Thread: MHEV DC to DC Converter Failures

Hi Mahn and thanks for a really interesting video.

But while the intention was to see how much current was being drawn through the DC/DC device at a cold start, the gentleman actually removed one of the biggest current draws before he even started.

These vehicles are renowned for using up huge amounts of battery capacity while the motor is off and a cold lead acid battery draws far more charge current than a warm battery does.

By charging the battery, he not only replaced the battery energy that would normally be done at the time of starting the motor, but because the battery charging would have warmed up the cranking battery, he removed anything from 70 to 100 amps of additional current load from his test.

That is still an eye-opening amount of current being displayed in that video but unfortunately he did not indicate what the DC/DC device was designed to provide and I have researched this very issue and also have not found any specific specs for these converters.


Thanks again for that very informative video.

Some background to the first video

So a mid-winter towing scenario might be 160A from the vehicle systems plus another, say, 50A for the caravan charger.
It will be good to monitor on the GAP.

And these are an improvement over my L322?

See also:

My ex-4.4 TDV8 L322 with its hydraulic power steering and mechanically-driven engine fan had a 220A alternator. I would expect a vehicle with electric power steering, electric engine cooling fan and more electrics (eg the soft close doors, electric upper and lower tailgates on my D350 L405) to require a DC-DC converter with far greater capacity, possibly around 300A.
The converter has cooling fins but without airflow I suspect that it gets quite hot. A 12V alternator has air flowing through it from the engine fan. I don't know where the converter is mounted on my L405 but I hope that it isn't the large aluminium box in the well under the driver's seat.

Originally Posted by Graeme

A 12V alternator has air flowing through it from the engine fan.

The alternator usually also has its own cooling fan. (The alternator in my L322 BMW M62 engine is water cooled although that may be stretching the truth a bit. It has engine coolant running around it and that’s around 95 deg C.)

On my MY23.5 D300 defender, the DC converter failed last August, it was after a night free camping. I suspect the car battery draw plus the caravan draw plus the Auxilery battery draw (two fridges running) are what broke the camels back. My Aux battery is charged by a Victron 30amp DC|DC charger, I don't know what the Land Rover trailer plug supplies to the Caravan.

In hind sight I have disconnected the carvan power, I may put a switch on it so I can select when it starts to draw power. I will also replace my Victron DC|DC converter with the newer XS model that can be regulated to supply a set current. Beyond that there is not a lot one can do except be mindful of heater usage ect.

For such a expensive car I think we could reasonably expect that the 12v DC power supply would have it's own overload protection. I don't know if anyone has had an out of warrenty claim on this part in Australia, but I suspect consumer law would protect you against any cost?

Very annoying!

Have you tried camping with the replacement converter? Did the engine keep running until the 12V battery got too low?

My aux is only a 60Ah Lithium via a 25A DCDC charger and my van has a single AGM, both via a VSR rather than engine-running sense, but I don't know how helpful that might be. I wouldn't appreciate being stranded by a failed converter, not that the 12V battery would survive any useful time without being charged.