A known weakness of the R1200GS is its AGM (absorbed glass mat) battery. After just one year of service that probably included spans of neglect, the original item in my bike stranded us one cold and forgettable night. It was replaced under warranty, but the BMW replacement battery also died, after two years, and this despite my trying to keep it well charged and happy.
These high-compression engines are almost impossible to bump-start too. They're unbelievable!
More cranking power and much longer life
Rather than risk more tears, I forked out for a real battery, in the form of an Odyssey PC535. This unit is not an exact replacement, so fitting it does require a few very small modifications. A good tutorial on doing so can be found here. My bike is a 2005 model, so some of the below might not translate to newer steeds.
I followed much of the advice in the above tutorial, however I found that re-orienting the Odyssey with the terminals towards the back of the bike seemed to work better for me. I had no need to route either of the cables across the top of the battery in this position. Nor did I need to trim the outside fins and thus void the warranty. I did cut a small notch out of one fin where the wiring loom passes it, but you could get by without doing that. I couldn't find any other clearance problems that caused me concern. My positive battery lead now loops around and back over the frame to approach the battery at a workable angle. Its terminal lug required virtually no bending to fit.
The negative lug was a different story. It needed to be twisted so that it approaches the terminal at 90 degrees compared to its natural direction. This was achieved with a pair of pliers little by little until it lined up. The lug is made of tinned copper, so it's soft and easy to twist.
You could avoid having to twist the negative lug at all by leaving the copper plate in place on the negative terminal instead of removing it, as directed by the above tutorial (these batteries come with easily removable terminal extender plates when sold). See feedback below for an example of this method. A small advantage of not using the plate is that it means one less physical junction for the current to bridge, and therefore slightly less voltage drop in the circuit.
The retaining bracket
The tutorial mentioned above recommends hammering out your stainless OEM bracket and re-bending it, which I couldn't bring myself to do. It's a nice piece of sculpture, and I prefer to keep it on hand in case I ever want to return to a standard-sized battery for whatever reason. Bending a new bracket is an easy job if you have a piece of metal strap and a drill handy.
I used a small length of used steel packing strap, of the sort that you see securing crates for shipping, etc. It's 3/4 inch (19mm) wide, thin and deliberately bendable. When I'd finished tweaking and drilling my custom bracket, I sprayed it with a bit of rust converter, then painted it with rubber paint to protect it from road muck. I whacked a bit of thin foam under the top section for padding too, which wasn't really necessary.
You only need to drill one hole into the bracket to line it up with the small bolt immediately above the battery, where the standard bracket attaches. In my case, I extended that section of the bracket to reach up to where the owner's manual clip would be, and drilled a second hole to suit (grey arrow in pic). This allowed me to get rid of the owner's manual clip altogether while using the battery strap and upper bolt as a securing point for tank bag webbing. You will probably have no such need, so just shorten the upper segment appropriately—so that it's more like the BMW item—and drill a single hole.
The Odyssey lump weighs 5.5 kilos versus 5.0 for the OEM unit, so you've picked up a pound of lead with this mod. But that 10% weight penalty brings vastly more performance and peace of mind, so in the greater scheme of things, I reckon it's well worth it. This battery cranks the engine much more vigorously. The difference is immediately noticeable. I've heard of some GS owners shoehorning the larger (taller) Odyssey into their bikes but believe me, for normal use there's absolutely no need. The PC535 is plenty.
To keep these batteries performing at their best and lasting as long as they are capable of (up to eight years, claimed), you do need to keep them charged up. This doesn't necessarily mean buying a charger, unless you do lots of small trips that are liable to deplete it. However, a digital multimeter is worthwhile, so you can check the effect of your usage pattern upon battery voltage. Odyssey recommend keeping the battery at 12.7 volts or above, though 12.6 is adequate according to the bloke who sold it to me. This avoids sulfation of the plates. My initial experience is that 12.7 is quite easy to maintain with normal riding.
Should you need to use a charger, do follow the guidelines of the manual that comes with the battery, and use a charger that provides appropriate voltages. I have mine hooked up (via a custom external connector) to a small solar panel, which is awkwardly positioned outside, hanging vertically and facing a tree. It gets a few of hours of angled sun at the most, and provides just enough of a trickle to keep the battery topped up, but nowhere near enough to charge it.
Update 2013: My battery is now 5 years old and it seems to be more difficult to keep it above 12.4 or 12.5 volts. Maybe it's just because I'm writing this in the middle of winter with the bike getting little use and the solar charger getting almost no direct light. No doubt the colder battery is less happy chemically too. Still heaps of cranking power, though.
Update late 2015: Another three years plus, and the Odyssey is still going strong. I haven't ridden the bike for two weeks, and I've just measured 12.71 volts on the battery as tended by the solar charger. It's early summer, though, so this supports my theory about the seasons affecting battery health.
I assume the clock, and possibly the immobiliser, draw current even when the ignition is switched off. But without this drain, the Odyssey will hold its charge for ages, even in winter, without self-depleting. So in theory you could just isolate the battery if you knew you weren't going to be riding for months, and it'd be good to go without any tending at all. The one downside of this practice is that you will lose your clock and trip-meter settings whenever you disconnect the battery.
Feedback, 2015: 'Thanks for your details on getting this battery in there. I was also able to install the battery backwards and I too trimmed only one notch in two fins to make sure there was ample clearance for wiring harness. I ended up slightly reforming the positive spade so it reached a little deeper and then ran the negative cable underneath it. There was no pressure on the two cables in this configuration. I also left the copper plate on the negative side so that I did not have to make that spade do unnatural acts and it also aligned the negative cable to pass under the positive cable. There was just enough clearance between the frame and the bolt head for it not to rub. Although to remove the negative terminal in the future, I will need to pry the battery up off the shelf to gain ample access to the bolt. Neat and tidy and the new battery really cranks. Appreciate the assistance.'