It would be FAR wiser to buy a battery powered docking station. I bought one that plugs into the wall but can run on batteries. Did you know you can easily fry electronics if you get the voltage wrong?! As batteries lose power they lose voltage. If your volts start to slip the amps increase to maintain power. The increased amp tend to burn out electronics. Battery powered devices are made to tolerate the amperage increase, can your speakers?

If you must, I bought a car jumper battery that you can plug those type of devices into it. It is heavy and expensive. It has a massive battery reserve and has a power readout so you can turn it off when the power get below 70%.