W202 C220 battery charging and drain

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I presume its the heavy cable connection on the alternator that I need for doing the diode test with the cable removed?
 
I've checked the voltage this morning, its 12.56v. Of course there's no load applied but will see what the test at the factors tells me.
 
Any battery approaching 5 years old should be replaced routinely at the slightest hint of trouble. Its easy to do and a relatively cheap elimination of a potential cause of multiple electrical system faults+ if its not the battery at fault you still have the other benefits of a new 100% unit for those cold winter starts come the end of the year. To test the alternator for diode leakage to earth best to disconnect it electrically completely-disconnect battery-----disconnect alternator ---insulate its loose cable connnections[ always live]---reconnect battery and leave over night-- reverse next day and test battery for voltage drop.-remember that main alternator cable is always live if the battery is connected
 
grober said:
To test the alternator for diode leakage to earth best to disconnect it electrically completely-disconnect battery-----disconnect alternator ---insulate its loose cable connnections[ always live]---reconnect battery and leave over night-- reverse next day and test battery for voltage drop.-remember that main alternator cable is always live if the battery is connected
Click to expand...

I've found some videos on YT for alternator diode testing using the multimeter's diode tester. Red lead to the alternator's battery output and black to the alternator casing should be open circuit and reversing the leads should indicate a 500mW - 800mV.

Alternator testing.
 
Cable and battery disconnected of course... 😁👍🏻
 
I would change the battery first, then if it doesn't cure the fault, start checking other things.
You could waste a lot of time checking things, when the battery is the culprit.
My wasn't 5 years old before it needed the battery replacing, so don't let the battery age cloud your judgement.
 
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Muckypaps said:
I would change the battery first, then if it doesn't cure the fault, start checking other things.
You could waste a lot of time checking things, when the battery is the culprit.
My wasn't 5 years old before it needed the battery replacing, so don't let the battery age cloud your judgement.
Click to expand...

Surely modern battery testers used by motor factor reveal all? If the test reveals weakness, then yes, its a new battery.

I checked the battery 30 minutes ago and the voltage is holding at 12.56v.
 
MercSpanner said:
Surely modern battery testers used by motor factor reveal all? If the test reveals weakness, then yes, its a new battery.

I checked the battery 30 minutes ago and the voltage is holding at 12.56v.
Click to expand...
Only way to check a battery is under load, it might read 12.5 volts but may lack current in a load condition, if you reconnect the battery on the car now and try to start it, place your multimeter over the battery terminals then try to start the car.
You may find the car struggles and voltage will drop to 8 to 9 volts, sure sign the battery is dead. If the car now starts you may find the battery is re-energised and will last for a while until we have another cold spell.
 
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Have had the battery tested. Its "BAD & REPLACE" sadly.

I measured the voltage before I took it and it was 12.46v as measured on test so I know my meter is accurate.

670A rated battery measured at 407A.
State of health: 60%
State of charge: 82%
 
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