It sounds as if the prosecution has a case, but not a formidable case. If the system battery is good, the PB should start up on battery alone. The PB should be capable of startup using the AC adapter alone. Do you know, by measurement, that your new battery is charging? The backup will not charge before the system battery reaches full terminal voltage, and takes anything up to another 24hr when it has been long out-of-use. What terminal voltage does the backup have?
During the currency of the 150 it is probable that a failure of the backup battery would have been fixed by total replacement of the interconnect board. That is certainly true of other members of the PB 1xx family, the PB 100 proper excepted. You are fortunate to have such a replacement, but you will be wise to measure the terminal voltages of the in-circuit and replacement batteries before you make the effort to replace the interconnect board. Apple's TI doc. for the 150 does indeed specifiy '60 mAh rechargeable nickel-metal-hydride battery' for the backup battery.
One important caveat is that the 150 uses a
low-power AC adapter (17W). Be sure that you do not already have MLB damage from a higher-powered adapter.
None out of my Apple Service Sources vol. I-III mentions the 150 directly, probably because it was too new (1994), but it is reputedly based on the Duo 230 architecture. However, the physical layout is still that of a PB 1xx series. They are not difficult to work with, the principal awkwardness being to disconnect the ribbon (video) cable that joins top and bottom case halves as you
separate them.
When you have the batteries to your satisfaction, remember to reset the power management. It is worthwhile to see whether
this will work before you begin the exercise above.
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