Request : Voltage at battery with running engine

Dear readers,
When starting my MGTF, I needed starting cables, the car has not been used for 2 months. So I figured, that is normal, and with starting cables, the car started right away.
After driving for about 10 kilometers, I released the clutch a little too fast at the traffic lights, so the engine stalled.
And would not start, a complete dead battery.
I called the Dutch " wegenwacht" , the AA.
They diagnosed that when the engine is running, the voltage at the battery is 13.1 volt, and with all electric on, 12,5 volt.
According to the AA that is an indication that the alternator has a problem, but I asked to measure the charging current, and that was about 20 amps. The alternator is I think 85 amps and fused at 80 amps, so 20 amps seams a bit low.
My question to you, before I go to the MG garage to diagnose a problem that may not exist: what is the voltage you measure at the battery terminals, with running engine, with lights on and with lights off ?
I can imagine that the long plus wire from alternator to battery, will cause some voltage drop, and also there is no wire from the minus to the alternator, MG used the chassis as conductor, there is only a short black wire from the battery minus pole to the chassis. Apparently the MG design has its faith in a good electrical connection between all body parts.
Thanks a lot, for measuring the voltage at the battery terminals.
Kind regards
Theo

I would expect somewhere in the region of 13.7 - 14.5 volts with the lights off and engine running with a healthy battery and alternator.
Lights on shouldn't make much of a difference, as the alternator should supply enough current to stop the battery draining, maybe a dip but not much more than 0.1 to .5 of a volt.
I would be looking at your battery first before the alternator, if you car runs ok after a jump start, then the alternator is supplying enough to keep the car running, plus trying to charge a dead battery.
One other thing is that if you stall the car and it wont jump or bump start, make sure that the immobiliser is cancelled..it caught me out a couple of weeks ago!

Sundance

Sounds more like a dead battery. 20 amps over 10km should be plenty enough charge to restart a warm engine.

could be you have loosed the engine earth lead this would give you the problems you are encountering. check the engine earth cable,:broon:

Good advice about earth lead. Check: with engine off check Volts at battery terminals then check positive to a good earth other than the earth point. Should be the same, if not, clean and remake earth to chassis. 13.8 V at the terminals with running engine is normal. Quick check to see if battery is goosed: check the voltage whilst cranking the engine: if the arse really falls out of the voltage then it's time for a new one. If you get 13.8 at the terminals then alternator is generally OK, though they can fail under load.

Hope this helps.

I bought a new alternator last year...I measured the voltage at the battery...first picture without the engine running with my metre connected.


And the second picture with the engine running.

looks ok to me, but you could still have a dodgy earth lead when the car is in motion, keep the leads connected and have someone look at the numbers while you mess with the earth cables should show you if there is a jump or decline in voltage

Dear readers
Thanks for your suggestions, helps me in analyzing.
Yhe engine earth lead seems a good check point, where is this mounted to the engine ?
Kind regards
Theo

The engine earth point as I recall is located in the engine bay bulk head . If you stand at the rear of the car with the MG badge facing you , remove the engine grille cover and it is more or less dead center about half way down the bulkhead . You can see it as it is the thicker black heavy duty type cable . I think that you will need a 13mm socket or ring key to fit the bolt . The other end I believe is attached to the gear box but I may be wrong. I did check my earth about two years back and I was surprised at how rusty it looked . There are earth headers all over most cars but the two I would check are the engine bay and the one by the battery .
John .

I got fed up of messing about with voltages and just bought a digital load tester. Takes most of the hassle out of battery testing.

One like this.
Amazon.co.uk link

it will be ok in the end it really can get you wound up, don't let it beat you.:bang: :bang: :bang:

If suspecting earth issue connect a jump lead between the engine/gearbox and the battery negative post (in addition to existing earth straps)

What is the voltage at the battery with the engine off and no load?

What does it drop to when the engine is cranked?