[sjb993]

Because I use my car relatively infrequently I was keen to periodically charge the battery. After discovering that my existing CTEK 3.8A charger wasn't charging my battery (even though the status LEDs indicated a full charge), I tried to find out why. Cutting a very, very long story short I found out that BMW have stated a charger with an output >20A is required. The only reports of successful MHT charging I could find were from 2 American guys on the General Discussions section of this forum, and they had both used CTEK 25A chargers. As there seems to be no alternative, I went ahead and bought a CTEK Pro25s.

It does seem to do the job. Before I hooked it up my battery had an estimated SoC of 55%. It then took roughly 11 eleven hours charging to raise the estimated Soc to 100%.

[paulockenden]

The number don’t make sense to me. 11 hrs at 25A is 275Ah. And if that tops up the battery from 55% it means the total battery capacity is over 600Ah.

But I think the actual OEM battery is around 80Ah.

Most strange.

[sjb993]

Here's the charging cycle shown in the manual.

My OEM battery is 92Ah.

[Soul_Glo]

Bravo to you sir!

[Soul_Glo]

Admin can we sticky this somewhere for all those struggling to figure out the charge situation. Thanks for considering.

[Z_Victor1]

Excellent work, well done.....!!!
It's a shame that charger is SO expensive though...

[sjb993]

Originally Posted by Z_Victor1

Excellent work, well done.....!!!
It's a shame that charger is SO expensive though...

It certainly is, but have you seen the price of the Noco GeniusPro25?!!! It's just as well that most MHT drivers, who use their cars more regularly than I do, shouldn't need one.

Thanks again for bringing to light the Service Information Bulletin.

[HighlandPete]

Originally Posted by sjb993

It certainly is, but have you seen the price of the Noco GeniusPro25?!!! It's just as well that most MHT drivers, who use their cars more regularly than I do, shouldn't need one.

Thanks again for bringing to light the Service Information Bulletin.

There's a post on one of the other charging threads, just yesterday, (very frustrated forum member), where the battery still won't charge with a high output charger. Same "clicking sounds" as the smaller charger.

Something still doesn't add up, where some can charge with a 5amp charger, some only with a 25amp, why no consistency?

[sjb993]

It is strange that some can't charge their batteries with >20A chargers.

In the case of reported success with 5A chargers, I wonder if those people are actually checking their state of charge. Or, are they relying on their charger's status indication LEDs? My 3.8A CTEK indicated that it had fully charged my battery, but it didn't increase the SoC at all.

[paulockenden]

But what is SoC as reported by the car actually measuring? Does anyone know? It can’t be voltage because that’s what the charger has measured and seen rise to the fully-topped-up value.

[HighlandPete]

Originally Posted by paulockenden

But what is SoC as reported by the car actually measuring? Does anyone know? It can’t be voltage because that’s what the charger has measured and seen rise to the fully-topped-up value.

This is also something I don't understand, as a charger like the CTEK MXS 5.0 moves to Step 4 at about 80% SoC. It must be getting changes in battery 'feedback' to move onto step 4 and through to step 7, float.

STEP 3 BULK
Charging with maximum current until approximately 80% battery capacity.

[paulockenden]

I suspect that SoC as reported by the car is just being too clever. If it knows the car was at 70% (say) and hasn’t been charged by the car itself, then it probably thinks it’s still 70%, even though you’ve charged externally.

But then that doesn’t explain why (sometimes) using a meatier charger will update the SoC value. Someone needs to put a hall-effect DC (current clamp) monitor on one of the charging leads and again on one of the battery leads and see what’s really happening.

[Z_Victor1]

Interesting to just check the battery voltage directly across the terminals in the boot before and after charging.
Whilst it won't give an accurate SoC, it'll give a rough estimate of the battery's state before and after.

[Bianchi Dave]

Interesting and timely thread. Having never had battery issues in any other car over 33 years of driving, today my M340i decided to throw a low battery warning.
Admittedly it hasn't been used much the last month or so and today was the first time it moved in 10 days!
So I'd better buy a charger - I guess the CTEK mentioned up thread is the best one to get??
Cheers

[NLJ]

Dumb question but why does one need to be worried about the battery for the MHT system?
Even if it is depleted, doesn't the running engine or coasting charge it back up?
The 12v batteyis what is important to get the car started is it not?

[sjb993]

Bianchi Dave and NLJ,

You may find this attachment interesting. It's a BMW Service Information Bulletin with instructions for dealerships on how to maintain MHT car batteries either in storage or on the showroom floor.

(FWIW - CTEK 25A chargers are used by BMW dealerships.)

[NLJ]

Originally Posted by sjb993

Bianchi Dave and NLJ,

You may find this attachment interesting. It's a BMW Service Information Bulletin with instructions for dealerships on how to maintain MHT car batteries either in storage or on the showroom floor.

(FWIW - CTEK 25A chargers are used by BMW dealerships.)

So realistically when do I have to worry about this?
If I leave for a 3 week vacation, should I worry about the 48v battery?
I've never needed to worry about my 12v for this period of time.

[sjb993]

Originally Posted by NLJ

So realistically when do I have to worry about this?
If I leave for a 3 week vacation, should I worry about the 48v battery?
I've never needed to worry about my 12v for this period of time.

I asked the same question in another thread and was assured by several members that it won't be a problem, based on their own experiences. It really shouldn't be a problem, if it was I think we'd have heard more about it by now.

But, just to be on the safe side, I'm going to get mine fully charged before I go away.

[HighlandPete]

Originally Posted by NLJ

So realistically when do I have to worry about this?
If I leave for a 3 week vacation, should I worry about the 48v battery?
I've never needed to worry about my 12v for this period of time.

One thing to consider before leaving the vehicle standing, how charged is the battery when you leave it? Could be quite low SoC, if not used regularly before the vacation.

48V system makes it more complex. BMW state the 48 Volt battery will keep the 12 Volt battery above a certain SoC, until both depleted.

As to how long even a 12 Volt system stays reasonably charged, BMW posted the following, during Covid for UK users. (Bold text added by me).

Battery

If, as part of your essential-only travel during the coronavirus outbreak, your engine runs for approximately 20 minutes once a week, the battery in your car is unlikely to go flat.

Car batteries, like any battery, discharge when not used. Your car takes some power from the battery even when locked and parked, to power essentials such as the alarm.

How long you can leave your car without starting depends on the condition of your car's 12-volt battery. A BMW with a healthy battery should last at least 2 weeks, without needing to be started up to re-charge.

Try to minimise excessive 'wake ups' of your car’s electrical systems. Avoid locking/unlocking or switching on the ignition without driving the car where possible. Start the engine before listening to the radio or using your headlights, heating or air conditioning system.

If there’s any doubt about the condition of the battery, or the car will not be used for longer than three weeks, we suggest you connect it to a BMW Battery Charger or a suitable trickle charger. Please note the car’s warranty covers the battery for manufacturing defects but not for depletion as a result of lack of use.

My 5-series definitely falls into the category for needed charging, if not used regularly. The closed circuit (sleeping) current draw is clearly quite high, even without issues, or warnings that it's above the design threshold.

[NLJ]

Originally Posted by HighlandPete

My 5-series definitely falls into the category for needed charging, if not used regularly. The closed circuit (sleeping) current draw is clearly quite high, even without issues, or warnings that it's above the design threshold.

I drive mine every day except the weekends when we use the family vehicle.
20km a day round trip. How do I check the state of charge for the 48v battery?
I have the LCI, does it show a percentage somewhere?

[Bianchi Dave]

Just another thought, and I can't seem to find a definitive answer -
I know batteries are not covered under the insured used warranty, but does the manufacturer 3 year warranty not cover it?

[HighlandPete]

Originally Posted by Bianchi Dave

Just another thought, and I can't seem to find a definitive answer -
I know batteries are not covered under the insured used warranty, but does the manufacturer 3 year warranty not cover it?

Isn't the issue defining whether it is a manufacturing defect, or simply a customer letting it go flat and it being unrecoverable.

There is documentation on how BMW dealers need to look after batteries on stock/showroom cars, to prevent too low a state of charge. Keeping charging calendars, etc.