[Mgs] Battery drain

PaulHunt73 paulhunt73 at virginmedia.com
Thu Jul 18 01:27:20 MDT 2019

That's a significant drain, even with alarm, radio and clock it should last for 2-3 weeks.

Your battery voltage off-car seems OK, but that is not a good indication of the amount of charge i.e. cranking power.  If you put it back on the car after three days and it cranked OK that does indicate the battery itself is OK - up to a point.

However the Lucas Fault Diagnosis Manual states that if a battery becomes 25% discharged then the on-board charging systems will never get it back to full capacity, and if it has become completely discharged they will never put back more than 50% capacity.  For that reason a flattened battery has to be boost charged for a period to restore full capacity.  Using a conventional charger this means charging at 16-17v for a couple of hours in a well ventilated space.  Or you can use one of the more sophisticated 'maintenance' chargers but only if they have a 'recovery' program, and not all do.  These charge using pulses of up to 20v.

But back to the drain.  If you have an analogue meter switch to its 12v i.e. VOLTS range and connect it in place of the battery earth lead.  With no drains on a car with an alternator you should see a few volts registered.  If it shows 12v there is a drain.  In that case the first thing to do is unplug the alternator, and if the drain drops to zero that is the culprit.  With the few volts registered unplugging the alternator should also drop it to zero.  But if still at 12v start disconnecting things like your radio, the purple circuit fuse, remove browns from things like the ignition switch, lighting switch, starter relay, hazard fuse and so on, looking at the meter each time.  If still there remove the browns from the starter solenoid, and finally the battery cable from the solenoid, but normally it will have been discovered by now.

You can also do this with a digital meter but they are far more sensitive and will show 12v even with just the normal alternator leakage, and may not fully drop to zero with that disconnected if there is dampness anywhere.

  ----- Original Message ----- 

  After leaving my MGB GT V8 for a few days - maybe 4 or 5  I find the battery is completely flat.  Did that 3 times.  So I took the battery out, charged it and waited 3 days.  The voltage dropped from 13.15V on Sunday 3pm to 12.91V 3pm Wednesday which does not seem to indicate that it is the battery's problem?
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