I've been wondering why my battery, which sits at merely 12.2V on a cold morning (4 years old), doesn't give me a problem.
In anticipation of some future problem I did a bit of servicing of the battery top terminals...BatteryTop | Flickr - Photo Sharing!
The yellow fuse, the left circled component in this next picture is the fuse for the glowplugs nuts | Flickr - Photo Sharing!
The two nuts circled on the right are also critical connections. Any corrosion on the terminals which those nuts hold together will give you problems due to the voltage drop created by the corrossion.
The glowplug fuse terminals should have clean terminal faces. Mine were reasonable, but I took the fuse off anyway to clean the terminals Closefuse | Flickr - Photo Sharing!
The lead which supplies the glowplug relay gets power via 4 terminal connections which could all have a slight detrimental resistance. You have the main battery +ve terminal clamp, then 2 nuts holding the distribution plate to the clamp, then a terminal where the distribution plate attaches to one side of the yellow fuse and then the other side of the yellow fuse to the cable which supplies the relay. 4 marginal connections which act like 4 resistors in series
In the end I cleaned all the terminals, and treated them with some Deoxit Deoxit | Flickr - Photo Sharing!
That's expensive stuff and any good quality connection preserver would do. If you don't have any, then just get the terminal faces clean and grease free and then clamp them together. Finally cover the joint with grease or vaseline to keep the contact isolated from the atmosphere.
It might be worth checking these if you're getting glowplug warnings and it might explain why some people seem to have a battery which checks out okay with a tester, but on the other side of a couple of marginal connections the voltage drops when the current goes up.