12V Battery charging when left for long periods

[Aircub]

I'm heading off for about three months and need to leave my car unused during that time. My main concern is the 12V battery. From what I understand, it's only directly charged by the car's main battery when the car is running.

I've been looking into solar trickle chargers as a potential solution to keep the battery topped up while I'm away. However, I'm not entirely sure if this is the best approach or if there are other, more effective methods.

I'd love to hear your thoughts and experiences! What do you guys think would be a good solution for maintaining the battery charge over a three-month period? Any recommendations for solar chargers or alternative strategies would be greatly appreciated.

Thanks in advance for your help!

 

[BarrieG]

My car was first registered in January 2025 and I first saw it at the dealer in May 2025 by which time it had only done 900 miles. The car was blocked in at the back of the dealer's multi storey car park, covered in dust, and clearly hadn't been used for some time. I have no way of knowing how long it had been sitting unused but the dealer just maneuvered it out on electric power and half way through the road test the ICE kicked in when I accelerated onto a dual carriageway.
Whether it had been unused for three months I don't know but it just worked as it should.

 

[Totti1001]

According to the owner’s manual, the 12V battery should be charged from the high-voltage battery if the vehicle has been stationary for a long time and the voltage of the 12V battery is too low. Unfortunately, I only have the German version of the manual, but this is referred to as ‘intelligent charging’ and I’m sure it’s also mentioned in the English version.

 

[Giller]

Ok. I have had an x power and switched to an S6 this year to which I fitted a battery monitor so have been able to see when the conventional battery is being charged by the main battery and can confirm it happens when the car is not running, I’m sure it’s the same for other models. Personally I’d not be concerned, I do have a battery jump starter bought years ago and carry that in case of any issue, might be something you should consider rather than battery maintainers?

 

[Totti1001]

I had the text from the instructions Google translated:


Intelligent Charging

When the vehicle is OFF and the system detects that the 12V battery is low, the vehicle will automatically charge the 12V battery under certain conditions to ensure the vehicle can start. This function will automatically deactivate once charging is complete.

Note: The system will interrupt intelligent charging if a fault occurs, the vehicle is started, or it is being charged via an external device.

Note: The vehicle's range will be reduced after intelligent charging.

Note: The intelligent charging function will be interrupted if the high-voltage battery has a low state of charge (SOC).

 

[MG Clive]

From the English manual. . . .

 

[HStrophyPHEV2025rc]

If it is of any comfort to others, here's my brief real-world experience. I left my car in the Heathrow long-stay car park for 2 weeks. During my holiday, I suddenly realised that I had accidentally left my wireless android adapter plugged into the USB socket. This definitely draws power because it has a bright green light. I was a bit worried that the car would not start when I came back to it.

Well, the good news is that the car started perfectly, when I came back (last weekend). For the record, I had approx 50% full (35 miles range) main battery at the time I left the car, so perhaps over the 2 weeks the car automatically took some power from it to feed the 12v battery.

I reckon my experience is very typical of most people's needs from the car, so hopefully this feedback prevents someone else worrying when they are on a 2 week holiday!

 

[Giller]

Well, the good news is that the car started perfectly, when I came back (last weekend). For the record, I had approx 50% full (35 miles range) main battery at the time I left the car, so perhaps over the 2 weeks the car automatically took some power from it to feed the 12v battery.

Not perhaps that's what's meant to happen, the problem is if the conventional battery fails and why many people carry a portable battery pack as back up.

 

[Biker67]

This sounds like a very well thought out feature.
And I guess it can be tested by looking at the voltage in the 12v battery via the MG App whilst you're on holiday.
i.e. if the voltage drops below 12v then increases, you know that the main battery is topping it up.

 

[Giller]

This sounds like a very well thought out feature.
And I guess it can be tested by looking at the voltage in the 12v battery via the MG App whilst you're on holiday.
i.e. if the voltage drops below 12v then increases, you know that the main battery is topping it up.

Not sure if it applies with a PHEV, but I'm sure someone who can confirm shortly.

It's critical with a full EV as the conventional battery is what kicks the whole process off providing the power for the computer and other systems.

I use after market battery monitors which connect with bluetooth and can see the process in real time, it's not constant but I get an alert if the batteries SOC drops below a set level, well that's the theory as hasn't happened yet, the S6 is on the left and you can see it's charging, the M3 on the left and its on a CTEK battery tender so shows as charging even though it's in maintenance mode.