12v battery replacement

I see the same thing. A short, sharp dip before the 20 minute charge starts, exactly like your graph shows. This does not always happen, just as it does not always happen for you. In addition, I always see a lesser dip, after the charge cycle completes. My voltage drops to 12.71 and it takes about 20 minutes (after the charge cycle completed) for it to come back up to 12.85.
So that is that.

I think I understand how my battery got "fixed". Stratification, where the acid/electrolyte becomes more concentrated at the bottom, happens
when the battery sits for a long time unused. This happened, while my car was at the dealer, waiting 8 months for the HV battery.
The recommended remedy is an "equalizing charge" ie. charging the battery for an hour or 2 at 16V. This is apparently an arrow in the quiver, for a marginal flooded battery, it is worth a shot.
At 16V, electrolysis results in vigorous bubble forming, which mixes up the electrolite. The caveat is to do this in a well-ventillated place, and
for flooded batteries only.

I have unwittingly done this. My shop charger has different AMP settings, at 40 AMPs it charges at around 16V. I charged the Rocket off the vehicle 2x 1 hours. So that is one mystery figured out.

Note, that for AGM batteries this is neither necessary, nor recommended. AGM-s do not suffer from stratification, and excessive gas formation is actually bad for them.

 

Kiwi, how do you capture your battery graphs from BM2?

 

I just do a screenshot and copy.

 

Your smartphone will have a means to capture the screen and save as a photo which you can copy and paste to the forum, just google that question for your particular model.

 

I went ahead and swapped the OEM Rocket battery for an AC Delco AGM. I did the
reset procedure recommended by Kiwi. Initially the BMS varies the charging voltage, it settled down now to 14.67V. It now charges this battery to a bit over 13V, if left sitting, on the usual 4 hour schedule.