Hi Shaun,
The charging voltage doesn't seem to change much, hot or cold it wont go over 14v but I will re-check this. I have done voltage drop from alt output to battery pos which is only about 100mv I think, but not from the alternator frame to neg. I will do this next. At one point I did feel that the battery pos lead was getting a bit hot near to the terminal but next time I checked it seemed ok. Perhaps I will change that anyway.
James,
Thanks for the suggestion. This car is normally left unlocked so I am not sure whether the siren will cause this problem or not, or indeed whether the car being unlocked is relevant? I will disconnect it anyway as they obviously dont need it, so anything has got to be worth a try. One advantage of this vehicle is that they are neighbours so I only have to walk across the road with my jump pack. The disadvantage is that they are pensioners and have absolutely no idea what they are doing at any time. The first time I connected my jump pack the hazzards started flashing which led me to think that was the cause, but he must have pushed the switch while trying to get it started even though it is in front of the gear stick. They also park it front first into a very tight garage even though I have suggested reversing it in.
Regards,
Bryan.