Chassis corrosion caused by common negative

[RGIvy]

Has anyone actually experienced corrosion caused by a common negative (neutral) using the chassis?

I know the theory that a DC-DC (B2B) charger based on common negative using the chassis instead running negative cables could cause some corrosion. Of course, this could only occur under very high current and probably with dicey connection somewhere. Some DC-DC chargers allow pretty high continuous current

The reason I ask is I've never actually seen it happen (corrosion that is), and the alternator is grounded to the chassis via the negative strap anyway.

I'm not asking about the inverter, this I'm assuming would always be connected to the battery by two short and thick cables.

 

[DerpsyDoodler]

As I understand it, as long as your house battery and chassis batteries have 1 and only 1 common ground to chassis, it should not be a problem (assuming you don’t randomly ground any of the components of any of the circuits to the chassis in another area. It should all be the same common ground. So, a ground distribution bar that you ground all of your house circuits back to (including EGCs), which then converges onto the same chassis ground as the chassis battery negative. I have no references to cite, but im nearly positive you can find everything I’ve just typed on this forum somewhere.

edit: you can also find this information in some inverter manuals, or referenced from links in some inverter manuals.

 

[RGIvy]

As I understand it, as long as your house battery and chassis batteries have 1 and only 1 common ground to chassis, it should not be a problem

Bear in mind that there are actually three negative attachment points:
1. Alternator
2. Starter battery
3. House (Leisure) battery

If you have a high current DC-DC charger (e.g. 60a or more) that's a fair amount of current being pumped through the chassis continuously. Bear in mind that we generally attach a positive cable to the alternator but no negative. The negative connection is through the engine and over the flexible strap to the chassis. Then from the chassis we go to a cable that usually connects directly to the starter battery. A high-current DC-DC charger is going to take power from the alternator meaning it's flow is through the chassis anyway.

So, loads of theories and opinions - I'm keen to know of any cases where using the chassis as a common negative actually caused structural problems. There are some opinions out there that it does, but practically I've never seen it.

 

[DerpsyDoodler]

So, loads of theories and opinions - I'm keen to know of any cases where using the chassis as a common negative actually caused structural problems. There are some opinions out there that it does, but practically I've never seen it.

i feel like we need to be very specific as to what is being communicated here. Chassis itself should not be used as a common negative (meaning tie to it at many points). What I am saying is:

house battery—> negative house bus bar —> chassis battery negative —> chassis ground.

another way of specifically wording it is:

house battery ties to chassis at exactly the same point that chassis battery ties to chassis. The idea is not to use the chassis for current flow, but as a common ground at a common point. If your dc-dc charger has it’s positive draw tied to the chassis battery positive, and it’s negative tied to chassis battery negative, that circuit should not pull any current through the chassis, at all. Chassis would be a high resistance path in comparison.

 

[carbon60]

i feel like we need to be very specific as to what is being communicated here. Chassis itself should not be used as a common negative (meaning tie to it at many points). What I am saying is:

house battery—> negative house bus bar —> chassis battery negative —> chassis ground.

another way of specifically wording it is:

house battery ties to chassis at exactly the same point that chassis battery ties to chassis. The idea is not to use the chassis for current flow, but as a common ground at a common point. If your dc-dc charger has it’s positive draw tied to the chassis battery positive, and it’s negative tied to chassis battery negative, that circuit should not pull any current through the chassis, at all. Chassis would be a high resistance path in comparison.

Is this only for high current applications? Because all vehicles use the chassis as return for all their factory wiring.

 

[DerpsyDoodler]

Is this only for high current applications? Because all vehicles use the chassis as return for all their factory wiring.

I believe the reason is because of different chemistries and different metals on the house side, that it will cause galvanic corrosion if it is used that way for the house system. I am not an expert on this matter, but that is my understanding.

Edit: and it may be that this only matters because I will be tying the house system to the chassis system. Regardless, my information is limited to understanding what I need to do for my designed-for scenario. Soooo....feel free to seek further advice and confirmation.

 

[Whinny]

As Carbon60 said the vehicle has chassis grounds all over the place.
Can it be done? Yes....
Should it be done....I would suggest no.

 

[DerpsyDoodler]

As Carbon60 said the vehicle has chassis grounds all over the place.
Can it be done? Yes....
Should it be done....I would suggest no.

Point is, the house system should be bonded to chassis at one place, and if using a dc-dc charger to gain alternator charging, house should tie to same chassis bond as chassis battery negative. Are there other just as correct ways to have the single chassis ground bond? Perhaps. But I know the way I’ve described is legit. Can’t speak for or against other ways.

 

[Supervstech]

Keep this in mind, vehicle loads are light, sporadic, and factory designed.
House loads can be hundreds of amps... for hours.
The ONLY multihundred amp load in a vehicle is the starter, and it shouldn't operate over 15seconds...
And it isn't powered through the chassis, it has a huge engine block with main battery directly connected to it...
For house loads... use copper.
Bond to the chassis, but DO NOT flow through it.

 

[RGIvy]

And it isn't powered through the chassis, it has a huge engine block with main battery directly connected to it...

Actually on my vehicle it is. The engine block has a flexible strap connecting the engine to the LHS on the chassis in the engine bay, and then the starter battery has a black cable bolted down on the RHS of the chassis in the engine bay. So the current is flowing through the chassis for both:
1. Starter battery
2. DC-DC charger to charge the house battery

It's been like this ever since the van was built. The starting process, which uses the biggest current, only lasts for a few seconds but definitely goes through the chassis. The charging of the starter battery and the house battery also go through the chassis and for prolonged period of time. In my case the DC-DC charger only draws 40a from the alternator which is clearly not enough to cause galvanic corrosion because I'm sure I would have seen it (had the engine out recently).

But I'm wondering if someone else may have a similar system that draws more power from the alternator. This is why I asked the question: has anyone actually SEEN damage caused by galvanic corrosion.

 

[JRG]

Actually on my vehicle it is.

What van do you have?

 

[RGIvy]

What van do you have?

Fiat Ducato, based in the UK. It's a German import.

 

[Supervstech]

What van do you have?

Sprinters and nearly all Mercedes are this way as well.
The battery main negative connects to the chassis, the block has a huge ground strap, the instruments use ground towers attached to the chassis in places...

 

[DerpsyDoodler]

Keep this in mind, vehicle loads are light, sporadic, and factory designed.
House loads can be hundreds of amps... for hours.
The ONLY multihundred amp load in a vehicle is the starter, and it shouldn't operate over 15seconds...
And it isn't powered through the chassis, it has a huge engine block with main battery directly connected to it...
For house loads... use copper.
Bond to the chassis, but DO NOT flow through it.

which is what I thought that was implied. Multiple bonds to ground resulting in a ground loop is exactly what needs to be avoided here. For instance, if not using a dc dc charger (or using an isolated one), house can bond to chassis wherever it wants, as long as all house grounds terminate on that chassis ground bond, thus not creating any potential for a ground loop situation in the house circuit. Is my understanding misguided?