The BMW Navigator V is a matter of two screws and off comes the cover...30 seconds.
I just called Garmin today and guess what, the batteries are back-ordered and I have to wonder why. Not that I need it till next April.
So when I get the new one, I'll fully charge it and then remove the battery from my Navigator V and just let it sit for a month or two before the riding season starts. That'll tell me if it is self-discharging without being connected to anything, be it the battery itself or the circuit board part of the battery pack that was poorly designed.
Dave, if you followed my post above, Li-Ion batteries last a long time and pretty well maintain their capacity, but then they are single cell packs and easier to control for a long life.
If you have the part # off the battery unit, try to match it from someplace like BatteryMart.com. I've had good luck finding batteries for my other GARMIN units (I have 4 other units, all different models) from them. Good prices, comparatively speaking.