[sjb993]

No, not to do with charging this time (although it is linked to it)!

All the discussions about charging MHT cars got me thinking about jump starting. I'm no expert on the MHT system, and the Owner's Manual doesn't mention any special procedures for jump starting MHT cars. I'd be grateful if someone would enlighten me.

I assume that, if the starter/generator doesn't turn over the engine, the 48v battery doesn't have adequate charge or is flat. That would suggest that another 48v source should be connected to the "starting aid terminals" in the engine compartment to jump start the engine - i.e. another car's 12v battery won't do the job. Is that correct?

If so, does that mean that in MHT cars the "starting aid terminals" connect the external 48v source directly to the 48v battery or starter/generator?

a) If it does, doesn't that conflict with the iDrive message that the 48v system is deactivated when the bonnet is open (you couldn't close the bonnet with jump leads attached)?

b) If it does, and therefore there isn't a direct connection between the "starting aid terminals" and the 12v battery, does that explain the 12v battery charging problems?

TIA.

[HighlandPete]

Originally Posted by sjb993

If so, does that mean that in MHT cars the "starting aid terminals" connect the external 48v source directly to the 48v battery or starter/generator?

a) If it does, doesn't that conflict with the iDrive message that the 48v system is deactivated when the bonnet is open (you couldn't close the bonnet with jump leads attached)?

b) If it does, and therefore there isn't a direct connection between the "starting aid terminals" and the 12v battery, does that explain the 12v battery charging problems?

TIA.

Don't forget there is also a 12V pinion starter. Technical data states this is used for the first engine start (below 60C) of the day.

Without reading up on this I imagine any jump starting will involve the 12V starter.

Also if the 12V 'pinion' starter is worked on, or replaced, there is a specific procedure to disconnect the 48V system, "Deactivate the 48V electrical system by opening the bonnet", and procedure to reactivate. The final function is closing the hood/bonnet. "Activate the 48V electrical system by closing the bonnet. The activation takes place automatically".

A prerequisite for activating and deactivating the 48V battery, the ignition must be switched off.

[sjb993]

Thanks for putting me straight, I didn’t know there was also a conventional starter. When the technical data says “first start of the day”, do you think it actually means first start of the journey, i.e. the 48v starter/generator is only used for Auto Start/Stop?

[HighlandPete]

Originally Posted by sjb993

Thanks for putting me straight, I didn’t know there was also a conventional starter. When the technical data says “first start of the day”, do you think it actually means first start of the journey, i.e. the 48v starter/generator is only used for Auto Start/Stop?

The 60°C reference, is engine temperature.

The starting procedures are barely noticed. The first engine start of the day takes place by the pinion starter motor. If the engine temperature is greater than 60 °C, the combustion engine is started by the starter motor generator.

[sjb993]

Thanks for the explanation.

[Torqueman]

Interesting. So if the temp is less than 60 is the pinion starter used for the stop start system until 60 is reached?

[HighlandPete]

Originally Posted by Torqueman

Interesting. So if the temp is less than 60 is the pinion starter used for the stop start system until 60 is reached?

Data is not saying that, just the first start up, if the engine is less than 60C.

We know a first start is more demanding, particularly as the ambient temperature drops.

[Talisman77]

Originally Posted by HighlandPete

Data is not saying that, just the first start up, if the engine is less than 60C.

We know a first start is more demanding, particularly as the ambient temperature drops.

Curious but what makes a first start more demanding than say, an engine that has returned back to ambient for a while after a short drive earlier in the day?

[HighlandPete]

Originally Posted by Talisman77

Curious but what makes a first start more demanding than say, an engine that has returned back to ambient for a while after a short drive earlier in the day?

An interesting question. From observation and experience with many engines over the years, (including hand start), typically it is easier to start an engine after the initial overnight cold start. Even when an engine cools down during the day. Current oils do lessen the energy demands to crank, making it less critical than years ago.

For an electric start, it involves both the internal engine friction and the battery chemistry, lower temperatures having a greater influence. For a hand start you feel it in the strength required. Anyone who has started something like a Petter PH2 diesel or a Fordson E27N Major petrol/TVO will know the dread of a cold low temperature start. Definitely a "three shredded wheat" breakfast required.

The particular question you pose, probably very little difference in reality, compared to an overnight oil drain and 'weakened' battery state.

Not sure BMW have stated an 'absolute' function for MHT, may not be describing every situation. Would be interesting to know if the car is locked and goes to sleep, then used later in the day in cold temperatures, whether the pinion starter is used.

[sjb993]

I assume the MHT's "starter generator" keeps the 48v battery charged. Does the car also have a 12v alternator to charge the 12v battery?

TIA

[HighlandPete]

Originally Posted by sjb993

I assume the MHT's "starter generator" keeps the 48v battery charged. Does the car also have a 12v alternator to charge the 12v battery?

TIA

The 'generator' does charge the 48V battery, which of course is in constant change in SoC, according to demand/regeneration.

Also charging the 12V battery with a voltage converter as part of energy management.

[sjb993]

Thanks again for the explanation.

So, when the car is parked up in my garage for days on end, does the 48V battery keep the 12V battery topped up above its minimum voltage? Are there likely to be any battery problems when leaving the car for extended periods e.g. when away on a 2-3 week holiday?

[peelerec]

Originally Posted by sjb993

Thanks again for the explanation.

So, when the car is parked up in my garage for days on end, does the 48V battery keep the 12V battery topped up above its minimum voltage? Are there likely to be any battery problems when leaving the car for extended periods e.g. when away on a 2-3 week holiday?

Just my anecdotal experience now that I work remote and moved to Northern California where overnight ambient temperatures are 28-38 deg F. Leaving my car parked outside exposed (not in garage)to the elements ; longest it’s been left without being turned on is 12 days. Car started just fine.

[HighlandPete]

Originally Posted by sjb993

Thanks again for the explanation.

So, when the car is parked up in my garage for days on end, does the 48V battery keep the 12V battery topped up above its minimum voltage? Are there likely to be any battery problems when leaving the car for extended periods e.g. when away on a 2-3 week holiday?

The 48 V battery does top up the 12 V battery if the voltage drops below the critical threshold. Will do this until both are discharged, according to BMW data.

From SIB 61 01 21

c) Prolonged used of the “Emergency charging function” causes the 48 V battery to become totally discharged. This occurs when the 12 V battery is not properly maintained and the SOC of the 12 V battery drops below the critical threshold value while the vehicle is in sleep mode. The vehicle is woken up and recharged from 48 V battery. If the 12 V battery is not properly maintained, this cycle repeats until both batteries become totally discharged.