Honda jazzz 2024: 12V battery charging when power on and enging running

[Bjorn]

I have a USB charger with display connected to the 12V outlet and have noticed something interesting (see image). In a typical traditional gas/diesel engine whenever the motor is running, 12V battery voltage is always typically 14.5-14.7, indicating that the generator is charging the 12V battery. Not so in my new Jazz. The voltage show on the USB charger display is either 12.6V or 14.6-14.7V even when the gas engine is running. If no electric heating is on, the voltage is 12.6. If I turn on electric heating (seat, steering wheel, cabin heat), the voltage may go up to 14.5-14.7V but not always, even with all electric heating on and gas engine running, the voltage may be 12.6V for a while before increasing to 14.6-14.7V
Conclusion: the 12V battery is not constantly charging when the power system is on. It looks like the voltage has to fall below 12.6 V before the generator/HV battery start feeding current to the 12V battery. Meaning that it is a good idea to have a 12V charger connected to the 12V battery and once in a while, charge the 12V battery to keep it healthy. I myself have a Ctek connected to the battery with eyelets

[John Ratsey]

Failure to keep the 12V battery 100% charged may help explain the problem of the 12V battery rapidly discharging if, for example, the radio is used for 1/2 hour without the vehicle fully powered up. Honda's approach probably improves power efficiency by a negligible amount but massively increases the risk of flat 12V batteries as sometimes the system will be powered down with the 12V battery just above the threshold for recharging.

[coldstart]

This behaviour (not permanently charging the battery while the engine is running) has been adopted by other manufacturers as well - supposedly to "recuperate" energy. My last car (a VW) only charged while coasting or if the charge level got too low which means the 12v battery was practically never fully charged even after a long journey.
Bjorn's discoveries let me suspect the same applies to the Jazz.

[Kremmen]

I used to have one of those power socket voltmeters plugged in

I noticed the voltage fluctuating and I got the impression that it's charging was similar to the HV battery

It would show circa 13.x or 14.x volts when being charged but when it was deemed full the voltmeter would drop to circa 12.x or 11.x volts before the cycle started again

[Nicksey]

I am sure Honda have done extensive testing regarding their electrical systems, and knowing how far technology has advanced then I assume battery charging has also moved on somewhat. I understand the 12v in the Honda is not as big as other cars, but there must be a reason for this.. and I don't believe it will just be a cost saving initiative.
Even in my previous cars I knew it was detrimental to the battery to sit with the engine off and listen to the radio, or keep the heating going. My only bug bear regarding the Jazz, is if you did want to listen to the radio there is no in built circuit cut out when the battery level drops that turns the radio off... something my previous cars always did