Thread: MHEV - Cranking Battery?

I have a RRS on order and trying to understand the same thing for the same reason.

The way I understand things at the moment is the 48v battery handles 'stop / start' duties and mild engine assist while driving so does do engine cranking via the 'starter / generator' but only while the ignition remains on.

The 12v battery (lithium I think) handles engine cranking via the conventional starter motor if the ignition has been cycled off and on. It also serves to run all other vehicle systems.

I think there is a DC/DC convertor between the 2 batteries allowing the 48v battery the charge the 12v battery.

It's early days for me so hoping someone with direct knowledge can confirm the above is correct.

Apologies for sort of hijacking the thread but I'm pretty sure they would both have the same common setup.

Yes, there are 2 starter motors. 12volt for cold starting and when the 48volt system needs charge and 48volt belt integrated starter generator (BSIG) which handles start/stop and harvests energy during braking. The 48volt system runs an electric supercharger to assist with turbo lag (you don't notice any compared to the D4) and torque for take off.

If you are looking for how to pick up 12volt for a dual battery system the best place is on the 12volt terminal of the DCDC under the cargo bay. This is connected directly to the 12volt battery positive and is convenient with factory earth points nearby.
I've just done this in my L663. If you join the FB New Defender group and do a search there are plenty of photos and descriptions.
Happy to post some pics of mine if you need. MHEV description attached if you haven't found before.

Thanks both, that is helpful. I had not picked up on the 2 starters.

I like the traxide set-up in my D4 and was pondering how to get something like it in the defender. I know Tim does not have a kit yet.

Your replies help.

Originally Posted by Drafrete

Thanks both, that is helpful. I had not picked up on the 2 starters.

I like the traxide set-up in my D4 and was pondering how to get something like it in the defender. I know Tim does not have a kit yet.

Your replies help.

I liked my Traxide units in both my D2 and D4. They just work for AGM.
If you are going lithium for auxiliary you'll, of course, need a DCDC. I had a Victron 30A in the D4 which ran uncomfortably hot.
I got a 50A Renogy for a good price so thought I'd try that for a while and it's working really well. 50A into my 200Ah battery works just great.
Might try the new 50A Victron down the track as it is a nice compact footprint.

Didn't know there was electric superchager involved.

Other things I don't understand is

1 does the existing DC/DC converter remains alive when the ignition is off allowing the 48v battery to top up the 12v battery when parked.
2 what the capacities of the 12v battery and 48v batteries are.

I do read reports of frequent low battery warning alarms from LR MEHV vehicles. So I'm assuming the answer to question 1 is 'No' and the answer to question 2 is they have relatively small capacities.

Originally Posted by Ferret

Didn't know there was electric superchager involved.

Other things I don't understand is

1 does the existing DC/DC converter remains alive when the ignition is off allowing the 48v battery to top up the 12v battery when parked.
2 what the capacities of the 12v battery and 48v batteries are.

I do read reports of frequent low battery warning alarms from LR MEHV vehicles. So I'm assuming the answer to question 1 is 'No' and the answer to question 2 is they have relatively small capacities.

1 - no idea but given the 48V system isn't required for starting from cold I would expect the 12V battery to be protected
2 - From the workshop manual the startup battery is H8 90Ah 850 CCA AGM, the 48volt lithium is 200Wh.
Had my D300 8 months and never seen a low battery warning. Had my first run with the auxiliary battery system running when camping last week and noticed that the Renogy DCDC stays connected for a short time after the car is shut down but no issues starting the next day.

That's fairly small for the 48 V system isn't it? 200wh = 0.2kWh = 4.2 Ah @48V

Compare to the main battery of 90 Ah which is about 1.1 kWh (1100 Wh) at 12V

If you have the DCDC charger, as soon as the main car voltage goes above threshold you will start to charge the auxiliary battery. I would imagine that when camping, if you wake the car up after using the auxiliary, the DCDC charger will draw immediately from the main battery, before you get to start the car. (input voltage goes high when the car wakes up, DCDC charger senses high battery voltage in the main battery and starts to push 40A or so to the auxiliary before the engine is running).

I think I would like to get a delay in the DCDC charger somehow so that the main batteries stabilise and the car starts before the aux starts to charge... Might need a delay timer? Just a thought.