flat battery AGAIN !!!

[Shortbusdriver1969]

I have a 96 2.0i 850. and just out of the blue I keep getting a flat battery
yes I have add-on HF radio front and rear strobes, but none are pulling a 'charge' on stand by, all equipment is wired to battery possitive
then grounded via earth on the body.
she can go days even weeks, then out of the blue flat battery
I have check the out put from alternater thats fine.
I am using brand new batteries as I thought one died I got another one megga heavy duty and that one also goes flat.
what could cause this ????
many thanks on advance :-)
Mac

[MrMopp]

Check your earth straps and make sure the battery isnt discharging back through the alternator would be the first checks

[Shortbusdriver1969]

funny you should mention earth, as the earth does not tighten up
fully wound up it still is loose not megga lose but moves about on the pole
could this be the cause ?

[MrMopp]

funny you should mention earth, as the earth does not tighten up
fully wound up it still is loose not megga lose but moves about on the pole
could this be the cause ?

Definately be my first port of call

[Shortbusdriver1969]

cheers matey :-)

[AlanG]

Had the same problem with my last 850 and traced it to a faulty boot light switch.

[Shortbusdriver1969]

Cheers Alan,
I have fixed the earth probs
any one know how I can check if its discharging through the alternator ?
Ta

[AlanG]

Am a time-served mechanic bud and never heard of that one,no disrespect to you. When life was in black and white,I used to pull a fuse at a time and constantly touch the earth lead against it's pole until the spark disappeared hence isolating the offending circuit. Not sure if that of any use to you

[t5 stealth]

hav u got an extracterfan in the n/s/rear wing,
& a switch on the dash ,
i had a early v70 come into work with same problem,turned out this fan was kickin in when the car was left for a wile.
just a thought lol

[MrMopp]

Cheers Alan,
I have fixed the earth probs
any one know how I can check if its discharging through the alternator ?
Ta

As a time served mechanic

Do a voltage drop test, engine running and with a load on the charge circuit (headlights etc) with a multimeter between the positive terminal on the battery and the B+ terminal on the back of the alternator should be between 0.2 and 0.4.

Also check from the earth pin on the alternator to ground for possible discharge through open circuit.

Also what is the voltage across the battery terminals it should be around 13-14v without the engine running and climb by 1-2v once you start the engine

[AlanG]

All very well informed and in no doubt correct. Putting it simply,when running put a voltmeter across the battery and expect a minimum of 13.5V at just above idle,1200 or so rpm. If so then it's charging. You've ruled out the battery so it's back to what I've previously said. There would seem to be a drain. You can always disconnect the batt and re-connect in the morning to be certain but it's a process of elimination. Do you get a spark when reconnecting? A slight one is to be expected with clocks,alarm systems etc but a noticeable one should raise suspicion. HTH

[Big Pete]

Had a problem on my 96 850R with the battery going flat quick, it was a new battery aswell. Traced it back to the battery ground that connects to the bottom of the front of the engine not being connected! Works like a dream now every time!

[Rufe]

Had the same problem with my last 850 and traced it to a faulty boot light switch.

i had the same thing with my C70. dodgy boot lock unit switching bulb on randomly.

Simon

[AlanG]

It's not uncommon matey,I just switched the bugger off. Almost as though Volvo fore-saw this.........never seen another car with a switch on the boot-light!!

[Shortbusdriver1969]

Hi peeps, I did sort out the earth connection and it was ok for a few weeks, then one day last week, AGAIN went out to the car and it was Flat again, GRRR
I will try the advice given over the week end. but some is mind blowing, the charge while running is fine 13.8 ish give some revs climbs to 14.
all very strange. I dont have a fan that is kicking in so thats a rule out.
might even be the radio, ? as I fitted the CD auto changer and the VOLa what ever the number is my mind blank, some times if I turn the car off with radio playing a CD it stays on a few seconds buzzing a wee bit,
but then goes off its all very freeky :-)