Dissconnecting battery when parked for over a week?

[PINBALL]

After returning from France for 2 weeks, found my 94 / 124 / E280 estate to have a completely flattened battery.

Luckily I had GEM homestart cover, & the nice man put his clever box of tricks on battery & it burst into life on just the second turn of the engine. He then flicked a switch & an array of led's lit up brightly to show nothing was amiss & car was charging fine.

Nice to know, as I often wodered if everything was ok on the charging circuits of the car.

Asked him why 6 month's old battery was so dead & he said nothing to worry about, as most mercs drain down after a week or so, and the best way to stop it from happening again was to loosen n slip off battery terminal negative side & re-connect on returning !

But what happens if you disconnect the wrong side - I wonder?

Is he right?

Any thoughts/advice out there please.
Regards,
Keith.

 

[jberks]

Yes, disconnecting either side (doesn't really matter but -ve has less wires on it) would stop the drain.
However, I'd question the 1 week business. I've left my various Mercs for a 2-3 weeks at a time and they've always started first flick of the key, there is a problem if they don't. True the alarm, clock etc draw a little current when parked for a week or so, but not enough to kill a fully charged healthy battery. You should be fine parking up for a month, never mind a week.
I'd find out how many amps are drawn when parked and work it out against the AH rating of the battery. Someone should be able to tell you what the draw should be and hence if you have a problem.
I'd also say that if the battery and charging system are fine and the battery was fully charged when it was parked, its possible that your starter is past it's best and drawing more current that it should.
I changed a battery on a car a few years ago. Jumped in, flicked the key and promptly blew the starter up!

 

[television]

One always disconnects the - negative terminal first as a safety issue
should the spanner touch any grounded object,no harm, If the + positive terminal were shorted to ground the battery could explode.

Malcolm

 

[Rory]

I have heard of this before on MB's that are not fitted with heavy duty batteries - I don't know if they're an option or standard on some models, but mine (C270 estate) is listed as having one.
Don't some of the saloons have 2 batteries, one under the rear seat? I guess this isn't possible in the estate, so hence they'd need a bigger battery.

I often leave mine for 2 wks and it starts fine.

 

[jberks]

One always disconnects the - negative terminal first as a safety issue
should the spanner touch any grounded object,no harm, If the + positive terminal were shorted to ground the battery could explode.

Malcolm

The risk is extremely remote, more likely you'll get a quick spark to remind you not top be so sloppy with your ratchet, but as it exists and there's no advantage to ignoring it, I fully agree with your advice.
My point was that it makes no difference to the car or electronics. To be fair, I always do disconnect the -ve first too but more through habit than anything else.
I have read 'high capacity battery' on various MB specs, but then 'high' is a subjective measure. Most MB's have one battery, located in various places, sometimes under the rear seat, sometimes under the boot floor etc. I presume moved around primarily for weight distribution purposes. I am told the W211 has an additional smaller battery under the bonnet too, solely used as a backup for the SBS braking system but as its hidden under a cover I haven't seen it so I may be wrong.
As I said earier, these problems only seem to affect older cars so I suspect it has more to do with ageing components than a generic issue. corroded earths, weak ignition and tired starters all play their part and hence need a bit more juice to get them going, especially when cold and damp.

 

[television]

The risk is extremely remote, more likely you'll get a quick spark to remind you not top be so sloppy with your ratchet, but as it exists and there's no advantage to ignoring it, I fully agree with your advice.
.

This is not my advice, it is a standard safety issue, followed by good battery fitters and garages. I have seen batteries that have exploded, or I should say the remains of one.
Every car handbook now warns of risk, you cant start the battery section of WIS without reading this warning. With any one not very used to hand tools, the spanner can weld itself on to the terminals and body. this is how an arc welder works.When you are working at the other end of the car, ie starter, that will give a spark, only because of the resistance in the lenth of the stater cable.
A spanner to ground has no resistance and a 60 amp battery would have 60 amps flowing through it

Malcolm

 

[malcolm E53 AMG]

I bought a couple of battery isolation devices a few years ago which clamp onto the negative terminal of the battery.

They are made up of two cast pieces, one of which is clamped to the battery neg terminal and the earth lead to the chassis is clamped to the other half. The battery only becomes earthed when a captive thumbscrew is used to tighten the two halves together.

It works a treat and I still use it on a car I don't run very often.

If you do an internet search I'm sure that there will be something similar available.

 

[anyweb]

it might be an idea to check out if you have any device that is consuming the power of your battery so fast,

i had a similar issue a few years ago which i couldnt understand until i retraced my steps,

my battery was going stone dead in 3 days of non use. Turns out i had removed the boot lid bulb for maintenance but when i refitted the wiring, i connected one end to the 'always on' connection meaning as soon as i closed the boot, the light was still on (hence the drain).

I confirmed this by removing the back seat and sure enough, it was on when closed.

simple fix to a funny problem

just a thought!

cheers
anyweb

 

[Stevie A]

Always disconnect the negative (usually black cable which is earthed to car) side. Don't muck about with the live red side for reasons mentioned above.

A battery running down after a week could be a bad connection somewhere which leaks juice out the battery over a period of time hence draining battery. Maybe an alarm or imobiliser could drain the juice too.

 

[PINBALL]

Many thanks for all those tips!
Must admit that when power was connected to battery, even before starting car, the alarm sounded even though I never activated it & only locked it by the key before leaving for 2 weeks.
Was told that alarm is so good that it detects any unusual power surge & sounds off accordingly.
Luckily I had the remote to hand and could switch it off quickly.

Cheers to all,
Keith.