I have a 2017 diesel Focus with a battery replaced a year ago. Left it for 10 days and came back to a flat (3V) battery. Charged up with smart charger and all seemed OK. A few weeks later I left it for 3 days and came back to a low battery (10V) so car wouldn't start. Charged up again and asked my local garage to have a look. They think it is a faulty diode in the alternator which is about £500 to replace.

When I put an ammeter in series with the battery, I get an initial reading of about 3A which drops over a minute or so and settles down to a steady 0.35A. Does that seem like the sort of current that a faulty alternator might draw?

I would like to pull the power lead off the alternator to prove it one way ir the other, but it is not at all easy to get to.

 

 

34 minutes ago, Timgm said:

. Left it for 10 days and came back to a flat (3V) battery. Charged up with smart charger and all seemed OK.

Once a car battery has been discharged below 8 volts it is usually permanently damaged and will never have the same capacity as it originally had. Why it was discharged down to 3 volts during the 10 day period is something that needs investigating.

37 minutes ago, Timgm said:

I left it for 3 days and came back to a low battery (10V) so car wouldn't start.

That is to be expected due to the previous event, and would confirm that the battery no longer has the capacity it originally had.

39 minutes ago, Timgm said:

They think it is a faulty diode in the alternator which is about £500 to replace.

  A short-circuit diode would cause and 'Battery Charging Light' to be illuminated while the engine was running. With the engine off, the short-circuit diode would be an almost dead short across the battery which would discharge it fully in less than 30 minutes, or until the diode blew completely and became open-circuit. Again there would be a charging light failure on the instrument cluster when the engine was running.

My view would be that the cause of the battery drain is not a result of an alternator issue.

46 minutes ago, Timgm said:

When I put an ammeter in series with the battery, I get an initial reading of about 3A which drops over a minute or so and settles down to a steady 0.35A.

In the first 5 minutes after switching off the engine the current drain can be as high as 17A falling to 3A over 15 to 20 minutes. After the vehicle is parked and locked it can take up to 30 minutes for all modules to shut down and enter sleep mode and the standby drain of about 0.035A (35mA) is established.

It is very difficult to measure the current of the battery when the car is in sleep mode as it is easy to cause it to start and wake up by simply walking near the car with the keyfob in your pocket or opening the bonnet.

 

Many thanks for that reply. I was aware that falling to 3V has probably done serious damage to the battery.

I also felt that a dide fault would be more drastic than the relatively slow discharge I am getting.

That is helpful about the length of time it can take before a car finally goes to sleep. I will see if I can put the ammeter under the car so I can see it after half an hour or more without opening anything and wirhout a key in my pocket.

Eventually I took the car to an auto electrical specialist who (as I had done), took out the fuses one by one to see which one led to a drop in the leakage current. None did. That left the only unfused item which was the alarm sounder. When he had found where the alarm was hidden away, he disconnected it and the leakage current dropped to 10mA. That identified the problem.

I am still concerned that the battery has probably been damaged by the original discharge to 3V, but will keep an eye on the battery voltage and see if the charge is being held.