/wiki/CANbus

Sounds like a Battery issue to me, as suggested above, warrants a Load Test on the Battery.

With the syptom "the headlights were quite dim and didn’t brighten when the engine was revved" , I'd look for some form of low charge. But equally ,it could be worth a battery check . For the cost, a simple battery meter ( something that plugs into the ciggie socket) might show up any non charging problems.That s for a problem that might not occurr whilst stationary and charge system under test, but a problem that occurrs intermittantly on the open road .

Edited 9 June, 201313 yr by VWD

Thanks for all the suggestions. Of course the car has started perfectly since! But I have noted all the comments to use on the next occasion the battery fails . . . and I've ordered a bettery meter.

Actually VWD's comment makes me wonder if the issue may be the sprague clutch on the alternator sticking "open". If that's happening, you should see slightly under 12v across the battery with the engine running. That's an indicator rather than a definite diagnostic though.

Didn't know it had one ,Ken. Just thought that with a car voltmeter might show some form of anomoly on the charge system when on the road . It don't have to be a meter, some sort of red/yellow/green LED array would suffice . Possibly better as most non electrical folks would have problems remembering the voltages expected . For Flavia - idle, expect somewhere around 12v and above about 2000 rpm anything up to 13.8v. Another good check is at about 2000 rpm, switch every electrical appliance on . IE- HEADS ( MAIN ) , heater , fog lamp (s) rear windscreen and anyother elactrical thing and watch voltage . Any drop - get batery checked ,and then,if that's ok, have a look at the charge system . ( I'm not an auto elec blokie ,just an electronics bloke with an interest in car electrics , from days of old, when diagnoses of a dynamo charge system was to short field and output and send output to a 100w household bulb. Bulb lit, system ok).