There are several battery packs used for notebook computers that will work listed on Amazon. Just be sure they have a 12Volt output. EverReady had as battery pack that will also work but you will need an adapter provided by them. The key is that you need an output of 12 to 15 volts to charge the TF or Keyboard. You might read Devcake's post on a DIY charger for the TF as information on the voltages required to charge the TF. There is also a post on external batteries that might help.