Wombat
Donation Time
The important thing to remember is that all power connections to lights etc should be on the Alternator (or generator) side of the ammeter if you want the ammeter to show what is going into or coming from the battery. Tom H clearly describes how the ammeter should work is wired as Rootes intended.
Ed, in your case, I am almost certain the lights are connected to the battery side of the ammeter, so with the engine stopped and the lights on, the current comes straight from the battery and does not go through the ammeter. With the engine running the current for the lights comes from the alternator, through the ammeter and registers as charging although the lights take most of it before it gets to the battery. I wired up a car so it behaved the same way, then realised the ammeter wasn't telling me what I wanted it to, so changed the wiring.
Ed, in your case, I am almost certain the lights are connected to the battery side of the ammeter, so with the engine stopped and the lights on, the current comes straight from the battery and does not go through the ammeter. With the engine running the current for the lights comes from the alternator, through the ammeter and registers as charging although the lights take most of it before it gets to the battery. I wired up a car so it behaved the same way, then realised the ammeter wasn't telling me what I wanted it to, so changed the wiring.