12 volt battery charging

[avhation]

Having returned after 16 days on holiday I found the 12v battery to be too flat to be bothered to unlock my MG4 Trophy LR. (The HV battery was left at 60% plus before trip.) I couldn't trickle charge the 12v battery, seemingly because the current draw was too high. I disconnected the positive terminal and trickle recharged the battery to 12v. My question is: at what voltage should the BMS kick in to top up the 12v battery, assuming that the HV battery is well charged?

 

[Surfdog]

I've found that when my battery drops to 12.3v it then takes a charge from hv battery up to about 12.6v. I've never had a problem with flat battery (yet) but a lot of owners on here have. I believe it doesn't do this with a low SOC but shouldn't be an issue at 60%. I take car was locked and lights were off as another problem some have, including me, is that I have no alarm to remind me to turn off lights when leaving car if lights have been set to manual.

 

[Tig170]

Having returned after 16 days on holiday I found the 12v battery to be too flat to be bothered to unlock my MG4 Trophy LR. (The HV battery was left at 60% plus before trip.) I couldn't trickle charge the 12v battery, seemingly because the current draw was too high. I disconnected the positive terminal and trickle recharged the battery to 12v. My question is: at what voltage should the BMS kick in to top up the 12v battery, assuming that the HV battery is well charged?

Keep going: A fully charged car battery voltage will measure around 12.6 volts.

 

[ukslim]

This sounds like a faulty 12v battery battery to me. 2 weeks away shouldn't leave you with a dead car. Get it replaced on warranty.

 

[Rolfe]

It better not. I'm going away for 2½ weeks in September and I will have to leave the car parked in the street for that time.

 

[t3rminalV]

Ours has sat for three or four weeks on occasion and has started up fine afterwards.

 

[Rolfe]

That's reassuring.

I left my Golf for 10 weeks at the docks in Bristol in early 2019 and it was fine when I got back. Just needed a bit of working on the brakes to get them going smoothly again. But when we got back to my house where my friend had left her Polo in my garage, we needed the jump-starter. And one of her wheels completely locked.

 

[avhation]

Keep going: A fully charged car battery voltage will measure around 12.6 volts.

Understood Tig. I have not used the car for 3 days now while monitoring the 12v voltage via the app. The BMS seems to have kicked in after 3 days when the voltage reached 11.9v, now showing 14v+ while being charged. When the BMS charging cycle ends I expect the 12v battery voltage to drop within a few hours to approx 12.6v in warm weather, unless the battery has deteriorated through being left in a discharged state.

My mk1 zx was no trouble after a 3 week trip last year.

I've found that when my battery drops to 12.3v it then takes a charge from hv battery up to about 12.6v. I've never had a problem with flat battery (yet) but a lot of owners on here have. I believe it doesn't do this with a low SOC but shouldn't be an issue at 60%. I take car was locked and lights were off as another problem some have, including me, is that I have no alarm to remind me to turn off lights when leaving car if lights have been set to manual.

Thanks

I've found that when my battery drops to 12.3v it then takes a charge from hv battery up to about 12.6v. I've never had a problem with flat battery (yet) but a lot of owners on here have. I believe it doesn't do this with a low SOC but shouldn't be an issue at 60%. I take car was locked and lights were off as another problem some have, including me, is that I have no alarm to remind me to turn off lights when leaving car if lights have been set to manual.

Thanks Surfdog. I was extra careful to switch off the HVAC and auto headlights, and to lock the doors.

 

[Tig170]

That's reassuring.

I left my Golf for 10 weeks at the docks in Bristol in early 2019 and it was fine when I got back. Just needed a bit of working on the brakes to get them going smoothly again. But when we got back to my house where my friend had left her Polo in my garage, we needed the jump-starter. And one of her wheels completely locked.

Although its not a competion I've got a VW T-Roc (1200 miles, albeit accident damaged), awaiting a small repair (Covid project, but got very busy!) - I started it the other week, after 3 years on the turn of the key (ironically it came up with a fault code :- the keyfob battery is low ) - the take home message is that some cars use no energy whilst parked up, but others do appear to leak energy over time. The VW has a VARTA battery, which I believe is the OEM battery on SAIC cars? (I do see a few Bosch components in the front of the MG4)

 

[Rip Van]

I think I'm going to find out about mine as it's been parked at an airport since 24th May and we return on 4th June.

 

[PeteC]

Ours has sat for three or four weeks on occasion and has started up fine afterwards.

Recently left ours for six and a half weeks and it unlocked and started up fine as well.

 

[Peter WA]

I've found that when my battery drops to 12.3v it then takes a charge from hv battery up to about 12.6v.

Hi Surfdog,

your comment is the first and only one so far that I found that confirms that the 'intelligent charging' of the 12V battery (as described in the user's manual) actually happens.

Could you please fill in some more details for me to understand better?

How do you monitor the 12V battery, are you doing it through the app, or do you have some external means? Were you ever able to observe during that top up? When charging the battery voltage should be anywhere between 13.8V and 14.6V, just wondering if that indeed happens during that top up.

Cheers,

Peter

 

[johnb80]

Having returned after 16 days on holiday I found the 12v battery to be too flat to be bothered to unlock my MG4 Trophy LR. (The HV battery was left at 60% plus before trip.) I couldn't trickle charge the 12v battery, seemingly because the current draw was too high. I disconnected the positive terminal and trickle recharged the battery to 12v. My question is: at what voltage should the BMS kick in to top up the 12v battery, assuming that the HV battery is well charged?

It's safer to disconnect the negative terminal if you have to do this again, if you inadvertantly touch the bodywork with the spanner, disconnecting the positive would create a dead short whereas disconnecting the negative it wouldn't matter.

 

[Surfdog]

Hi Surfdog,

your comment is the first and only one so far that I found that confirms that the 'intelligent charging' of the 12V battery (as described in the user's manual) actually happens.

Could you please fill in some more details for me to understand better?

How do you monitor the 12V battery, are you doing it through the app, or do you have some external means? Were you ever able to observe during that top up? When charging the battery voltage should be anywhere between 13.8V and 14.6V, just wondering if that indeed happens during that top up.

Cheers,

Peter

Yes, I can see through my app what the voltage is. At the moment it's at 12.7v. I've never seen it above this unless the main battery is being charged when it jumps to about 13.8v and drops back to 12.7 once charging complete. The lowest it seems to go is 12.3v and then when I check it a few hours later, without a main battery charge, its back up to 12.7. So the intelligent charging seems to work perfectly on my car and have had no issues with battery in over 4 months. So it appears there is a problem with the intelligent charging system on all the cars with flat batteries, as, in theory, this should never happen as long as car is not in low SOC.

 

[Robf]

A lot of members (me included) are purchasing battery boosters rather than chargers. If faced with a flat battery you can instantly start the car rather than waiting for the battery to charge (access by manual key), the car main battery then charges the 12v battery. I keep mine inside the glove compartment, they are very compact.

Amazon.co.uk : battery boost