I am using a dry cell battery and a Classic Instruments voltmeter. The gauge is definitely faulty and reading 1v to 1.5v lower when checked against a multimeter. My auto electrician says the gauge does not cause other problems. With the multimeter I check the battery and get around 12.8 volts. On initial start I then checked on the multimeter and it was charging at 14.2 which is OK also. Turn on lights and air con and charge drops to 13 or so volts. When driving with air con & fans on the gauge drops to 11 volts and you can even see it flicker up/down with the turn signals operating. After a run last week with air/fans on I got home,parked and then shortly after needed to move the car and battery flat!! So I guess it was not charging sufficiently and was running on the battery some of the way and not enought charge to keep it up to 13v. With it charging correctly on the multimeter when first started the alternator must be kicking in OK but then or some reason the charge drops off. Could the battery be causing this problem and not accepting charge or something faulty in the wiring/connections. I do recall some months ago working under the car and tightening something and accidentally hit the power wire at the same time the screwdriver was against the chassis and shorted out. Got a good spark off that but not sure if that would damage anything internally. As said earlier still charges correctly at least at start up but something happens paticularly when air/lights/fans turned on.