Adequate Electrical Grounding

gordo019

Member
I believe my Sterling B to B Charger is now properly installed with my House Battery and 12 volt system (wiring diagram attached). I have moved my grounding entirely to the Starter Battery negative terminal (exception: my 1000w inverter has its own equipment ground cable). Is my entire system adequately grounded? I am assuming that the starter battery is grounded by Mercedes Benz.
 

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avanti

2022 Ford Transit 3500
You do not mention the model year of your van, but if it is new enough to have the master disconnect terminal down by the acceleration pedal, then I think you have an issue:

If you literally connected those big black wires directly to the chassis starting battery negative terminal, then it looks to me that you have created a sneak ground path that bypasses the aforementioned OEM disconnect. Someday, somebody will use the terminal to power down the van for service or storage, and assume that it is completely dead. However, your added wiring looks like it will provide an alternative ground path from the battery to the chassis. This is not good.

Far better to use one of the OEM-sanctioned ground points. There is one under the driver's seat, and another under the vehicle back by the rear axle.
 

avanti

2022 Ford Transit 3500
BTW: Note that you will have a similar problem with the + side of the circuit if somebody disconnects the OEM ground but fails to turn off your house master switch. I.e., the house battery may power the chassis through the B-to-B charger, even with the disconnect open. This is not as bad as the other problem, but it should be understood and documented.

This problem also occurs in my Trik-L-Start setup. I am scratching my head deciding what to do about it.
 
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gordo019

Member
I have a 2016 Sprinter with the master disconnect switch near the gas pedal. Are the OEM-sanctioned ground points my best option? Sterling prefers a direct connection to the starter battery negative terminal but I don’t want to disable the master disconnect button.
 

calbiker

Well-known member
I don't see any sneak path to chassis gnd. There's only one connection to chassis (green gnd symbol). This is just a safety electrical connection to the chassis of the inverter and does not normally come into play.

The B2B charging + cable is 2 awg and the neg cable is 4 awg. I would make them both 2 awg.

If you literally connected those big black wires directly to the chassis starting battery negative terminal, then it looks to me that you have created a sneak ground path that bypasses the aforementioned OEM disconnect.
 

avanti

2022 Ford Transit 3500
I don't see any sneak path to chassis gnd. There's only one connection to chassis (green gnd symbol). This is just a safety electrical connection to the chassis of the inverter and does not normally come into play.

The B2B charging + cable is 2 awg and the neg cable is 4 awg. I would make them both 2 awg.
The issue is that it is not unusual for 12VDC devices to bond ground and chassis internally. (Whether this is good practice is another question). A single such device (present or future) in the vehicle with a path (such as mounting screws) from the device to the vehicle chassis will create a path. If you are willing to guarantee that no such device will ever exist, then you are correct. Such a guarantee is very difficult, however.
 
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calbiker

Well-known member
One could determine if device case is connected to the neg power terminal by taking a ohm reading before the device is mounted in the vehicle.

In addition the OP can double check for single point chassis ground by removing the master disconnect and then measuring battery voltage (from + bat terminal to chassis). Reading should be less than 5V.
 

calbiker

Well-known member
A more efficient design would be to connect the B2B + cable to the alt, not to the starting battery. If I made the calculation correct, this cable can conduct 78A. A 80A fuse would be too small.

Current going into the B2B:

I_in = I_out * (V_out/V_in) / efficiency

I_in = 60A * (14.4V / 12.2V) /0.9

I_in = 78.7A
 

gordo019

Member
I appreciate the comments but this is heady electrical stuff for me as a novice. I am still unclear about these four questions related to installing a B to B charger:

1. Where is the best place to connect the negative cable coming from the Sterling charger? OEM-sanctioned ground points or direct starter battery negative?

2. Where is the best place to connect the negative cable coming from my negative bus/house battery negative back to the starter battery.

3. How do I not screw up the functioning of the MB master disconnect switch near the gas pedal?

4. Where would be the good location for a master on/off switch to disable the Sterling charger unit?
 

avanti

2022 Ford Transit 3500
I appreciate the comments but this is heady electrical stuff for me as a novice. I am still unclear about these four questions related to installing a B to B charger:

1. Where is the best place to connect the negative cable coming from the Sterling charger? OEM-sanctioned ground points or direct starter battery negative?
The Mercedes Upfitters manual says to always use a sanctioned ground point. That is what I would do--the chassis provides better ampacity than any practical wire.
2. Where is the best place to connect the negative cable coming from my negative bus/house battery negative back to the starter battery.
Same as #1.
3. How do I not screw up the functioning of the MB master disconnect switch near the gas pedal?
By following the above advice. The problem will only occur if you connect the ground directly to the battery terminal, rather than a chassis ground.
4. Where would be the good location for a master on/off switch to disable the Sterling charger unit?
Dunno.
 
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calbiker

Well-known member
If you have isolation concerns, here's what I would do:

1. Remove cable from neg bus to starter battery neg terminal.

2. Add 2 awg cable from neg bus to chassis.

3. Move B2B ground from starter bat neg terminal to neg bus. I'm not sure if the B2B "likes" this move. The B2B ground reference has now changed and varies with cable IR drops.

There are now no sneak paths that can discharge the starter battery if the ground disconnect by the pedal is activated.

On the other hand, you can keep the grounds like in your diagram and do the ohm and voltage measurements I suggested to ensure starter battery is really floating when ground disconnect is activated.

Besides all that, I would connect the B2B input to the alt + and not starter bat +. This fuse should be 100A, not 80A. You can contact Sterling to notify them of their error.
 

gordo019

Member
If OEM grounds are used instead of the starter battery negative, are all negatives of my B2B integration still considered “common”?

Sterling’s instructions are: “Important: Negatives should all be common. We recommend joining the BB’s negative to the starter battery negative for improved efficiency."
 

calbiker

Well-known member
They will still be common grounds. They are not isolated grounds.

Either method, common ground is maintained.
 

gordo019

Member
Thanx for grounding diagrams. I am hoping the ground point I am using is correct. It is located near the rear right side corner of the driver's seat pedistal. Two other grounding wires already access this floor bolt location and it's the only floor bolt location I can access easily. I think there is another floor grounding point in the box but it is buried under pre existing MB electronics.
 

mlim

New member
How is the Sterling B2B working out connecting the ground to the OEM Sprinter ground under the driver seat instead of directly connecting the Sterling B2B directly to the starter battery ? I have the same question in my install.
Thanks.
Michael
 

gordo019

Member
The OEM ground has worked fine. No blown fuses or over heating issues for 20 months. The Sterling B2B has kept our house battery charged without any issues or any additional solar panel charging. We have relatively low 12 volt electric consumables installed. The Sterling B2B has been one the best choices we have made with our system. When our deep cycle house battery declines and needs replacement, I am looking into replacing it with a lithum battery and will continue to use the Sterling B2B for recharging with the Sterling lithum battery charging profile.
 

mlim

New member
Have you check how many amps are coming out of the BB1230? I talked to a Sterling technician and he ran into someone who only got 20A coming out of his BB1230. He suspects that it's because the starter and home battery grounds were not connected directly to each other, hence the lower efficiency.
 

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