[paulgato]

I think I destroyed my alternator today.

I was trying to see if it was possible to stop the alternator putting out power by 'softly grounding the field wire' through a light bulb to de-energise the field winding. This is so I could finally install an alternator ON/OFF switch on the dash. The IGnition wire to the alternator was, as usual, disconnected from the alternator and I connected the alternator end of that wire (the wire coming out of the alternator two-pin plug) to ground, via a 5w light bulb. The bulb lit and the battery voltage dropped to 12.7v, but the bulb did not extinguish, and when I disconnected the bulb from ground again the voltage went right back up to 14.5v.

I then tried a higher wattage bulb. Unfortunately (doh!) the only other bulb I had was a 55w headlight bulb. That didn't light at all, but after that the alternator was putting out power all the time, regardless of whether the IGnition wire was connected to the alternator or not. Even after switching off the engine and restarting, the alternator was putting out power as if the field winding was getting current from somewhere other than the IGnition wire. Also, from that point on, grounding that wire via a 5w bulb did NOT light the bulb any more.

I then found that there was a 'parasitic drain' on the battery of about 800mA with ignition key out. (It was about 25mA when I'd measured it a few months back.) Sure enough, there was also about 800mA travelling down the main alternator output wire with everything supposedly 'off'. So somehow I must have blown/shorted at least one diode in the alternator by dragging more current through it than it could take. I'm not too sure on the exact circuitry inside the alternator so I'm guessing somewhat, but in any case a diode has certainly blown (whether I blew it or not, but I guess I must have done) and if a diode has blown then as well as a parasitic load (and 800mA is MASSIVE) there is also the risk that the alternator could be putting out AC current, which would damage the battery and possibly damage other components too. So in any case I need a new alternator.

Temporarily, I have disconnected all the wires from the altenator except the ground strap (being very, very careful to insulate the main fat output wire, as that runs direct to the battery and I don't think it is fuse-protected in any meaningful way!)

Ah well, I was experimenting, and sometimes experiments go wrong. I did try to learn as much as I could about what I was doing before going ahead with my test, but I didn't really find much online to help me. It's not actually that bad: I replaced a faulty alternator a year or so ago and it was neither expensive (a used alternator from an excellent company called StevensVWspares for £40 delivered) nor was it difficult. I am optimistic that changing it again will be even easier, and no more expensive.

I know I can safely leave the alternator field wire (IGnition wire) disconnected to disable the alternator, or disconnect it and reconnect it via a switch on the dash, because that connection and disconnection is something that happens anyway when you turn the key. I'm not really introducing any novel circuitry there, nor asking any component to experience any voltage or current it doesn't ordinarily experience. But shorting the alternator to ground via that wire is something else entirely. I knew I was taking a chance, ...and I lucked out.

It would have been cool to be able to switch OFF a running alternator, but it seems I can't do that after all. It would have been cool, and that might have opened up the possibility of charging the battery when braking or when slowing down in gear with my foot off the accelerator pedal, but in fact the most important thing is to be able to switch the alternator back ON if battery voltage gets too low, and to be able to do that without having to stop and duck under the bonnet with the engine running to reconnect the IGnition wire, possibly in the dark and in the rain with other traffic zooming by. Once I've got a new alternator in there I'll fit that switch so I can re-enable the alternator while driving along. If I really want to stop the alternator I can always kill the engine for a few seconds and then restart it.

So, not a great day!

However, now I have no alternator - and it will take a few days to get hold of a new one - I am using this slight emergency to prompt me to get the auxilliary 'donor' battery system finally installed in the rear so I have more confidence that I won't be stranded in the next few days with a totally dead car.

One other thing (things have not been going so well lately!) is that the 90Ah AGM starter battery I bought six months ago has been showing signs of losing capacity in the last couple of weeks.

I guess it is POSSIBLE that there was a fault with the alternator before I killed it by experimenting. (Probably not.) That 800mA parasitic drain for example might have been going on for a while, and might have been giving me the impression that the battery was failing. More likely the battery was just not up to the job of 25% - 50% daily discharges. It was very cheap. Still, it came with a 4-year warranty and the seller sounds like he's going to honour the warranty. I may get him to send me a proper deep-cycle-capable battery instead though (to use as a starter battery) and I will pay the difference. I'm thinking of an Odyssey Extreme PC1220 70Ah battery, which can apparently cope with 400 cycles down to 80% DOD, so should laugh in the face of 25% daily discharges, and should last for years. (With a deep cycle 'donor battery' running via the dc/dc converter as well as a deep-cycle battery under the bonnet, neither battery will be discharged more than 25% under my daily driving pattern.)

......