Backlight not working, advice and/or schematics would be wonderful

I've got 3 Motorolla MC9090 Barcode Scanners that all are not lighting up. Their displays aren't dead, just no backlight. I find it strange for it to happen to 3 of them though (all from the same client). I was reading somewhere that it might have something to do with the backup batteries that pull a charge from the removable charge packs.