Last year, 237 changed their backup battery EVERY SINGLE MATCH, and I suggest you all do the same. It is the only way to keep the camera running well enough to track and complete our task well last year (and yes we did do it at competition).
Now, if you
read the manual (5.3.5 Electrical System Rules <R53>) ... you will see that it says that you can build a circuit to charge the backup battery off the main battery during the match. We are probably going to try to do this and see how often we have to change our battery.
Backup Battery Charger Schematic (pdf) 1-12-06 (not supported, use at your own risk) < right from the IFI homepage