How do you power cycle it ? If you have a working battery this shouldn't be necessary as when you remove all power, the capacitors drain quickly and then the PMU is automatically reset in a matter of seconds. You have a rebuild battery I assume ?
What sad mac code do you get ?
I use my Mac Portables regularly but never leave the main battery installed. However they always power on immediately after inserting the battery and pressing the keyboard.
I have seen Mac Portables from fellow collectors that used the Mac Portable without rebuild battery on a heavy DC PSU and got the logic board damaged this way ( PMU errors, unstable, often not willing to power on,... ).
We also never know what previous owners did with the computers before we got them...