Havent got one of those, sorry. But have a battery intelligent charger that will say if the battery is knackered, when i charge it, what do you charge it with? If both batteries are gone at same time id be tempted to say fault with the backpack. But unless you have a spare backpack bit difficult to say.
Anyone know of common backpack electrical failure problems with backpacks?
Oh, pole2pole, you never mentioned how old the inidividual components are, etc. more info the better.