In a PowerMacintosh 7200/90 I swapped the internal 3.6 Volt Lithium battery for an external 3 x AAA (= 4.5 Volt alkaline) battery case mounted on the backplate of the computer's case. The setup works fine for half a year now, in everyday use in a shop (that is why I chose the external battery case: no one wants to get the computer out of it's corner just to change batteries). The PRAM buffer and clock chip of this specific machine seems not to be picky about it's supply voltage, but no warranty for other computers.