|Put 26V into a device expecting 20V and you will allow the smoke to escape. When the smoke escapes everything stops working. I would stick with the recommended power supply. |
Unless the battery was on a trickle charge, as soon as it started to run down all sorts of strange things could start happening. Bear in mind also a car battery only shows 14-13 V when not under load. When under load the voltage can drop to below 12V. This is not a problem with cars as car eclectics are not that sensitive to voltage fluctuations. Computers are very sensitive to variations in voltage.
There are adapters that you can get to run electronic devices like sat-navs and mobile phones from a car battery with there own built in voltage regulator but they are usually only rated for 12V. The recommended power supply will also incorporate a voltage regulator.