If it's PS1 they run on the same 9V style battery as the early 5V PS2 boards. Make no mistake, it's made in 1998 which is easily 2 years before any PS2 equipment hit the market.
A simple battery test for a PS1 is to power it up but leave it in "reset" mode (don't hit the direction button to move it). let it idle like this for a few minutes at 12 or more volts and then shut off the power. If the engine completes the 7 to 10 engine shutdown sound cycle and you hear the faint "popping" noises afterwards, the battery is charged and the engine is ready to run.