12v battery and Noco booster

[Kithmo]

IIRC from my early auto electrical training in the late 70s, ISTR a 12v wet cell car battery was made up of six 2.1v cells, in other words it's a 12.6v battery. I would expect it could be slightly higher if it's just been charged, but left stood with no load (disconnected) it should settle to around 12.6v. With a light load when connected, I would think anything above 12v should be fine. Mine varies between 12.3v and 12.6v on the app depending on when I check it.
I don't think short journeys have any effect on the 12v going flat in an ev as there isn't the heavy load on start up like there within an ICE car's starter motor and the initial switching of the HV battery relays is a very short burst of whatever amps are needed to throw the contacts and this miniscule amount is soon recovered by the DC to DC converter. I actually measured the current drawn to activate the relays on SWMBO's old Gen 2 Prius at around 30A for less than a second.
We do nearly all short journeys and have not had any problems with the 12v on any of the Priuses, the ZS ev, SWMBO's MG5 and now the MG4 so far (touches wood).

 

[keithab]

I left mine for over 2 weeks albeit at my house. It was absolutely fine when I came back to it. I did see the charge % had dropped by a couple of % though. Would that be normal? I left it with 80% charge and it had dropped to 78

 

[siteguru]

That suggests it was topping up the 12V battery during that period.

 

[keithab]

That suggests it was topping up the 12V battery during that period.

Is that we should expect then? I’ve had the car just under 2 months so still learning about it.

 

[siteguru]

Yes - as long as the HV battery SoC is greater than a set value (not specified, but anecdotal evidence suggests that it is 50%, which is way too high*) then it'll top up the 12V battery. When SoC falls below the limit then it'll sacrifice the 12V battery in order to protect the HV battery.

* Even 20% SoC limit would be much higher than necessary, but at least it would be workable. It's not often you'd park up the car for several days/weeks with the SoC less than 20%.

 

[tsedge]

Yes - as long as the HV battery SoC is greater than a set value (not specified, but anecdotal evidence suggests that it is 50%, which is way too high*) then it'll top up the 12V battery. When SoC falls below the limit then it'll sacrifice the 12V battery in order to protect the HV battery.

* Even 20% SoC limit would be much higher than necessary, but at least it would be workable. It's not often you'd park up the car for several days/weeks with the SoC less than 20%.

Really it should top up the 12V at any state of charge >10% with a warning on the app if high idle drain is detected.

 

[Rolfe]

I parked Caliban up for just over two weeks at 69% charge and he was still on 69% (and fully functional) when I got back. Although I did notice that the SoC dropped to 68% after only a short time driving.

I can see the point of sparing the HV battery if things are getting dodgy, because it's going to be a lot easier to get the car started if all it needs is a 12v jump start, but even 20% is overkill (though as @siteguru says, reasonable in practical terms). 50% is bloody ludicrous.

 

[AndyL61]

I left my Trophy for a full week at the boatyard car park while we cruised around the broads. We arrived with 20% SOC , 1 week later , car was fine still 20% , never considered that you need to leave 50% in it , why should you ?

 

[Deleted member 11291]

Caliban

Hi Rolfe been meaning to ask whats the significance of the character relative to your MG4, just curious?

 

[siteguru]

Do you mean Caliban? (That's covered in another thread about Car Names).

 

[Rolfe]

Hi Rolfe been meaning to ask whats the significance of the character relative to your MG4, just curious?

It's a series of names.

First car, 1982 Fiesta 1.1L. Miranda, "she who is worthy to be admired".
Second car, 1993 XR2, Ferdinand for obvious reasons.
Third car 1998 Peugeot 306 GTi6, Ariel to carry on the theme.
Fourth car, 2009 VW Golf GTi Mk6, Prospero to continue, also because he was very prosperous-looking.

I remarked once or twice that I was running out of names and it was looking as if car no 5 was going to have to be Caliban. I was half-joking, but when I was sitting there in the car on the dealer's forecourt and he was pairing it to my phone for me, he asked me the car's name. I was a bit giddy with new-car euphoria at the time and answered "Caliban".

 

[Deleted member 11291]

Thanks for the clarification.

 

[strange powers]

I had the 12V go flat end of last week - I had left it plugged into the home charger in our garage for about 48 hours (lesson learned!) and it was entirely unresponsive when I went to drive it.

I had been meaning to pick up a battery booster just in case, so I popped to the Halfords in town, bought a Noco GB40 and topped up the charge that was already in it.

It was a bit of a fiddle getting the negative clamp on, but not too hard. The unit didn't detect a charge in the battery so I had to push the "boost override" button. The MG4 immediately came to life - hardly any of the Noco power was depleted. Dead easy!

I topped up the Noco unit charge - literally 10 mins - and popped it back in the glove compartment.

And weirdly, it solved my radio issue that hadn't been cleared by a full battery disconnection at the service garage...

 

[PeteC]

And weirdly, it solved my radio issue that hadn't been cleared by a full battery disconnection at the service garage...

That's odd. My radio had stopped working before my 12V battery went dead not long ago. I used a booster to get the car started and the radio came back to life immediately.

 

[SMGYorks]

I've just got the MG4 back from MG after its first year service. Apparently the 12V battery is fine but "a little down on charge". The guy who serviced it suggested that every now and then, I fully charge the HV battery to 100%, and once it gets there it will charge the 12V too. They also said earlier in the day that some of the updates that had been installed might improve 12V battery health.

I really get the sense that no-one at MG really has a clue. Before they said I should drive the car every day to make sure the 12V stays charged, and now that I should routinely charge to 100%. :/

 

[Rip Van]

I've just got the MG4 back from MG after its first year service. Apparently the 12V battery is fine but "a little down on charge". The guy who serviced it suggested that every now and then, I fully charge the HV battery to 100%, and once it gets there it will charge the 12V too. They also said earlier in the day that some of the updates that had been installed might improve 12V battery health.

I really get the sense that no-one at MG really has a clue. Before they said I should drive the car every day to make sure the 12V stays charged, and now that I should routinely charge to 100%. :/

I smell ?

 

[Rolfe]

That's pretty tragic. I'm a positive novice around EVs, but having read stuff here and elsewhere, even I know better than that.

 

[Ian Key]

I really get the sense that no-one at MG really has a clue. Before they said I should drive the car every day to make sure the 12V stays charged, and now that I should routinely charge to 100%. :/

More like the person who serviced the car needs to go on an EV course.

 

[johnb80]

I really get the sense that no-one at MG really has a clue. Before they said I should drive the car every day to make sure the 12V stays charged, and now that I should routinely charge to 100%. :/

Thats totally wrong. The 12v battery lives a life of luxury compared to ICE vehicles. It is charged from the traction battery using a DC to DC converter, effectively a battery charger like you would plug in to the mains at home. This charger is capable of charging at its maximum rate as soon as it is switched on, it's not dependent on engine RPM like an ICE car, the 12v battery doesnt have to recover the considerable start up loads either compared to ICE. If the 12v is going flat either some of the electronics have not gone to sleep properly, the battery is duff or the DC to DC charger is faulty, any of these are easily detected by very basic test equipment. The garage sounds to be incompetent and maybe another garage should be sought.

 

[SMGYorks]

Thanks for your feedback everyone.

I'm planning to leave the car unused for 48 hours to see if the problem reoccurs. I'll have driven roughly 100 miles in the week beforehand, and I'll leave it not charging, with less than 50% in the HV battery.

However, I want to be absolutely sure that I'm not inadvertently leaving something on which could drain the 12V battery. Does anyone have a "turning the car off fully" checklist? e.g. Select parking brake, get out, close door, lock with key fob. Also, is there anything I should avoid doing during that 48 hour period that might unintentionally wake the car?

Thanks!