When connecting car starter battery to DC / MPPT charger, should I complete the loop to the car ground?

[uksuntimes]

I'm curious about if I have completed my install correctly. So here is the simplified version (I also have solar but am excluding from this for now for simplicity)

1. Car battery + goes to Renogy MPPT/DC charger on the Alternator+ Input
2. MPPT/DC charger is connected to + Positive Bus Bar and - Negative Bus bar
3. LifePo4+ goes to + Positive Bus bar and - Negative Bus bar
4. Currently, I then also connect the negative bus bar to the car chassis/ground as well
5. There is a wire that tells the charger when the engine is running, and then proceeds to use the alternator to charge from. When the car is off, this stops.

Is connecting the bus bar to the battery negative but also the chassis/car battery negative correct?

The MPPT has a function where it will use the solar input to trickle charge the car battery if the house battery is at 100%. I am assuming that this will only work if I have completed the 'ground' connection to the chassis as well, but am worried I might create some kind of loop or phantom power drain of the main starter battery as a result.

 

[justinm001]

In almost all cases it makes the most sense to chassis ground both the house and starter battery systems. It doesn't cause any power drain because the positive isn't connected anywhere. This is for safety so a wire is lose it'll hit chassis ground and blow the fuse.

Yes its correct. You want to connect the negative wire to everything but also have the ground as a backup to complete circuits.

It does/can create ground loops which can cause interference with RF signals. This can cause static in devices or hums.

 

[uksuntimes]

Thank you for that knowledge, I really appreciate it.

I just did a test whereby I disconnected the house battery and the solar inputs entirely, and only had the starter battery connected. It appears that it does light up the MPPT charge controller and sees a draw of 1amp from the starter battery, even though the car is not on and it apparently isn't meant to be drawing anything from the starter battery. Which I find odd/concerning.

However, I am going to put that down to not having my main house battery or solar connected and that the Renogy MPPT simply will take the power if nothing else is available. I'll have to get everything connected again and then see if there are still any amps being pulled. Hopefully not. But that's a different thing.

Thank you for at least confirming that having all my negatives connected, including to the chassis/starter battery is ok.

 

[12VoltInstalls]

am assuming that this will only work if I have completed the 'ground' connection to the chassis as well, but am worried I might create some kind of loop or phantom power drain of the main starter battery as a result.

Best practice is to do one homerun from the busbar neg(-) to the car battery neg(-).
Why?

1. The vehicle chassis may not have a good- or large enough- connection to the vehicle battery neg(-) post
2. In a ‘fault,’ you want to guarantee that the fuse/breaker will open rather than smoking a wire while the fuse ‘misses’ the current which would otherwise blow the fuse reestablishing a safe condition.
3. With postmodern canbus systems transients or ghost voltage/current can give the various vehicle control modules a panic attack requiring hospitalization
4. Vehicle bodies and chassis are not the best conductors in a a circuit
5. Alternate ground paths can create multiple avenues through components that weren’t designed for the current or even current direction; continuity (current’s ability to pass) happens with a difference in voltage potential, not just a “black cable.”

 

[uksuntimes]

Best practice is to do one homerun from the busbar neg(-) to the car battery neg(-).
Why?

1. The vehicle chassis may not have a good- or large enough- connection to the vehicle battery neg(-) post
2. In a ‘fault,’ you want to guarantee that the fuse/breaker will open rather than smoking a wire while the fuse ‘misses’ the current which would otherwise blow the fuse reestablishing a safe condition.
3. With postmodern canbus systems transients or ghost voltage/current can give the various vehicle control modules a panic attack requiring hospitalization
4. Vehicle bodies and chassis are not the best conductors in a a circuit
5. Alternate ground paths can create multiple avenues through components that weren’t designed for the current or even current direction; continuity (current’s ability to pass) happens with a difference in voltage potential, not just a “black cable.”

This is a really good point and I will look at running a direct cable at my earliest.

There is already a dedicated positive coming from the battery to the back of the car, so I would hope I can follow the same route back for the negative. But this is how the car comes from the factory so I anticipate likely finding my own path back through the car and the firewall.

 

[12VoltInstalls]

It does/can create ground loops which can cause interference with RF signals. This can cause static in devices or hums

Homerun to vehicle battery from busbar in a setup as described nearly always mitigates that possibility. It certainly keeps the late model millivolt sensing happy.

 

[justinm001]

Homerun to vehicle battery from busbar in a setup as described nearly always mitigates that possibility. It certainly keeps the late model millivolt sensing happy.

Yes homerun the wires BUT both sources should be chassis grounded for safety. Otherwise you could have house + loose cable touching the frame and the fuse wouldn't blow until you touched the chassis and a ground source like a metal inverter or mppt. Then when you complete the circuit the fuse will blow.

 

[uksuntimes]

Thank you, all good info.

I have a number of fuses, switches and breakers in my system as well, including a class T for the lifepo... am doing my best to over spec on safety as best I can. No desire to fry myself or my car for the sake of some lights and power

I also fully disconnect my solar panels as an extra precaution when doing work.

If I am to understand correctly, by having a direct cable to the car negative AND the chassis ground connected, the current would follow the path of least resistance and thus if any spikes did somehow manage to get through, they would primarily take the main cable as their path rather than the chassis?

 

[justinm001]

Thank you, all good info.

I have a number of fuses, switches and breakers in my system as well, including a class T for the lifepo... am doing my best to over spec on safety as best I can. No desire to fry myself or my car for the sake of some lights and power

I also fully disconnect my solar panels as an extra precaution when doing work.

If I am to understand correctly, by having a direct cable to the car negative AND the chassis ground connected, the current would follow the path of least resistance and thus if any spikes did somehow manage to get through, they would primarily take the main cable as their path rather than the chassis?

Correct.

 

[12VoltInstalls]

both sources should be chassis grounded for safety.

If I am to understand correctly, by having a direct cable to the car negative AND the chassis ground connected, the current would follow the path of least resistance and thus if any spikes did somehow manage to get through, they would primarily take the main cable as their path rather than the chassis?

No. Electricity takes all available ‘paths.’

On the DC side a homerun from busbar to battery, add an equal-sized cable and connect at/near the vehicle battery contacts the body/chassis.
You can add one Ground stacked on top of inverter green/bare/ground connection at busbar (see below) but imho the heavy cabling should be done at the vehicle battery.

On the AC side see your inverter manual but the green/bare/ground should go to the busbar.
Read inverter manual carefully: some are N-G bonded, some are not, some will zap out if you bond it.

 

[justinm001]

No. Electricity takes all available ‘paths.’

On the DC side a homerun from busbar to battery, add an equal-sized cable and connect at/near the vehicle battery contacts the body/chassis.
You can add one Ground stacked on top of inverter green/bare/ground connection at busbar (see below) but imho the heavy cabling should be done at the vehicle battery.

On the AC side see your inverter manual but the green/bare/ground should go to the busbar.
Read inverter manual carefully: some are N-G bonded, some are not, some will zap out if you bond it.

Yes it takes all paths BUT 99.99% takes the least resistive path. So if you have the chassis connected to the negative AND a proper sized cable 99.99% will take the wire path. This small small amount that doesn't take the path won't cause interference or other issues. Only other issue is with a shunt but the frame ground should be on the load side of the shunt.