This is just an educated guess, but possibly yes, test the leads to the normal charger too, because mine for an example supplies 8.7 not 8.5, and as the charge of the battery goes down the voltage of it does too, so let’s say you supply 8.5v, as soon as the battery goes down the voltage will too and it’ll turn off, BUT, before you supply more voltage, check the rated amps of the batteries you’re using and make sure they supply enough current for what the PS2 requires, that’s the more likely issue here, but yes older models were less efficient and needed more volts even without the disk drive from what I read online