Battery flattens over night no charge light.

[Simon B]

Hello,

I've got a problem with my2.8 Isuzu convrted 110, the battery flattens itself totally overnight - goes down to 1volt so the dashlights dont even illuminate. There are other symptoms;

• Battery charging light had been taking longer than normal to go out
• Battery charging light now does not illuminate, but will if I short the wire to earth - so bulb circuit ok
• With a new battery engine starts ok voltmeter shows a good charging voltage that goes up and down dependant on load.
• I may have run the engine for a short time with the sensor wire disconnected, it had pulled out during some other work.

I suspect either diodes or voltage regulator, it's a hitachi alternator and they seem very expensive so did not want to rush in to changing it if there's an easy fix.

[western]

I suspect your point 4 may have fried a diode or 2 in the rectifier, might be worth having the alternator bench tested by a auto electric place.

[Simon B]

Hello Western,

How does the battery flatten? Through the sensor wire or the connection to the main lead.

[Ed Poore]

It might flatten through the sensor wire. Fundamentally this provides the excitation current required by the alternator in order for it to work. I doubt disconnecting it and running the engine would have had any effect on it.

When you turn on the ignition the battery is connected through the charge indicator bulb to the field coils of the alternator. These are just wire coils with one side earthed (usually) so without the engine turning these just provide a path to ground hence the bulb illuminates. Once the engine starts the alternator starts turning and then the electro-magnetic effects kick in and the current flowing through the field coil is "amplified" by the alternator (the mechanical rotation of the engine converts mechanical energy into electrical in the alternator).

A functioning alternator should be chucking out 14.4V in order to charge the battery properly hence you should check for that. If the charge light takes some time to extinguish after starting the engine then it's reasonably likely that something within the alternator has shorted out (diodes may have fried etc.) with a multi-meter you could open up the alternator and measure impedances if you wanted to help isolate the problem - sometimes it can be fixed reasonably easily sometimes it's not worth the hassle. I had a situation where wet mud had managed to arc-weld some diodes together, that wasn't really worth fixing (could have been done) but for £40 a new alternator arrived.

[Simon B]

New alternator ordered here;

Milner

Hope that sorts it. Then MOT time! (again)

[Simon B]

Fitted today, light on with ignition light off when engine running. 14.88 volts at battery terminals. Glad that's sorted.

[Simon B]

New alternator fired again, annoyed! Might look at trying to repair it this time and finding out what is shorting it out.

[Simon B]

Minor update, put an ameter in series and found the sensor wire drawing 4amps. The main battery cable (without sensor wire attached) draws no current. For now, I'm fitting a relay to switch off the sensor wire when the igniton is off. Dont want to buy a new alternator just yet so will look at how much a voltage regulator is.

Simon.