I wouldn't trust the backup battery meter at all

. Though on little power the unit is able to go into sleep mode and "usually" save all of your data.
Strangest thing ever: While I was waiting for my replacement Axim to arrive I removed the battery from the broken device. I waited 6 days. When the new device arrived I switched both batteries. The Backup battery registerd at 10% power while the normal battery at 30%~. I let the device charged while I went out to buy a new backup battery (its a standard size calculator cell). When I arrived back up home both batteries were at 100%. First device I ever used that could recharge it's backup battery

.
BTW: I replaced the battery regardless.