Plugging a car to L1 or L2 charger even HV battery is at 100% charge going to trigger DCto DC charger charging your 12 volt battery if you want to use an external charger 120 v .have to be good quality charger like NOCO 2 genius for AGM battery setting not a just charger for any battery plug positive to red terminal visible in SE and negative to any clean metal / bolt around not to direct 12 v battery terminal this bay pass bbs not trigger any electronics i thing with 69% battery you still be able wake up SE electronics as voltage is 12 .3- 12.4 volts to make a charge of 12 v battery from HV but not press a BRAKE doing this to me the easiest way plug a car to charger L1L2 to charge 12 V battery as those 12 v batteries after 4-5 years need replacement in EVs depends of use or garage control temperature or climate where car is used
A voltage on 69% is in a border to not wake up electronics on SE 12.4 volts Acording to your reading is low don’t wait for long keep that 12 volt battery healthy as you don’t know when your SE hit a road
Just for information as I own bmwy I3 for 6 years driving 4000 miles a year my 12 V battery last 7 years bc was CPO bought 1 1/2 year old used garaged limited driving in winter in NY as 20 Ah AGM 12 V battery last that long as I sold a car with that old battery in perfect condition .Mini Cooper 12 V battery is 50 Ah AGM battery is so big as both cars used the same electronics I’m assuming should last 5-7 years easy as you not aloud to go below 50 % discharge as AMG batteries should never be discharged below 50%