@tomcircuit Actuallay I missed the reason for both the diode and the Zener. I a next version I will remove them. I am also considering moving to USB-C. But still have a number of PCB to use first.

This is very nice, and these old BP-7/8/9 packs are all dead/leaking by now, so a replacement is long overdue. I notice that you include the MAX40203 (Ideal diode, U2) and 5.1V Zener (D2) in the circuit, but I am not sure why. 

In my TI-59 charger circuit the zener diode and MAX40203 were included to take care for the case when the original TI AC adapter was connected with the LiPo pack in place - the peak voltage from the AC adapter could be higher than the maximum rating of the MAX40203 and end up damaging it or, worse, finding a 'sneak path' and over-charging the LiPo.  

In the BP-7/8/9 packs, there is no AC charger connection anymore (the two-pin connector was on the old pack itself, not on the calculator body) so there is no possibility of an external high-voltage input. So, I think that you could safely remove D2 and U2, and jumper across pads 1 and 5 of U2. Your thoughts?

My thoughts are I'm working on exactly this --> Using the original chargers, which are rated anywhere from 5 to 7VAC, requires rectification, smoothing, and a step down to operate a lipo charging IC. Battery (or the smooth input voltage) can be stepped up to 9VDC for the calculator. Quiescent is an issue but not worse than the original NiCad packs.

https://github.com/calcpsu/cptibp8/blob/master/cptibp8-pcb/cptibp8-pcb.pdf