For starters a reminder to the Newbies this thread relates to the original Soul so some of the following is that model specific.
We are currently getting around 10-14 days (depends on the temperature) out of the weedy little 12V battery before the systems relatively high parasitic loads have discharged it to the point the car won't unlock properly or boot up into drive mode, i.e. all the symptoms described previously.
Before I continue it is worth mentioning a few things:
SOC = State of Charge
DOD = Depth of Discharge.
OBC = on board charger (HV circuit)
DC-DC converter = pseudo alternator (12v circuit)
Lead acid batteries take many hours to fully charge especially if the bulk voltage is too low and in my car the output from the DC-DC converter at 14.3V is low 14.6v or even at these temperatures, 15V would be better.
FYI
The higher voltage increases the amount of time that a high level of current can be absorbed before the batteries internal voltage equals that of the charger and which point current flow rapidly drops A decent multi stage battery will raise the SOC to around 80% at this stage but feed the battery a lower voltage and it will be less often around 60-70% Soc before the current tails off.
I suspect the DC-DC converter is simply an Alternator emulator rather than a true multistage charger which is a shame.
Lead acid batteries have reduced capacity as they get colder:
Lead acid batteries when new are rated at 100% capacity at 25C.
At 0C they are typically only good for 80% at -20C around 50-60%
Emphasis on the above is when new!
All batteries degrade slowly over time through use, this can be reduced by being permanently on float charge when not in use and by taking as little as poss. out when being used.
Lead acid batteries last for years if only discharged by 5-10% between full recharge cycles, hence why your average car battery lasts ages, cranking an engine takes very little out of it.
Put any battery through a prolonged DOD and leave it like that for a few days, (which is typical of what is happening in this situation) and permanent damage due to sulphation begins to occur.
A sulphated battery never regains 100% capacity. It is a vicious circle the more deeply and more often a lead acid battery is heavily discharged the shorter its overall life will be.
Due to the constant gradual loss of capacity over time the constant discharge from the parasitic loads means each time there will be a proportionally slightly greater DOD hence it can appear to have gone flat sooner the next time, maybe by only a few hours a time but slowly that becomes such a pita it has to be replaced. Our battery may be approaching that point now.
We know that in the accessory mode the 12v DC-DC converter does not operate.
I thought I would therefore just double check that when the OBC is connected to the EVSE charger and charging that the 12v battery is also being charged by the 12v DC-DC converter
The good news is yes it does, the bad news is potentially that charge window will not for long enough.
Yesterday I connected one of my BM2 monitors (I do electrical installs on camper vans and use lots of these) to the Soul
I had the timer set for 00:30 to charge the car to 80%.
That can be seen on the blip in the graph, as the 12v battery drops slightly, but because the car was already at 80% obviously nothing charging wise happened.
This morning I jumped in around 09:30 and to force a charge I set it to charge the traction battery to 100% starting at 11:00AM.
You can see that whilst charging the traction battery it also started charging the 12v battery until I stopped the charge at 12:45.
During lockdown with significantly reduced lack of use this has amplified the problem:
Depending on the state of the traction battery it is pretty obvious a top up charge of the traction battery to 80% or 100% or whatever may not be long enough to charge the 12v battery.
an hour or two will give it a boost but that is not good enough if the battery still needs another few hours to get to 100%
I have couple of charging options.
I have a Solar panel on the garage connected to a 12V Mppt controller and I have a Ctek MXS 5.0 that I use for maintaining various spare batteries.
The Soul will be added to that cycle, probably weekly.
I am also going to investigate fitting a 12v socket alongside the mains charging connectors so I can easily connect it.
HTH.