12volt battery low on MG3 Hybrid Trophy

[Radish]

I purchased an MG3 hybrid Trophy in July 2024, it has been fine since then but the last few weeks it shows 12V battery low when ignition switched on. I have contacted the dealership for an answer and am still waiting to hear. Anyone else had this problem and been given an answer ?

 

[Coulomb]

It doesn't mean the 12V battery has 14.1V.

When the auxiliary battery is charging it is at essentially the same voltage as the DC-DC output.

The voltage is not a measure of the energy stored in the battery. Though the one hour rested voltage of a lead acid battery gives a rough indication of its state of charge.

 

[Coulomb]

so are the main batteries then charging the 12V one?

Yes. But the main battery (some 100 cells in series) is at about 400V, and the auxiliary battery needs to be at about 14.1V (depending on temperature) to be charged quickly, so you can't connect them directly. So a piece of power conversion electronics called a DC-DC converter (often shortened to just DC-DC) is used. Basically, the main battery voltage is chopped into square waves (alternating current, AC), stepped down in voltage (and stepped up in current) by a high frequency transformer, rectified back to DC again, and this is applied to the auxiliary battery. Any 12V loads are supplied first, and whatever is left over charges the auxiliary battery.

A typical DC-DC in an EV can push out over 150 amps, more than most ICE alternators can.

 

[Coulomb]

I tested the 12V battery with the power all off...

That's a good idea, because when the car is in Ready mode or charging the main battery, measuring the voltage at the battery terminals will just tell you the output voltage of the DC-DC converter, and nothing about the health of the auxiliary battery.

Ideally, you should let the auxiliary battery rest with minimal loads (lights off, doors closed, apart from bonnet or boot to gain access to the auxiliary battery terminals) for an hour or more. That's because after a battery is charged, it takes some time for the chemicals to settle down to a steady state. So the battery voltage immediately after charging might be say 13.1V, but that gradually reduces to say 12.4V over a hour. If you take the 13.1V value you might think that the battery is totally full and in great health. The 12.4V measurement says that actually it's doing OK, but could use more charging, or if more charging doesn't increase the rested voltage, it's on the way out and needs replacing within the next year or so. Lower rested voltage (at around 25°C) gets more concerning.

 

[garyh]

I've not had any issue with the 12volt battery. It is a recorded issue with the Toyota Yaris mk4, when owners leave it sitting up for a few days, but I haven't heard of any issues on this forum. Have you had the battery tested? Have you hard wired a dascam or similar?

I purchased an MG3 hybrid Trophy in July 2024, it has been fine since then but the last few weeks it shows 12V battery low when ignition switched on. I have contacted the dealership for an answer and am still waiting to hear. Anyone else had this problem and been given an answer ?

I have same problem when I came back from hols.Despite long journeys since,I still get the message 12V battery low