12V Battery Size

[WigglePig]

Absolutely I wouldn't expect to see constant 14.2+ while charging but a flooded lead acid will never reach full charge capacity if it is only ever presented with a float charging voltage. The at-rest terminal voltage of the battery is barely 12V in my vehicle where I would definitely expect to see more like 12.5 or so in a fully-charged lead acid battery. The only conclusion I can come to is that the 5's charging system never fully charges the 12 V battery.

I did a small experiment by somewhat discharging the 12 V battery and then reconnecting it to the vehicle to see how it behaves; 13.8 V is the maximum I ever saw.

 

[Coulomb]

I did a small experiment by somewhat discharging the 12 V battery and then reconnecting it to the vehicle to see how it behaves; 13.8 V is the maximum I ever saw.

Ah, my misunderstanding. Yes, that is too low for recharging the 12 V battery in what might turn out to be a short trip.

Someone above mentioned a firmware update to fix that, and it sounds vaguely familiar. Sounds like you need that. I find it amazing that MG seem to be happy to let faults like this go on until the customer complains, hoping that the abused 12 V battery will outlast its paltry 12 month warranty.

 

[MickeySw]

Someone above mentioned a firmware update to fix that, and it sounds vaguely familiar. Sounds like you need that. I find it amazing that MG seem to be happy to let faults like this go on until the customer complains, hoping that the abused 12 V battery will outlast its paltry 12 month warranty.

Update SC052 is meant to increase the 12V aux, however that doesn’t seem to be the case? Post #34 below..

MG5 Confused over update versions

 

[Paulie68]

14.2 isn't good for a lead acid battery for too long. 13.8V is a recognised float voltage, i.e. one that will maintain the battery at full charge, but is safe to leave at that voltage long term (even years, in the case of an uninterruptable power supply).

Those voltages are for 25°C, it needs to be a little higher (e.g. 0.5V higher at -3°C) in colder temperatures.

13.8 is for standby float, that's not the case here as the battery is getting constantly used. 14.2V is too low for successful charging and will allow sulphating and premature failure. The usual compromise for charging in cars is 14.4V, but this is arguably still a little low, 14.6V is ideal at 25°C, with the UK climate being usually colder, 14.7V would be more suitable most of the time.

 

[MickeySw]

Seemingly update SC052 was released by MG as a CCS rapid charge fix, not to increase the 12V aux voltage in general, maybe the CCU only boosts the voltage during a DC session?

 

[Coulomb]

14.6V is ideal at 25°C

Since the gassing voltage at 25°C is 14.4V, I would think you would not want to exceed that (at 25°C). You can't easily replace water in a sealed battery.

 

[Paulie68]

Since the gassing voltage at 25°C is 14.4V, I would think you would not want to exceed that (at 25°C). You can't easily replace water in a sealed battery.

Don't disagree but the gassing voltage is a range and 14.4V is at the bottom end of it, more suited to very hot operating conditions. It's a trade off, no risk of water loss at 14.4V, but the battery will be more prone to suffering from sulfation and capacity loss over time. 14.6V is better for a typical warm British summer day, 14.7V for our usual non summer temps. ICE engine bays get very hot, which is most likely why the voltage is traditionally set at 14.4V, different for an EV with almost no engine bay heat at all.

 

[WigglePig]

13.8 is for standby float, that's not the case here as the battery is getting constantly used. 14.2V is too low for successful charging and will allow sulphating and premature failure. The usual compromise for charging in cars is 14.4V, but this is arguably still a little low, 14.6V is ideal at 25°C, with the UK climate being usually colder, 14.7V would be more suitable most of the time.

I have been wondering if this is why we see so many people reporting 12V battery failures.

 

[MickeySw]

Seemingly update SC052 was released by MG as a CCS rapid charge fix, not to increase the 12V aux voltage in general, maybe the CCU only boosts the voltage during a DC session?

Came across this by accident: our MG5’s 12V aux (without SC052 update) is topped off @14.4V DC when the car is AC charging overnight.

However when driving, the DC/DC only supplies it with the usual 13.8V (ish), another reason to charge small amounts but frequently..

Interestingly though, the voltage also drops slightly when the car is ‘READY’ during an AC charging session, will have to look at this in detail during the next rapid charge..