voltage dop alternator to battery

[kully arwand]

please help. my alternator pushes out 14.40 volts( tested off car) but my battery test only shows a 12.20 volts. and my battery warning light stays on very dim you can hardly see, until you switch on heater etc and it gets brighter.but the voltage at battery stays at 12.20.volts. am i loosing 2 volts or is this common. i have checked all the fuses none blown. all advice very welcome, ps my very first thread in 8yrs

[bsdnazz]

Have you checked the 100A fuse near the sump?

If it blows then the alternator won't be able to charge the battery.

[mandrew]

there is a main fuse in on tom the back of the alternater to the battery
its on the chassis and can be seen when your under the car /or some times its at the back of the alternator its a 100 amp

[E Ponym]

Are you comparing like with like?

You measured a good alternator voltage off the car but are you testing the alternator in situ?

The voltage at the battery will be low if there is low charge in the battery, it will rise as the battery gets charged and should peak at about 14.2v.

The charging light is part of the circuit for the alternator, it allows a small current to pass that energises the coils in the alternator so it can start to charge, as the voltage rises the light should go out. If the light is on when stopped but goes out when the alternator turns then that part of the circuit should be OK. There could be a bad earth involved.

[kully arwand]

thanks for the reply's 100 amp fuse is fine,i used a digital battery tester that gives you crank, battery voltage and recharge value, so battery is 12v plus, and recharge is only 12.20, bad earth sounds good any common area to look first, both earths on center console look dry,

[E Ponym]

Test for bad earth would be to use one of a pair of jump leads and connect the battery earth to the engine block (do the RV8s still have the crane loops) - try an alternator mount.

There are usually two wires from the alternator to the battery +. One of these carries all the current, the other is connected to a separate (probably smaller or thinner) terminal on the alternator and is a battery voltage sensor.

What is the battery voltage about 10 mins after you have stopped the engine? If the running voltage is higher than this then it does indicate the battery is taking a charge.

ETA - the usual culprit for a bad earth is the braided earth between the engine and the chassis. This will be tested by the above test.

You could also use the multimeter to check the earthing, instead of using a jump lead connect the meter on the volts range between the battery - and the engine block. If you see more than a couple of millivolts then the earth is suspect.

Edited by E Ponym on Tuesday 12th April 19:59

[kully arwand]

FOUND THE FAULT. SMALL CRACK IN 100 AMP FUSE UNDER THE CAR. REPLACED AND WORKING FINE .. THANKS FOR ALL ADVICE