Charging the 12v battery using the HV battery?

  • So i Have what I believe to be a 12v battery in my 2014 EV nearing the end of its life. It is four years old and I was hoping for another year or so, but knowing the issues of a 12v battery going bad and leaving one with a brick, I plan to replace that battery in the next couple of weeks. In the meantime I've got a question. I can certainly install a battery charger, but I'm trying something else. The 12v battery has been down to 11.7v after sitting for a few days. After driving for a bit or after charging the HV battery, the 12v battery goes up to 12.6v. Then after one day it is down to 12.3v and after a second day it is down to 12.1v. It was suggested to me to just turn the car "ON" for an hour of two and this will charge the 12v battery. I have tried that and it seems to work great. After two hours with the car "ON," the 12v battery is once again at 12.6v. This is a very simple process if it is okay to do. As a side note, the HV battery charge only drops by 3 or 4 percent in those two hours.

    So my qu3estion to the group is "Is there any downside to using this procedure as long as I remember to turn the car "OFF" after two hours?" I do set a timer to remind me.

    Len

    Len

    2014 EV Coupe 28,000 miles
    2014 EV Cabriolet 28,000 miles
    2015 EV Coupe 27,000 miles

  • Nope, there isn‘t really a downside to this. Of course it‘s not a ideal solution, but for a few weeks, until you‘ve got the new 12 V battery, it‘s not bad. :)

    smart BRABUS #1 in gelb, 04/2023; > 20.000 km
    smart fortwo Coupé 451er ED 3,7 kW-Lader, 05/2013; > 285.000km (bei 250.000 km 22 kW nachgerüstet)
    smart fortwo Cabrio 453er EQ 22 kW-Lader, 04/2021 - 08/2023; 42.000 km
    smart fortwo Coupé 453er EQ 22 kW-Lader, 05/2018 - 05/2021; 109.000km

    Elektrisch, weil's Spaß macht.

  • So here's another question regarding this. For the last week or so I have been watching the 12v battery charge and turn on the car for an hour or two whenever it gets down around 12.1v. I'll be picking up a new 12v battery in two weeks. I do not need to drive the car in that time, so I was wondering if it makes more sense just to hook up a trickle charger and leave it on 24 hours a day. It would make things a little easier for me, but I can also live with turning the car on once or twice a day too. Another option would be to just disconnect the negative cable on the 12v battery, but I'm a little leery about doing that and leaving it for two weeks.

    What are your thoughts?

    Len

    2014 EV Coupe 28,000 miles
    2014 EV Cabriolet 28,000 miles
    2015 EV Coupe 27,000 miles

  • I'd do whatever is more convenient, as long as it's not disconnecting the 12 V battery permanently.

    smart BRABUS #1 in gelb, 04/2023; > 20.000 km
    smart fortwo Coupé 451er ED 3,7 kW-Lader, 05/2013; > 285.000km (bei 250.000 km 22 kW nachgerüstet)
    smart fortwo Cabrio 453er EQ 22 kW-Lader, 04/2021 - 08/2023; 42.000 km
    smart fortwo Coupé 453er EQ 22 kW-Lader, 05/2018 - 05/2021; 109.000km

    Elektrisch, weil's Spaß macht.

  • Another option would be to just disconnect the negative cable on the 12v battery, but I'm a little leery about doing that and leaving it for two weeks.

    What are your thoughts?

    Even disconnecting the cable may not stop the voltage drop if there is an internal fault in the battery. At 10 years old your battery is long overdue to be changed.

  • So I replaced the four year old 12v battery yesterday with an OE battery from Mercedes - $225. Pretty easy job. After removing the old battery and testing it, I'm pretty sure that I could have gone longer with it (it read 12.6v), but the EV bricking history and that one 11.7v battery reading a month or so ago convinced me the be safe rather than sorry.


    Len

    Len

    2014 EV Coupe 28,000 miles
    2014 EV Cabriolet 28,000 miles
    2015 EV Coupe 27,000 miles

  • Moritz Leicht June 21, 2024 at 3:56 PM

    Changed the title of the thread from “Chargibg the 12v battery using the HV battery?” to “Charging the 12v battery using the HV battery?”.
  • I didn't do a lot of price checks for other batteries, but I don't think there was a huge difference in price for batteries coming from shops outside the dealership. Plus many of the generic batteries don't fit perfectly like the Mercedes battery does. The hold down clamp on some aftermarket batteries has to be "adjusted" to make it work and for me it's just easier to get a battery that is an easy swap.

    Len

    Len

    2014 EV Coupe 28,000 miles
    2014 EV Cabriolet 28,000 miles
    2015 EV Coupe 27,000 miles

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