My original battery replacement attempts failed too. Battery wouldn’t charge.
Turns out that the first battery may not have been a genuine one, as the seller told me he had the same problem with the battery before he listed it on ebay for sale - as new but not used! suffice to say, surprised and not happy, but he gave me a refund.
I then ordered a new battery (claiming to be genuine and compatible) which it looked to be the case when I opened it (same labelling and markings as the original battery within the SP4).
I attempted to change it the first time, and it didn’t detect the battery!
Then I checked very carefully the seating of the battery onto the SP4 chasses - I made sure I lined up the tiny hole on the battery tab with the anchor on the SP4 chassis (not easy and took me a long time, as the tab needs to be very slightly arched up) and made sure all the contact brushes from the PCB above it were aligned precisely and not bent (I noticed they were bent from the original battery removal, which needed some pulling and pushing of the adhesive under the old battery tab).
Anyway, all good after that …