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2017 i3 Rex 12V Battery Voltage

Posted: Tue Jul 07, 2020 11:53 am
by roco1953
When I check the voltage of my 12V battery it shows permanently 14.2-14.4V. (using 12V car outlet)
Does it mean the battery is permanently charged via DC/DC from High Voltage Battery ?
If not in charge mode the voltage should read around 12.5-13V.
Am I correct ?

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Tue Jul 07, 2020 1:01 pm
by MKH
The 12v car outlet isn't powered by the 12v battery like in an ICE car, it is powered by the DC/DC converter, so checking the voltage from the outlet only shows the DC/DC converter voltage. But yes, the DC/DC converter does charge the 12v battery.

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Tue Jul 07, 2020 5:15 pm
by alohart
MKH wrote:
Tue Jul 07, 2020 1:01 pm
The 12v car outlet isn't powered by the 12v battery like in an ICE car, it is powered by the DC/DC converter, so checking the voltage from the outlet only shows the DC/DC converter voltage.
I think a more precise way to state this is that all 12 V loads and outlets are on the 12 V circuit. Power sources for this circuit include the 12 V battery and output side of the DC-DC converter. The voltage measured anywhere on this circuit, including at the auxiliary power ports, is that of the 12 V battery, ~12.7 V resting voltage for a fully-charged AGM battery, unless the DC-DC converter is on, in which case the 12 V system voltage is that of the output of the DC-DC converter, ~14.3 V. When the high-voltage system is on (e.g., after unlocking or opening a door, the frunk, or the hatch), the DC-DC converter is on. Both the high-voltage and 12 V systems time out and shut off after a period of inactivity.

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Wed Jul 08, 2020 8:13 am
by roco1953
Thanks for explaining this issue. But is brings me to another question:
How can I check the 12V battery with a Voltmeter when the DC/DC kicks in as soon as Open the door, trunk,etc ?
I just want to make sure my 12V batteryis healthy and does not need to be replaced.

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Wed Jul 08, 2020 12:10 pm
by MKH
How can I check the 12V battery with a Voltmeter when the DC/DC kicks in as soon as Open the door, trunk,etc ?
Open the Frunk, remove the storage bin and side covers to expose the battery location on the left side (as you face the car), Wait an hour. Come back (without the key in your pocket) and test at the battery posts.

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Wed Jul 08, 2020 1:36 pm
by alohart
MKH wrote:
Wed Jul 08, 2020 12:10 pm
Open the Frunk, remove the storage bin and side covers to expose the battery location on the left side (as you face the car), Wait an hour. Come back (without the key in your pocket) and test at the battery posts.
Because the DC-DC converter will begin charging the 12 V battery when the frunk is opened and because the high-voltage system will remain on for at least 30 minutes prior to shutting off automatically, the 12 V battery won't be at its resting voltage after an hour. I'd wait overnight for an accurate reading.

Parking in an apartment parking garage, I don't like leaving our car unlocked overnight, so I connected leads to the 12 V terminals beneath the cargo area floor and routed them out below the rear of the car. I was able to connect these leads to a voltmeter without unlocking the car. Pain in the butt!

Re: 2017 i3 Rex 12V Battery Voltage

Posted: Wed Jul 08, 2020 11:35 pm
by Steve999
I bought one of those inexpensive battery load testers off Amazon, to check my battery life (and my other cars too since they are sitting idle with the current pandemic issue). After some difficulty getting to the terminals and making sure car was in 'sleep' mode, here are the numbers ( 2015 Rex, original battery) :

SOH - state of health: 50%
SOC - state of charge: 85%
RES - Internal resistance : 16.71
Volts- 12.48
CCA, measured/ Rated: 148/300 ( used DEKA AUX 18 L specs)

Has anyone tried this, and if so, anyone know or have experience with AGM batteries as to what is the minimum values before battery should be changed? Only thing I found was best if changed if SOH < 40%. Nothing to indicate any problem with the battery so far ( strange faults, etc.) I will likely change the battery in the next 6-12 months in any case, but it would be cool to try and do measurement/metric based maintenance vs shotgun approach...