question

brokenfeet avatar image
brokenfeet asked

Connect "ground" of load output to vehicle body? MPPT 75/15

Hi everyone!

Is there any reason I should or should not connect the ground/- pin of the load output to the body of the vehicle?

I'm having issues with my fridge (Dometic CB40 compressor, 60W) shutting down due to low voltage, so I connected the fridge as the only load to the load output of the MPPT 75/15. I used 6mm² cables, roundabout 1.5-2m length. In this first setup, ground of load output was connected directly to the ground pin of the fridge.
With that setup I was able to measure the current (up to 7A) that the fridge used. Unfortunately I still had a misterious voltage drop of 0.5V-1.0V along this connection, so the fridge would shut down due to low voltage (<10.5V).

Now I have connected the ground of the fridge to the vehicle body and the voltage seems to drop significantly less, at least the fridge doesn't swithc of any more....

But it seems that I can't measure the load current anymore?


Grounding
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

2 Answers
wkirby avatar image
wkirby answered ·

The current measurement shunt inside the MPPT is a low side shunt, so if you connect the negative of the load to a negative point other than the load output negative then the shunt will be bypassed and will not measure any current.

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

xmd avatar image
xmd answered ·

I have my load negative wired to the vehicle body, and it seems to work fine (in that it can accurately measure the amps on the Load output). However, I'm having a different issue which is that the load output won't turn on reliably in all cases: See https://community.victronenergy.com/questions/88996/mppt-smartsolar-load-output-cant-turn-on-if-load-a.html I supopse this could be a related issue?

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Related Resources

Ground, earth and electrical safety


Additional resources still need to be added for this topic