[Tommygunn]

Originally Posted by sjb993

I assume you have an MHT car as you are going for the CTEK Pro25SE? That being the case the charger is used to charge the 48V battery via a 12V to 48V up converter. The 48V battery charges the 12V battery. So, you definitely need to connect your charger to the terminals under the bonnet (and charge with the bonnet open).

Yes my car is a 21 plate 340d MHT and I connected the CTEK PRO25SE to the terminals under the bonnet and left the bonnet open.
I then locked the car.

There were no clicking/switching noises whatsoever and as pointed out in another post the hazard warning light and the light under the light switches on the dash remained on until the battery was fully charged.

Full charge took approximately 1.5hrs

My question is: is it safe to leave the charger connected long term or am I better to disconnect it and connect it say once a month to charge up the battery ?

What are others with the PRO25 doing regarding above ?

I'm a little confused after reading the BMW bulletin here: BMW MHT Battery Charging.pdf

It talks about damage to the 48volt Hybrid battery but appears to be a bit contradictory and confusing.
Am I safe in thinking that the 48Volt battery will also charge from the 12V battery after the 12V battery is fully charged provided the bonnet is left open and the charger is left connected ?
Can anyone explain what the bulletin is actually trying to say ?


Finally I recommend the PRO25SE model over the PRO25 as it has a hanging bracket on the unit and the charging cable is around 5 meters long. Only negative is that the mains cable is short which limits hanging options.

As the PRO25S/SE is expensive, it might be a good idea to compile a list of other chargers that work for the MHT.
If anyone comes across one that works then just add it to the list to help others.

[Emily172013]

People, I think this is being a little over thought. Since the late 80's I've used a Battery Tender Plus on my three BMW's and my other cars and many motorcycles with both AGM and lead batteries w/o a problem. I put it on my 2020 M340i every three/four weeks for the night mainly in the winter when I not using it, doors open or locked, hood open or closed. It's all good.

[sjb993]

Originally Posted by Tommygunn

Yes my car is a 21 plate 340d MHT and I connected the CTEK PRO25SE to the terminals under the bonnet and left the bonnet open.
I then locked the car.

There were no clicking/switching noises whatsoever and as pointed out in another post the hazard warning light and the light under the light switches on the dash remained on until the battery was fully charged.

Full charge took approximately 1.5hrs

My question is: is it safe to leave the charger connected long term or am I better to disconnect it and connect it say once a month to charge up the battery ?

What are others with the PRO25 doing regarding above ?

I'm a little confused after reading the BMW bulletin here: Attachment 3310588

It talks about damage to the 48volt Hybrid battery but appears to be a bit contradictory and confusing.
Am I safe in thinking that the 48Volt battery will also charge from the 12V battery after the 12V battery is fully charged provided the bonnet is left open and the charger is left connected ?
Can anyone explain what the bulletin is actually trying to say ?


Finally I recommend the PRO25SE model over the PRO25 as it has a hanging bracket on the unit and the charging cable is around 5 meters long. Only negative is that the mains cable is short which limits hanging options.

As the PRO25S/SE is expensive, it might be a good idea to compile a list of other chargers that work for the MHT.
If anyone comes across one that works then just add it to the list to help others.

The charger charges the 48V battery and not 24V battery - it does so via a 12V to 48V step-up converter. The 48V battery charges the 12V battery.

My car isn't used regulary and so I leave the charger connected to the car, and use a timer socket to turn it on overnight on for 4 hours per day.

[Tommygunn]

Originally Posted by sjb993

The charger charges the 48V battery and not 24V battery - it does so via a 12V to 48V step-up converter. The 48V battery charges the 12V battery.

My car isn't used regulary and so I leave the charger connected to the car, and use a timer socket to turn it on overnight on for 4 hours per day.

I'm confused as I've got the charger in "Normal" mode which is for charging lead acid and AGM battery which is why I presume it's charging the 12V battery in the boot.

Have you got yours set to "Lithium" and how is it connected to enable charging of the 48V lithium Hybrid battery ?

From what you are saying does it mean that when the car is driven it charges the 48V Hybrid battery first which in turn then charges the 12V AGM battery ?

[Tommygunn]

Originally Posted by Emily172013

People, I think this is being a little over thought. Since the late 80's I've used a Battery Tender Plus on my three BMW's and my other cars and many motorcycles with both AGM and lead batteries w/o a problem. I put it on my 2020 M340i every three/four weeks for the night mainly in the winter when I not using it, doors open or locked, hood open or closed. It's all good.

Is your M340i the MHT variant ?

[TurtleBoy]

Originally Posted by Tommygunn

I'm confused as I've got the charger in "Normal" mode which is for charging lead acid and AGM battery which is why I presume it's charging the 12V battery in the boot.

Have you got yours set to "Lithium" and how is it connected to enable charging of the 48V lithium Hybrid battery ?

From what you are saying does it mean that when the car is driven it charges the 48V Hybrid battery first which in turn then charges the 12V AGM battery ?

You should have the charger set to AGM since it is charging the 12v battery and not the 48v. Most chargers are not capable of charging the 48v battery since they need to be putting out at least 20amps to do so. If you had a charger capable of that it would all happen automatically when the 12v battery was charged the charging would be switched to the 48v battery using the converter.

Edit: Here is a document and some more information about the 48v system that has been posted in other threads. There is a bidirectional charge controller that can charge the 12 volt battery from the 48 volt system, or charge the 48 volt battery from the 12 volt system. Leaving the hood up tells the controller to shut down the 48 volt system and allows both batteries to charge from an external 12 volt source. With the hood closed, the 48 volt system charges both batteries while driving.


Link for mobile app users -48v Electrical System

48V vehicle electrical system.pdf

[Emily172013]

Originally Posted by Tommygunn

Is your M340i the MHT variant ?

No, I ordered the car in Oct. 2019, received it Nov. 2019, shortly before they stopped making them in Germany.

[TurtleBoy]

Originally Posted by Emily172013

No, I ordered the car in Oct. 2019, received it Nov. 2019, shortly before they stopped making them in Germany.

Then your experience doesn't apply since it is a different system.

[Tommygunn]

I'm still confused regarding the charging process using the CTEK PRO25S charger.

If the charger is first charging the 48V hybrid battery which then charges the 12V battery then is it really a good idea to leave the charger connected long-term since having full charge all the time on the 48V lithium battery is not a good idea and will decrease the lifespan of the battery ?
Usually lithium batteries are best stored at around 50-60% charge.

[TurtleBoy]

Originally Posted by Tommygunn

I'm still confused regarding the charging process using the CTEK PRO25S charger.

If the charger is first charging the 48V hybrid battery which then charges the 12V battery then is it really a good idea to leave the charger connected long-term since having full charge all the time on the 48V lithium battery is not a good idea and will decrease the lifespan of the battery ?
Usually lithium batteries are best stored at around 50-60% charge.

As mentioned above, it is not charging the 48v battery under most circumstances unless you have a charger capable of it. The 12v battery is always being charged/mainttained from the charger when there is one connected. Either way, keeping the charger attached will have no negative effect on the lifespan of the battery but should increase the lifespan of your 12v battery. Just because the charger is connected does not mean the batteries are being charged as the IBS and a controller is involved in decided when they need to be charged and to what level.

[zazzau]

Originally Posted by Tommygunn

I'm still confused regarding the charging process using the CTEK PRO25S charger.

If the charger is first charging the 48V hybrid battery which then charges the 12V battery then is it really a good idea to leave the charger connected long-term since having full charge all the time on the 48V lithium battery is not a good idea and will decrease the lifespan of the battery ?
Usually lithium batteries are best stored at around 50-60% charge.

The 48v doesn’t get charged directly. That said, Lithium-Ion batteries and AGM batteries have different needs for maximum life. The latter needing to be fully charged for maximum possible lifetime from the battery; the former does not need to be fully charged. So for a vehicle that’s idle for an extended period of time, a charger/tender could be beneficial in maintaining the charge of the AGM battery while parked and being drained by systems in the car that never actually go to sleep. It’s one reason why display cars at the dealership have a specialized tender permanently hooked under the hood.

[Tommygunn]

Originally Posted by TurtleBoy

As mentioned above, it is not charging the 48v battery under most circumstances unless you have a charger capable of it. The 12v battery is always being charged/mainttained from the charger when there is one connected. Either way, keeping the charger attached will have no negative effect on the lifespan of the battery but should increase the lifespan of your 12v battery. Just because the charger is connected does not mean the batteries are being charged as the IBS and a controller is involved in decided when they need to be charged and to what level.

According to the BMW charging bulletin, any charger above 20A is capable of charging the 48V battery too. The CTEK Pro25S puts out 25A max.

I have contacted both CTEK and BMW UK regarding long term use of a 25A charger on a MHT vehicle and if it results in any possible issues with the 48V battery and will post their replies here.

I'm aware that when the battery is fully charged the charger will just monitor the battery and charge it up if it drops below a certain level.

[TurtleBoy]

Originally Posted by Tommygunn

According to the BMW charging bulletin, any charger above 20A is capable of charging the 48V battery too. The CTEK Pro25S puts out 25A max.

I have contacted both CTEK and BMW UK regarding long term use of a 25A charger on a MHT vehicle and if it results in any possible issues with the 48V battery and will post their replies here.

I'm aware that when the battery is fully charged the charger will just monitor the battery and charge it up if it drops below a certain level.

Don't forget it is the vehicle that controls which battery is being charged and to what level not the charger. There should be zero concern about the 48v battery life. Like other LiOn batteries they could be saving a reserve at the top or bottom or the PCU/ECU will cut off charging a lower level like many EV charging processes recommend. Think about all of the hours the 48v battery can be charged while you are driving, it is not a simple alternator connection there is software and various controls involved.

[Emily172013]

Originally Posted by TurtleBoy

Then your experience doesn't apply since it is a different system.

Maybe not, all I know is that it too has a LI battery under the hood.

[TurtleBoy]

Originally Posted by Emily172013

Maybe not, all I know is that it too has a LI battery under the hood.

The dual storage systems use a 12v lithium ion battery. The charging for those is “normal”.

[Emily172013]

Originally Posted by TurtleBoy

The dual storage systems use a 12v lithium ion battery. The charging for those is “normal”.

Thanks, that's good to know.

[Tommygunn]

Originally Posted by TurtleBoy

Don't forget it is the vehicle that controls which battery is being charged and to what level not the charger. There should be zero concern about the 48v battery life. Like other LiOn batteries they could be saving a reserve at the top or bottom or the PCU/ECU will cut off charging a lower level like many EV charging processes recommend. Think about all of the hours the 48v battery can be charged while you are driving, it is not a simple alternator connection there is software and various controls involved.

Would that not be the case when the car is being driven ?

I wouldn't think the car could control charging of it's batteries when it's asleep ?

[Tommygunn]

So far I've had no luck finding any answers from BMW UK direct or their dealers

I got a reply back from CTEK ( I had sent them the BMW bulletins regarding battery charging on MHT cars which I got from previous posts on here ), see below:


"It seems you got the same experience of BMW dealers as me.

I got to see those bulletins back when I did the investigation on why a MXS 5.0 won't charge BMW mild-hybrids. The BMW workshop/dealer where I live only gave me nightmares in regards of how incompetent the employees were.

Another customer tried contacting the general agent for BMW here in Sweden but got no good answer from them either.

BMW and other manufacturers don't mention this and that's what triggered me to investigate it. With 10 years experience as a diagnostician for other German brands, when my investigation concluded, I figured out that mild hybrids requires at least 20A chargers because of the fact that they automatically charge the 48V battery. So when using something weaker it won't charge at all, not even the 12V.

As I previously said, from the chargers point of view there shouldn't be any issue. However I cannot know for certain how it will affect the 48V battery. It's a lithium battery so there shouldn't be a problem but I'm not 100% sure. Connect it as it states in the user manual and make pauses now and then is what I would've done."

I hope this helps.


Med vänliga hälsningar/Best Regards,
Mirsad Fazlic


Basically I have two options:

A- Leave the CTEK PRO25SE connected all the time
B- Charge battery to full using the PRO25SE then remove and repeat every 2-3 weeks

How are other owners of the CTEK PRO25S using it ?

[Tommygunn]

Originally Posted by sjb993

The charger charges the 48V battery and not 24V battery - it does so via a 12V to 48V step-up converter. The 48V battery charges the 12V battery.

My car isn't used regulary and so I leave the charger connected to the car, and use a timer socket to turn it on overnight on for 4 hours per day.

When using it with a timer socket does the charger revert back to the start stage for charging each time it is switched on by the timer socket and then proceeds through the other stages until it reaches maintenance mode ?

Leaving the charger connected without the timer socket will mean it stays in maintenance mode all the time and will be monitoring the battery and only charging it up if it falls below a certain voltage.

Do you have any idea which battery it is monitoring in the maintenance mode, the 12V or the 48V lithium one ?

[zazzau]

Originally Posted by Tommygunn

Leaving the charger connected without the timer socket will mean it stays in maintenance mode all the time and will be monitoring the battery and only charging it up if it falls below a certain voltage.

Do you have any idea which battery it is monitoring in the maintenance mode, the 12V or the 48V lithium one ?

If leaving it plugged permanently then you also want to leave your hood closed (latched) so it only charges/monitors the AGM. This is effectively how it’s done in the showroom at the dealership.

[TurtleBoy]

Originally Posted by Tommygunn

Would that not be the case when the car is being driven ?

I wouldn't think the car could control charging of it's batteries when it's asleep ?

Yes, that is what I'm saying. It takes care of the charging at all times as it essentially doesn't sleep for that.

[TurtleBoy]

Originally Posted by zazzau

If leaving it plugged permanently then you also want to leave your hood closed (latched) so it only charges/monitors the AGM. This is effectively how it’s done in the showroom at the dealership.

Definitely do not do this as you will drain your 48v battery. If you close the hood for a long period of time while on the charger your 48v battery will likely need to be replaced. Many dealers found this out and is why they disconnect the 48v from the system when having the hood closed.