Inverter grounding to batt or chassis

[pauldub]

I'm looking for some input regarding the ground (negative, not neutral) connection for my 1800 watt inverter. I need about 10 of wire to run it back to the battery or I can ground it to the motor home frame with less than 2' of wire. I suspect that the frame will provide a lower resistance return to the battery than any reasonable size wire; however, not knowing the resistance of the frame, it hard to be certain. Thoughts?

[pauldub]

OP here. I'll have half my battery bank on the curb side of the motorhome and the other half on the street side. To wire the batteries in a balanced fashion will take extra wire length. This is most of my motivation to ground the inverter DC return directly to the frame. I'll use a tin plated lug which is galvanically compatible with the steel frame. I can also use a chrome plated bolt so that the bolt will be galvanically compatible with the steel and tin.

[wa8yxm]

This varies some from INverter to inverter on my Xantrex Freedom the case is negative and needs to be bonded by a big heavy wire to something least the negative wire fail.. Long story as to why you have issues should that happen.

I've not bonded it yet but will.. Soon.

[2oldman]

DrewE wrote:

2oldman wrote:
Something about using the frame for high amperage just doesn't seem right.

Why not? It's done practically all the time for starter motors.

Ok, but starter motors are run for only seconds. They also don't put 4/0 or bigger cable to the starter (at least not that I've seen) because it's not expected to run but seconds.

[BFL13]

I measured the R of my 5er's frame to be 0.013 ohms. Adding the copper pipe got that down to 0.0014 The pos path was reduced to 0.006 with wires.

The R stays the same. More amps means more voltage drop.

Attaching the linking wires to the frame is the "weak link" where corrosion at the frame can develop

[DrewE]

2oldman wrote:
Something about using the frame for high amperage just doesn't seem right.

Why not? It's done practically all the time for starter motors.

[2oldman]

Something about using the frame for high amperage just doesn't seem right.

[BFL13]

The frame does have low R. The missing info is how long and what gauge the battery neg to frame wire is. If also 2 ft, then that leaves only 6 ft difference wire vs frame.

A complication is the inverter's chassis neg wire to frame, if used. That needs to go to a different frame spot from the neg input wire. the chassis ground is for reducing radio and TV interference.

For just 10 ft, if you use fat wire for the neg and pos inputs it would not make much difference in voltage drop running the microwave IMO, but how much is too much? Don't know.

For just wire economics for the least voltage drop you can use the frame for the neg and double up on the pos input. (Parallel both 10ft wires on the pos, and use the frame for the neg.) Now you will have way less R on the circuit.

EDIT I posted a few years ago, how I added a copper water pipe to the neg path in parallel with the frame for my converter-battery run in the 5er. Got R down to hardly any. Then I added a fat pos wire to parallel that path, and got R down to even less.

Point of no returns is when the reduced R lets the converter do its rated amps. Reducing R further is a waste because the converter is current limited.

There must be an equivalent point with the inverter. Say when you can already run the MW at 50% battery SOC long enough to do whatever just before it alarms at 11v.

No point reducing R more if you don't need to run the MW when the batteries are below 50%.

[MEXICOWANDERER]

The issue is to ensure a line-to-chassis leak gets earth and chassis grounded. Together. The case of the inverter is the point of interest.

Shunting of noise atmospheric emissions to earth is also a consideration. Most inverter noise is carried by battery negative and positive power wires.