12V battery drained in 48 hours whilst locked

[T1 Terry]

One of these is very handy https://www.ebay.com.au/itm/2841233...bjtd8i0gnGqQh48ZwAYz5UKhYQ==|tkp:BFBMgN-bgfNm not saying to buy from that seller or anything like that, this is just an example link, but the Uni-T 203+ is a great tool for testing DC current flow without needing to disconnect any wires or terminals

Probably not technician standard accuracy, but it gives the used a good indication of just what is happening through a cable ...... and it can do all the other multimeter functions function the general handyman might need .... and the price is quite reasonable for the level of quality they provide .....

No, this was not a paid infomercial, the presentation would have been a lot flasher

T1 Terry

 

[Martinonline]

According to the manual, if there is a fault with the low-voltage battery charging circuit then the car will display a warning symbol. Unless someone has access to the charging circuit and software then finding the fault would be difficult.

 

[Rolfe]

I'm guessing it would be the same point it stops the V2L, 25% if I remember ...... I can't imagine it has a heap of different low SOC cut offs for each different function.

Either the MG smarts has determined the 12v battery is faulty because it can't seem to get it fully charged, or the "Intelligent" charging if faulty

T1 Terry

The lowest you can set the protected charge to for the VtL is 20%. However the car is supplied with a default setting of 50%, or at least mine was. As I discovered to my cost the first time I camped in it. The second morning the SoC was 48% and the VtL wouldn't come out to play. I had to wash in tepid water (with about a thousand midges swimming in it) and have Coke for breakfast instead of tea. I had no idea how to change it.

Back home, the forum directed me to the right screen. And I still couldn't change it because the slider wouldn't slide. Turns out you have to tap it. And the lowest setting it will accept is 20%.

 

[T1 Terry]

The lowest you can set the protected charge to for the VtL is 20%. However the car is supplied with a default setting of 50%, or at least mine was. As I discovered to my cost the first time I camped in it. The second morning the SoC was 48% and the VtL wouldn't come out to play. I had to wash in tepid water (with about a thousand midges swimming in it) and have Coke for breakfast instead of tea. I had no idea how to change it.

Back home, the forum directed me to the right screen. And I still couldn't change it because the slider wouldn't slide. Turns out you have to tap it. And the lowest setting it will accept is 20%.

I was close, missed by that much as Maxwell Smart would say

T1 Terry

 

[Martinonline]

It would appear that other manufacturers have problems with maintaining the 12V battery capacity. Hyundai and Kia have recalls for the Ioniq 5/6 and EV6 to software update/replace the ICCU (Integrated Chargung Control Unit). The part that maintains the 12V battery can fail completely causing, eventually, everything to stop. One fault cause was put down to voltage spikes at the starting and stopping the charging. An ICCU is believed to cost £4,000 as it controls all charging/V2L processes.

 

[Joningy]

I got a tray from AliExpress and the booster fits underneath it.

@Ian Key My Armrest Storage Box & Center Console Lower Storage Tray, from Ali Express turned up yesterday Ian and fitted (perfectly) today, the armrest is by far the more useful, but both delivered in a week from China , both for £12.49p, including all delivery costs, pick up in China, flight to UK, all delivery journeys this end, don't know how they do it for the price. Cheers for the tip off!

 

[Joningy]

One of these is very handy https://www.ebay.com.au/itm/284123338576?_skw=uni-t+203++multimeter&itmmeta=01KEC16XXPV4SGE4G4H025XFEK&hash=item4227121750:g:Z2AAAOSw5QNjjra4&itmprp=enc:AQAKAAAA8FkggFvd1GGDu0w3yXCmi1efbUw2Heyg7Whmk5txM5FfohDQGOFeFSTSKsfeZ3dj3BL/hhNPJ+8bCe6OP89f7oR/vCCoqp2bWtnUfBMbJsj6T7TdU9uURqvDsDrj5VFQhDxHqtYUFqObmjaratM5BHE2UktMUOhRC+O4loT26QzS1D12myYJkYCCqXS0uNzoIovVI2yjbGoTjjqHGsYNLiOEsU0AOhtZzN0nM3nF44tOda6JEBnz7roE6dDedGyVArMHrmeOrW/dreuyjyNBDQBxlV3zJ99kCC9wQBhEbjtd8i0gnGqQh48ZwAYz5UKhYQ==|tkp:BFBMgN-bgfNm not saying to buy from that seller or anything like that, this is just an example link, but the Uni-T 203+ is a great tool for testing DC current flow without needing to disconnect any wires or terminals

Probably not technician standard accuracy, but it gives the used a good indication of just what is happening through a cable ...... and it can do all the other multimeter functions function the general handyman might need .... and the price is quite reasonable for the level of quality they provide .....

No, this was not a paid infomercial, the presentation would have been a lot flasher

T1 Terry

Got 3 basic testers Terry, all reading similarly, so good enough for what I'm doing!

 

[Joningy]

12v battery down to 3v again this morning (09/01/26)

Phoned local Penton MG, because this points towards the HV battery not charging while the car is in switched off, I was told this is correct.

I was under the impression the HV battery did charge, so long as the the HV battery SOC was above 20 to 30% capacity.

AI says: "Yes, the high-voltage (HV) battery can and should charge the MG S5's 12V battery even when the car is switched off, provided certain conditions are met."

• "Automatic Maintenance: Modern MG EVs are designed to monitor the 12V battery's health while parked. If the system detects the 12V charge is running low, it will automatically activate the DC-DC converter to "top up" the auxiliary battery using power from the HV pack."
• HV State of Charge (SoC) Threshold: This automatic charging typically only occurs if the main high-voltage battery has sufficient energy (usually above 20-30%). If the HV battery is too low, the car will prioritize its own preservation and stop maintaining the 12V battery.
• Charging While Plugged In: The 12V battery is also maintained whenever the vehicle is actively charging from an external EVSE (charger).

So which is correct? I tend to think MG know what they are saying, but why does AI tell a conflicting story?

 

[siteguru]

Nobody knows what the "Low SoC Threshold" is ... well, MG China probably know, but they've not told anyone else. Try doing the same test with the car's SoC at around 60% (i.e. above 50%).

 

[Joningy]

Nobody knows what the "Low SoC Threshold" is ... well, MG China probably know, but they've not told anyone else. Try doing the same test with the car's SoC at around 60% (i.e. above 50%).

The car was charged to 80% 2 days ago and was still the same this morning @siteguru , but in that time the battery drained to 3v, parked in the garage.

So there's either a serious parasitic drain, or IF the HV battery is supposed to monitor & charge the auxiliary battery whilst asleep, then that appears not to be happening.

But that's still a serious drain to flatten the fully charged 12v battery in a few days battery, OK the 12v is a pewny 35Ah, but should hold up better.

 

[Martinonline]

@Joningy, if you are asking whether your dealer is correct or the AI then I believe the AI is correct except, as @siteguru says no-one really knows at what level the HV stops charging the low voltage battery.

You may have 2 problems - a parasitic drain and failure of the low voltage battery charging circuit when the car is not powered.

I assume that you do not receive any low voltage battery charging circuit warnings when the car is powered?

 

[Lovemyev]

But that's still a serious drain to flatten the fully charged 12v battery in a few days battery, OK the 12v is a pewny 35Ah, but should hold up better

You’re totally correct, the battery size could be a bit more generous I agree, but it’s the same size battery on all S5’s so although small, it should be capable of doing the job.
Otherwise everybody else would be the same boat of course.
That is a big drop in voltage.
When a 12 volt battery is constantly running flat on any car, then it can only be one of a few things that is causing it.
1. The battery IS receiving a charge, but it is unable of retaining that charge, due to an internal fault.
2. The battery itself is fine, however because it is NOT receiving a charge, it is soon depleted.
3. Both the battery is fine and it’s receiving a charge correctly, but in its “standing state” there is a parasitic drain, depleting the SOC.
It’s purely a process of elimination and you have done a lot of these checks yourself.
I guess you have already done this, but what happens if you charge the battery, then leave it disconnected, does the voltage still drop over the same given time scale to levels you have witnessed recently?
If not, then it’s not the battery (although by now I think it will need replacing regardless, given the number of times it has been deeply discharged).
If your Amp clamp reports a significant battery drain when the negative terminal is disconnected and the car is fully asleep, then there has be a parasitic drain surely.
This only leaves the question, is it receiving a back charge from the HV battery?
The difficult process for diagnosing/discovering the cause of a parasitic drain, is, again, trial and error.
Normally this is done by systematically removing fuses / relays, in a “one by one” fashion and then returning them, until you finally find the circuit that is the cause of that parasitic drain.
This can be as simple as a relay that is failing to unlatch.
Now they have a clear path on which item(s) are causing the drain.
Then breaking down which item(s) are fed by they fuse etc and isolating each one.
The dealer will no doubt be asked by MG CS to check if the car is charging of course.
I hope with your help and the checks you have already performed, the dealer has already a good head start and with some direction to head down.
Although MG CS will want to perform the same tests as you, that I am 100% sure.
As they do tend to dismiss other peoples findings unfortunately.
It will be very interesting to hear what they discover, good luck .

 

[emmrecs]

12v battery down to 3v again this morning (09/01/26)

Phoned local Penton MG, because this points towards the HV battery not charging while the car is in switched off, I was told this is correct.

I was under the impression the HV battery did charge, so long as the the HV battery SOC was above 20 to 30% capacity.

AI says: "Yes, the high-voltage (HV) battery can and should charge the MG S5's 12V battery even when the car is switched off, provided certain conditions are met."

• "Automatic Maintenance: Modern MG EVs are designed to monitor the 12V battery's health while parked. If the system detects the 12V charge is running low, it will automatically activate the DC-DC converter to "top up" the auxiliary battery using power from the HV pack."
• HV State of Charge (SoC) Threshold: This automatic charging typically only occurs if the main high-voltage battery has sufficient energy (usually above 20-30%). If the HV battery is too low, the car will prioritize its own preservation and stop maintaining the 12V battery.
• Charging While Plugged In: The 12V battery is also maintained whenever the vehicle is actively charging from an external EVSE (charger).

So which is correct? I tend to think MG know what they are saying, but why does AI tell a conflicting story?

I think, from my experience, AI is correct and the dealer is wrong!

As I've reported elsewhere, having also experienced the "dead" 12v battery problem (at least 4 times) and the dealer being unable to find any fault, despite an initial 2 full days of diagnostics, I've been keeping a daily log of 12v battery levels, distances travelled between battery checks, etc. etc., all of which have now been passed to the dealer where the car is currently undergoing yet another diagnostic session (they reported yesterday that the battery is showing a 64% SOH, so further tests today prior to them contacting MG Technical). But during that logging process I noted a number of days when the voltage, recorded when the car was completely "off", showed as 14.x, strongly suggesting the HV Battery was trying to keep the 12v charged.

Certainly, since the first time I saw those levels I have not experienced another 12v battery "failure".

 

[Joningy]

Thanks @Lovemyev @emmrecs @siteguru & @Martinonline and all other contributors.

It may well be the charge controller, but it certainly charges the auxiliary battery, during HV charging, it also works correctly whilst in "ready" or drive mode, it certainly is NOT charging whilst switched off and garaged.

As I mentioned a fair bit earlier in this thread, whilst the negative lead was disconnected for 24 hours in my garage, there was NO real detectable battery drain, so I think the battery IS fine, so that potential fault is perhaps eliminated, but too many total depletion cycles of any lead acid battery will eventually cause irreparable sulphation, hence the very poor, skinny 12 month warranty, this illustrates the crap quality of the MG 12v battery.

It was AI which suggested the 20 to 30% cut off for the HV battery to stop charging the auxiliary battery to protect it from a dangerously low SOC.

I searched the MGS5 owners manual earlier today, but was unable to find any info on charging the auxiliary battery by the HV battery!

Knowing the incredibly high workload of MG tech guys, causing incredibly laggy delay times in sorting customer problems, I think if I were an MG service centre operative, I'd probably be giving the heads up to the techies at head office of the problem so far, after all, when the car goes in next Wednesday (14th Jan) it'l be the 4th visit in it's under 4 months life, but 2nd in just over a week for this problem.

I've had lots of new cars over the decades, but cannot remember any of them needing a single return visit to the dealer for a fault, so 4 visits for a variety of reasons is very poor from MG.

I still like driving the MGS5, it's great value @ just over £24k brand new, very comfortable, extremely responsive in sport mode if you want to use all of the available power, but whether I remain with MG in the future, is under consideration, unless they pull their finger out.

 

[Joningy]

More AI, this time from ChatGPT.

Yes — on the MG S5 EV, the high-voltage (traction) battery does maintain the 12 V auxiliary battery even when the car is switched off and locked, but only periodically, not continuously.

How the MG S5 EV handles the 12 V battery when off​

DC-DC converter behaviour​

• The car uses a DC-DC converter (instead of an alternator) to step down HV battery voltage to ~14 V for the 12 V system.
• When the vehicle is off and locked, the DC-DC converter:
  
  • Wakes up intermittently
  • Checks 12 V battery voltage
  • Tops it up if needed
  • Then goes back to sleep

This is normal MG EV behaviour and is shared across MG4, MG5 EV, ZS EV, and S5 EV platforms.

---

What this means in real terms​

What​

• The 12 V battery is maintained automatically
• Short periods parked (days or weeks) are usually no problem
• Normal parasitic loads (alarm, telematics, keyless entry) are covered

What​

• It is not a constant charge
• If the 12 V battery is:
  
  • Old
  • Sulphated
  • Internally failing
    …the system may not rescue it

In those cases the car can still end up with a flat 12 V battery despite a full traction battery

---

When the car wakes itself up​

The S5 EV will briefly energise systems:

• After locking
• At scheduled intervals
• When remote services / telematics check in
• When HV battery SoC is healthy

You may hear:

• A faint click
• Relays engaging
• A brief whirr

That’s the DC-DC converter doing its job.

---

Important edge cases to be aware of​

HV battery very low​

• If the traction battery is critically low, the car may stop maintaining the 12 V
• This is a safety strategy

Long storage (many weeks)​

• MG still recommends:
  
  • Driving periodically or
  • Using a smart 12 V maintainer if the car is parked for a long time

After software updates​

• Some MGs temporarily increase standby drain post-update
• This can expose a weak 12 V battery

---

Practical advice (especially useful)​

• If your S5 EV is more than 2–3 years old, have the 12 V battery: (Mine is less than 4 months, there isn't an MGS5 EV 2-3 years old, so AI boobed on that one!)
  
  Load-tested (not just voltage-checked)
• AGM replacements last longer and cope better with EV duty cycles
• A Bluetooth 12 V monitor can confirm the top-up cycles are happening

If you want, I can:

• Explain how often the top-up typically occurs
• Recommend good AGM 12 V replacements for the S5 EV
• Help interpret Bluetooth monitor voltage graphs

Just tell me

 

[T1 Terry]

To load test a lead acid storage battery, divide the capacity by 20 (capacity is rated at C20, the load constant for 20 hrs) and find a load that equals that number of amps (a light bulb, watts divided by volts, in this case 12v, will give you the amps it will draw as a load)
While the load is across the two terminals (battery isolated from other loads) measure the voltage across the terminals each hr ..... it should hold above 12v for 10 hrs if it is up to scratch.

The problem may be, they have used a start battery as a storage battery, completely different construction method. Start batteries have a lot of thin material plates to provide big current (amps) for a short period. Storage batteries, in the lead acid format referred to as "Deep Cycle" have much thicker plates, designed for a long slow current (amps) draw over the rated time, generally C20 or capacity drawn at a constant load over 20 hrs ..... till the terminal voltage is around 9v .....

Start batteries have bugger all storage capacity, if it has a 30Ah rating marked on the case, it is a deep cycle battery, if it is a start battery it will have a CCS number (cold cranking amps) and an RC number, (reserve capacity) of how many minutes a constant 25 amps can be drawn till the voltage drops to 10.5v .... although, there never seems to be mention of the load being on or off when the 10.5v is measured .....
As there is generally some reference to voltage recovery over time, I presume it is with the load still across the battery .....

Hope that helped

T1 Terry

 

[Archev]

To load test a lead acid storage battery, divide the capacity by 20 (capacity is rated at C20, the load constant for 20 hrs) and find a load that equals that number of amps (a light bulb, watts divided by volts, in this case 12v, will give you the amps it will draw as a load)
While the load is across the two terminals (battery isolated from other loads) measure the voltage across the terminals each hr ..... it should hold above 12v for 10 hrs if it is up to scratch.

The problem may be, they have used a start battery as a storage battery, completely different construction method. Start batteries have a lot of thin material plates to provide big current (amps) for a short period. Storage batteries, in the lead acid format referred to as "Deep Cycle" have much thicker plates, designed for a long slow current (amps) draw over the rated time, generally C20 or capacity drawn at a constant load over 20 hrs ..... till the terminal voltage is around 9v .....

Start batteries have bugger all storage capacity, if it has a 30Ah rating marked on the case, it is a deep cycle battery, if it is a start battery it will have a CCS number (cold cranking amps) and an RC number, (reserve capacity) of how many minutes a constant 25 amps can be drawn till the voltage drops to 10.5v .... although, there never seems to be mention of the load being on or off when the 10.5v is measured .....
As there is generally some reference to voltage recovery over time, I presume it is with the load still across the battery .....

Hope that helped

T1 Terry

It was so much simpler in the horse drawn days. Vertical horse- moving- OK - not moving- add extra horse(s).
Horizontal horse - replacement required.

 

[T1 Terry]

It was so much simpler in the horse drawn days. Vertical horse- moving- OK - not moving- add extra horse(s).
Horizontal horse - replacement required.

Now, in our modern world, you need to add, flogging a dead horse, seems to be most popular pastime among upper management and politicians ..... both result in a similar outcomes ..... the next in line will try the same approach and expect a different outcome

T1 Terry

 

[Lovemyev]

Now, in our modern world, you need to add, flogging a dead horse, seems to be most popular pastime among upper management and politicians ..... both result in a similar outcomes ..... the next in line will try the same approach and expect a different outcome

T1 Terry

This is very true, people in government appear to spend more time dishing the dirty on each other and back biting, instead of focusing on getting the country out of the complete mess they have got it into.

 

[Joningy]

To load test a lead acid storage battery, divide the capacity by 20 (capacity is rated at C20, the load constant for 20 hrs) and find a load that equals that number of amps (a light bulb, watts divided by volts, in this case 12v, will give you the amps it will draw as a load)
While the load is across the two terminals (battery isolated from other loads) measure the voltage across the terminals each hr ..... it should hold above 12v for 10 hrs if it is up to scratch.

The problem may be, they have used a start battery as a storage battery, completely different construction method. Start batteries have a lot of thin material plates to provide big current (amps) for a short period. Storage batteries, in the lead acid format referred to as "Deep Cycle" have much thicker plates, designed for a long slow current (amps) draw over the rated time, generally C20 or capacity drawn at a constant load over 20 hrs ..... till the terminal voltage is around 9v .....

Start batteries have bugger all storage capacity, if it has a 30Ah rating marked on the case, it is a deep cycle battery, if it is a start battery it will have a CCS number (cold cranking amps) and an RC number, (reserve capacity) of how many minutes a constant 25 amps can be drawn till the voltage drops to 10.5v .... although, there never seems to be mention of the load being on or off when the 10.5v is measured .....
As there is generally some reference to voltage recovery over time, I presume it is with the load still across the battery .....

Hope that helped

T1 Terry

Hi again Terry, thanks again.
My DIY solar storage battery bank is the flooded lead acid type you refer to, i.e. colossal thick plates, each 2v cell weighs 50Kg, I have a bank of 24 of the heavy sods, very difficult to manoeuvre, but solid performance.