You must have an internal battery that can a charge at least 1% in order for the external battery to be used. If the internal battery is dead, your external battery will not be used. Again, the internal battery does not need to work well, just work a little.
You didn't mention the model you have but if it's a 366 there is an issue where sometimes it does not know there is an external battery is connected and then uses the internal battery only. The simple fix if this happens is to do a soft reset on the tablet. With the tablet OFF (not standby or hibernate) - press and hold the power button. The LED will light that it's starting to turn on but keep holding the power button and then the tablet goes back off. All together you will hold 5-8 seconds. This will perform a soft reset and if the external battery is connected properly, it will be recognized. Of course this doesn't help if your internal battery is dead.
Another common problem with external batteries on the 366, 375 and 368 models is the contacts get oxidized either from humidity or from touching them with our fingers or other environmental issues. Use a pink pencil eraser to clean the contacts on the back of the tablet and carefully clean the battery contacts. Be careful you don't short out the contacts with the brass part that holds the eraser to the pencil.
Good luck, -Dave