Someone just asked the same question on the 650 single forum ----------.
But , yes, they are good, and no need to go for the large BMW one , the smaller BMW 51914 ? starts my hi comp 1000 cc G/S no bother at all, and the --20-- size does the same for the GS and my 1000cc 75/7.
Oldest is 3 1/2 years old and still performs perfectly.
When the time comes to buy a battery for my R100GS-PD rebuild, it'll probably be the Motobatt MBTX20U, £66 on eBay from Buskers. Height 155mm (175mm with spacer), length 175mm. width 87mm.
I bought a Motobatt battery for my GS PD. Good battery but I think the cable attachments are a bit flimsy. I've just bought an Odessy battery for my HPN but its not as wide as the Motobatt but has better cranking power and is a dry cell. It would flop about a bit in the GS battery box unless you pack it out. Model number was a PC 680. Lighter as well.
Cheers Paul ,I went out to start the bike today after a ride yesterday and the battery would not start the bike so I bit the bullet and opted for the Large motoblatt £79 free delivery,its the one with the same dimensions as the tray, so hopefully it will do the Job .