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W202 C220 battery charging and drain

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MercSpanner

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Nov 5, 2018
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Location
Co Durham
Car
C220
Probably not the right category but I couldn't see one for electrical systems.

Anyway, its a 1994 W202 petrol and there's 14.2v across the battery with the engine running so a bit low.

I charged the battery running the engine yesterday and today, the battery was totally dead, completely drained.

Any pointers on what I should investigate would be much appreciated.

thanks
 
How old is your battery ?
14.2v is ok when your car is running.
Have your battery checked properly by a specialist or leave it on charge overnight, let it stand, then check the voltage next day without it connected to your car, if it is below 11.5v then it will have a bad cell.
 
You should see 14.1-14.2v when the engine is idling and 14.5-14.6 when revving. This is the alternator output (via the voltage regulator), not the battery.

The battery voltage should be checked with the ignition switched-off. You should get 12.6-12.7v (fully charged) after a long drive. In the morning it should still be 12v or higher.

If you are measuring via the poles in the engine bay (i.e. not directly on the battery terminals), the measured figure could be up to 0.2v lower due to resistance.
 
Thanks for posting!

I believe the battery is no more than 5 years old (my uncle's car).

I've taken the battery off and its now on charge. Its an electronic charger and shuts off when it detects that battery is fully charged.

When/if its fully charged, I'll let it stand for a couple of days and if its kept a full charge, will take it to a local motor shop for testing.

A battery with a faulty cell should still have some voltage across the poles. All the dash electrics were dead so it seems there is a drain.
 
The Electronics & Audio is a more appropriate section. I’ll ask the mods to move this.
 
MercSpanner said:
Anyway, its a 1994 W202 petrol and there's 14.2v across the battery with the engine running so a bit low.
Click to expand...

Possibly a touch low but there are a couple of variables to be aware of.

A charging voltage check should ideally be done with the battery already fully charged as if it's not, a heavy charging current will drag the voltage down a few tenths.

The voltage regulator may have a built in negative temperature coefficient to compensate for the batteries internal resistance falling with increase in temperature. The optimum safe charging voltage being lower as temperature rises.

You will therefore see the highest charging voltage when the battery is fully charged and cold.
 
MercSpanner said:
A battery with a faulty cell should still have some voltage across the poles. All the dash electrics were dead so it seems there is a drain
Click to expand...
What. Completely zero measured with a multimeter ?
If it is down so low it probably won't recover.
There could be lots of things causing a drain, interior lights left on, an old alarm or radio etc.
But I suspect a dead battery, lots of cars haven't been used very much over the last 12 months and batteries have been the first to pack up and die.
 
Yes, completely flat according to the mulimeter and no dash panel lights on when I turned the key.

When I used the jump leads to get it started, I let it tick over for an hour, stopped it and the battery was able to crank and start the engine. I then took it on the dual carriage way for a good run. This all resulted in getting the voltage up to 11.8v.

The battery's been off the car and on charge sine 4pm yesterday and its still charging now at 9.30am.

This is the charger I have.
 
People seem to think they may have something draining the battery, when it is simply a knackered battery.
The car is clearing charging the battery, but it's not holding the charge.
 
Tayna sell on ebay and they have an Exide EA640 for £63. The Oldham battery that's on charge is a 650A.

I part matched on the Exide website.
 
I have lost track on how many batteries I have bought from Tayna , car's , van , motorcycles , mobility scooter, kids electric motorbike....

Never had a problem with their products or service.

I do not work for them :)
 
Have checked the charging progress.

Disconnected the charger and there's 12.9v across the terminals but the charger has not indicated the battery is fully charged yet so the charger is back on.
 
Edited following 190's post below.
 
Last edited:
MercSpanner said:
Disconnected the charger and there's 12.9v across the terminals but the charger has not indicated the battery is fully charged yet so the charger is back on.
Click to expand...

It's perfectly normal for a battery to read a voltage as high as 12.9 volts and still not be fully charged.

Due to the surface charge effect, a battery will read abnormally high when it's first taken off charge. To use a voltage measurement as a reliable indicator of state of charge the battery needs a rest period of several hours or preferably overnight. Only then will you get a true voltage reading.
 
I intend to let it charge till 4pm today so its had 24 hours. I'll stop charging then whether its full charged or not according to the charger and let it stand till Monday and see what the voltage is. If the charge has held, its a trip to the local car shop for a battery test.
 
The charger is now Green light so the battery it seems is now fully charged after just under 22 hours of charging.

12.73v is showing across the poles. Will let it stand till Monday then off the car shop for a test.

Noted on the rectifier diode pack. Is the alternator on a W202 C220 petrol (111.961 engine) an easy get to?
 
while the voltage regulator/ brush assembly is field replaceable the diodes are not- if its part of the diode assembly thats the fault then a reconned/ exchange alternator is your best bet


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