No (or very low) charging, after battery replacement

EDITED: Extremely slow charging, after I installed an iFixit battery replacement.

Note that the slow or semi-random charging had been present prior to the battery update, and began when the iPad last upgraded to some version of ios 12. (approx. a year of bad charging behavior, where it would jump from 4% to 100 % in a very short time.)

Initially, after the battery upgrade, it appeared to be stone dead - screen NOT lighting up at all. Left it on a charger for a couple of days, still nothing.

Connected it to a Mac MINI, and when I pressed the button, the device came on! but it showed 1 or 2 % power. The Mac MINI said that the device was “drawing too much power”, and I should disconnect it. Put it back on a charger for several days. It climbed to a staggering 4% battery level. Everything appears to work, touch screen, buttons, etc. Except for the sad state of charging.

Any obvious culprits?