Should I do it? '14 SL w/36K for $11.5K?

[GerryAZ]

I also turned to car of e then plugged in to charge the car, and a few seconds after I saw voltage go up to 14-15V for about a minute or so then back to rest. So that leads me to believe the car is capable is charging the 12 V battery when plugged in, though when and how often I don’t know. I was just making sure the car was even charging the battery.

The issue is when you leave the car plugged in but not charging for a long time, either because charging has finished or because a timer has been set that won't allow charging to start for many hours. Also, the charge delivered to the 12 volt battery while the main one charges is enough to maintain the existing charge of the 12 volt battery, but is not sufficient to recharge it.

To correct some misinformation: The car charges the 12V battery while the traction battery is charging at the same rate as when the car is being driven. When the traction battery is not actually charging, the DC-DC converter is not active so the 12V battery will slowly discharge. There is more residual load on the 12V battery when the car is plugged in and not charging than when unplugged. This means the 12V battery will be fully charged when the traction battery finishes charging, but will slowly discharge if the car is not unplugged. Also, the car will periodically turn on the DC-DC converter to charge the 12V battery from the traction battery when parked for extended time if it is not plugged in. The DC-DC converter output is about 14 to 15 volts until the current flowing into the 12V battery drops below a threshold; then the output drops to float charge at about 13 volts.

 

[IAMTHESTIG]

Just thought I'd follow up after having the car almost 2.5 years now, 52K miles on it. Not a single problem with it, drive it daily. Our routine hasn't changed much, so its still been a great runner. Electricity is still $0.08 or $0.12 per kW/h so its only costing me about $15-20 a month to run. Battery degradation hit hard after I first got it but has been pretty minimal since, it appears. Battery was 84% SoH when I bought it but dropped down to ~82% a few months after. It's around 80% SoH now, seemingly dropping about 0.5% a year... LeafSpy always sees the SoH higher in the cold winter months then it drops about 2% in the hot summer, only to go back up about 2% again in the winter.

I've been taking care of the battery by trying to keep SoC between 50% and 70% when its sitting at home, so sometimes that means charging it for a few hours then unplugging it and plugging it back in to have the timer charge it in the early morning, or just charging it the next day when I get up so it is ready to go when I am. Sure its annoying to have to go unplug/replug but I think it is helping the battery last longer. It doesn't sit with 80% or higher for any longer than an hour or two really, and that is only in the morning and pretty much only in the cold winter months. I find keeping it always below 80% and even 70% in the summer provides more than enough range for our needs.

The only thing at this point I wish I had done differently was install a 240V power for it, as I'm still doing 120V so it takes a long time and maybe got a cheaper one. Though I could still do this and maybe will, it just hasn't been a frequent need to want to have fast charging. I owe just under $6K on the loan but came into some extra cash so I think i'll just pay the loan off so I don't have to have a car payment anymore. Yay. No regrets on the car though... We don't drive our Mazda 3 very often but maybe a couple times a month - which is especially great now with gas prices being $5/gal.

Anyway thanks to everyone for the advice, it has served me well. I'm happy with the car and imagine this car should easily last us another five years or probably longer. Only real fear is one or more cells crapping out, especially now that it is out of warranty.