Thread: MHEV DC to DC Converter Failures

My D350 L405 was only producing 60-70A near the end of 2x20km drives today.

Originally Posted by Graeme

My D350 L405 was only producing 60-70A near the end of 2x20km drives today.

Hi Graeme, did you by any chance note the voltages during your trips?

No, I didn't. My previous voltages were taken from a display not visible when driving and I didn't think to have the IIDtool display the converter's o/p voltage, assuming that it can. I'll try again.

I didn't get to check right at start-up but after a few minutes idling while on a phone call the current was briefly up to 117A then down to 80-90A for the 1st km or so before settling to around 60A. The converter supply voltage from the IIDtool was 14.8V for a few moments initially before dropping to 14.5V where it stayed for the 8km drive. I stopped a couple of times to check my display in my rear side window which showed 14.3V every time.
At start-up to head home, voltage was 15.0V, then 14.8V for a few seconds then back to a solid 14.5V.
After the engine was switched off the voltage soon dropped to 12.5V and my display showed 12.2V but I had since opened then closed the soft-close door.

Edit: The 12V battery is original and now 5 years old.

Originally Posted by Graeme

At start-up to head home, voltage was 15.0V, then 14.8V for a few seconds then back to a solid 14.5V.
After the engine was switched off the voltage soon dropped to 12.5V and my display showed 12.2V but I had since opened then closed the soft-close door.

Edit: The 12V battery is original and now 5 years old.

Thanks Graeme for the feedback and it is starting to confirm my suspicions that you may be able to charge a Lithium Auxiliary battery directly off one of these vehicle DC/DC units.

This would mean you would not need a DC/DC device and while my existing isolators would work fine, after seeing the video Mahn posted, I have decided to modify the operation of the isolator so that on cold morning starts, the isolator will not connect the auxiliary battery to the Defenders power supply for a short period of time, to allow the Defenders PTC Heater to run and shut down before adding the high current load a low auxiliary battery would apply to the power supply.

This would greatly reduce the chances of over loading the Defenders DC/DC unit.

Please keep sending info about your operating voltages. The info is very much appreciated!

My Kings 25A DCDC charger's operating manual states that it doesn't connect for up to 2 minutes after receiving supply power and that it disconnects supply for a little while every 2 minutes to check the Kings Lithium battery's status, so perhaps a DCDC charger might still be required. I suspect the start-up delay is just when its internal 2 minute timer expires so it could sometimes switch on immediately. However delaying switch-on might go a long way to not adding more initial load for the converter.

Originally Posted by Graeme

However delaying switch-on might go a long way to not adding more initial load for the converter.

Yesterday morning I set my GAP IID to live values and watched the DCDC Current output, It started at about 100 amps but when i started driving it went up as high as 175ish amps until it settled down to 60A for normal driving.

This morning I started the car and let idle for 10mins before heading off. It only got to 105A and stayed there for 10ish minutes then settled to 60A and stayed there.

So my totally unscientific conclusion is to warm the car up for a bit before heading off in the morning if you can. Especially if you have been drawing on your aux battery overnight. I've had 2 fridges running 24/7 in mine with minimal driving.