Cdni3
Posts: 28
Joined: Wed Nov 25, 2015 4:54 pm
Location: Vancouver, BC

12V Battery Questions

Fri Sep 30, 2016 8:26 pm

Does anyone know if it is possible to either put a second 12V battery (deep cycle battery) to run a 12V ARB cooler/freezer (2.4 amp draw) for an extended period of time and have it charge from the main EV battery? Does the REX charge the 12V battery along with the main EV battery?

alohart
Posts: 769
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI, US and Uppsala, SE

Re: 12V Battery Questions

Sat Oct 01, 2016 9:51 am

Cdni3 wrote:Does anyone know if it is possible to either put a second 12V battery (deep cycle battery) to run a 12V ARB cooler/freezer (2.4 amp draw) for an extended period of time and have it charge from the main EV battery? Does the REX charge the 12V battery along with the main EV battery?

The 12V battery is charged by the high-voltage battery pack via a DC-DC converter that reduces the high-voltage down to ~14V, but only when the car is in the Ready state or is being charged. So running your cooler/freezer with the car off would discharge the 12V battery.

To the 12V system, adding a second 12V battery in parallel with the existing battery would appear the same as if a single much larger capacity 12V battery had been added. Generally, 12V systems aren't sensitive to different battery capacities. However, if your cooler/freezer discharged the 12V batteries too much, the car would not be able to enter Ready state which could strand you. If you connected the auxiliary battery with a switch that could isolate it from the standard battery, you could discharge the auxiliary battery without also discharging the standard battery which would prevent being stranded. However, reconnecting a very discharged auxiliary battery to the standard battery could cause the voltage of the 12V system to suddenly drop and a high current to flow from the higher voltage standard battery to the discharged auxiliary battery, both of which might cause problems. Others with more electrical knowledge will likely comment.

The REx generator charges the high-voltage battery pack, not the 12V battery directly.
Aloha,
Art

Honolulu, HI, US: 2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE
Uppsala, SE: FOR SALE: 2000 Honda Insight hybrid

jadnashuanh
Posts: 3174
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: 12V Battery Questions

Sat Oct 01, 2016 1:40 pm

I do not know the power capacity of the DC-DC power supply, but it is entirely possible that it is not big enough to recharge a second battery during a typical trip. It probably could handle it (more like a trickle charger) while connected to an EVSE. The computers and local electronics are powered by the 12vdc circuit, and even if you isolated the second battery, once you reconnected it, the current surge could be problematic and disrupt things.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

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