PIAA 540s dimming at idle

[Jethro]

I installed a set of PIAA 540s on the H&B tank guard of my 1200 GS. These are the 55 watt extreme white driving lights. They work quite nicely and I've settled on aiming them onto the same light pattern as the stock light (anything higher and I'm getting flashes from oncoming traffic). These are wired with relay and fuse per the directions with power off the battery terminal. I've noticed, at idle the lights ar slightly dimmer than anything above idle. Is this normal?

[ca1ic0cat]

I've got IPF lamps mounted and when I'm at idle the lamps dim as well. That's a stock GS alternator, seems its efficiency gets low below 1200rpm. I have not had a problem with the battery going flat or the regulator dying though.

[Edd]

The alternator on the 12GS (or any alternator for that matter) doesn't reach full output until it gets above 1800 rpm. Below that you are pulling from the battery.

[marc]

Alternator RPM and engine RPM are not the same. Don't know what engine RPM is required for sufficient alternator output on the GS.

[krusko]

Its normal.
The headlight on my 2005 GS gets dimmed a bit when the engine is on idle, and it dimmes quite a bit when I step on those servo brakes (engine on idle).
Plus if you have your heated grips and some other accessories running, the alternator output is not sufficient to supply all the necessary power, and you will end up draining you battery if the engine is on idle.

This is why BMW changed alternator to a higher output one in the later GS model, I would think.