Thread: 12V and 24V on same vehicle. Can they share same earth (-ve)? Or keep separate?

Originally Posted by drivesafe

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Hi spudboy and to your question, as others have posted, the negative of all batteries and appliances would normally be connected to a common ground or earth, like the chassis.

If you do not set up a common ground or earth you will end up with one or two FLOATING circuits and in extreme cases could cause shorts to occur in the positive and negative circuits.

This is just a suggestion but if you are setting this up from scratch, try using specific colours for the two different voltage positives.

Start by using Black for all negative circuits.

Use Red for the main 12v supply and Red with a trace colour for 12v circuits.

Then say Yellow for the main 24v supply and Yellow with a trace colour for any 24v circuits.

Thanks for that reply. I like your idea about the colour coding.

In my case it will be pretty simple as there is almost no interaction between the systems. Everything in the back of the truck is 12v. Everything in the cabin is 24v, and I am not modifying anything that the manufacturer has installed. It is only the solar charging area where the 2 systems 'touch'.

My plan is therefore to run a single heavy duty black 'earth' cable from the 24v to the 12v battery banks.

I'll then run another red/yellow heavy duty wire from the +ve terminal of the solar MPPT controller to the +ve terminal on the 24v battery bank, as well as a black wire from the MPPT controller -ve to the 24v battery bank -ve.

That way, both battery banks have a common ground, and there is a direct conection between the MPPT controler and the 24v system (i.e. the chassis is not used as part of the connection).

We'll see how that goes!

I am planning to put in a fuse on the +ve line, close the the 24v battery bank. Is that all I need? Is it worth putting in a 2nd +ve fuse close to the MPPT controller? Also: I read a few articles where they thought it was good to fuse BOTH the ground and the +ve lines, but that was on a yacht building site.

Thanks
David

Correction - the convention is to use a Capital "V" for Volts (because its someone's name - Voltaire) and d.c. and a.c are both lower case with a dot between them and at the end. Yeah - anal I know, but its what I am paid to check and if you are going to be corrected then you should be corrected correctly. Correct? http://www.aulro.com/afvb/images/smilies/wink.gif

Originally Posted by matti4556

Correction - the convention is to use a Capital "V" for Volts (because its someone's name - Voltaire) and d.c. and a.c are both lower case with a dot between them and at the end. Yeah - anal I know, but its what I am paid to check and if you are going to be corrected then you should be corrected correctly. Correct? http://www.aulro.com/afvb/images/smilies/wink.gif

Nearly correct. V is for volts. Only unit that is upper case is Celsius. Why? Who knows. Most units are named after people who were instrumental in their discovery, but only Celsius gets the capitalisation treatment.
dc and ac are the correct way of expressing direct and alternating current ( no dots). All according to SI unit standards. Your company may adopt a different standard, but international standards are what the scientific community uses... just saying

I used to keep it simple and just use the term positive and negative. It is only recently since hanging around this place that I have started using +ve or -ve.

After drivesafe's post on this I shall go back to how I used to do it.

Short version.

Yep go it.

Normal version.

Yep, but there are some cautions to be adhered to with regards to cabling and earthing. To help prevent accidental misconnection and rf noise.

Long technical version that probably has no probablility on coming up with what you are trying to do. (too lazy to type on a phone)

Summary.

Since your not building up a radio transmission unit with ultrasensitive electronics just run with drivesafes info and ensure good earrhing connections throughout.

Originally Posted by spudboy

Thanks for that reply. I like your idea about the colour coding.

In my case it will be pretty simple as there is almost no interaction between the systems. Everything in the back of the truck is 12v. Everything in the cabin is 24v, and I am not modifying anything that the manufacturer has installed. It is only the solar charging area where the 2 systems 'touch'.

My plan is therefore to run a single heavy duty black 'earth' cable from the 24v to the 12v battery banks.

I'll then run another red/yellow heavy duty wire from the +ve terminal of the solar MPPT controller to the +ve terminal on the 24v battery bank, as well as a black wire from the MPPT controller -ve to the 24v battery bank -ve.

That way, both battery banks have a common ground, and there is a direct conection between the MPPT controler and the 24v system (i.e. the chassis is not used as part of the connection).

We'll see how that goes!

I am planning to put in a fuse on the +ve line, close the the 24v battery bank. Is that all I need? Is it worth putting in a 2nd +ve fuse close to the MPPT controller? Also: I read a few articles where they thought it was good to fuse BOTH the ground and the +ve lines, but that was on a yacht building site.

Thanks
David

If you are not using the chassis as a return path why are you going to connect the -ve terminals of 12v & 24v together?

I would fuse both your power generator (panel or controller) and storage (battery).

Because the solar panels (when split between 12v & 24v charging) will share an earth. This means that the 12v and 24v circuits are no longer independent from each other.

Also - in the long run I'd like to have a 24v > 12v converter, so the house batteries get charged from the 24v alternator when the engine is running. Same issue as the solar panels, in that the 2 systems will need to interact.

I have fuses on the +ve sides of the 24v batteries, the solar panels (special ceramic fuses) and the 12v batteries. I was just wondering if fusing the ground line was of any use, seeing as I read about someone building a boat who did that, that's all.

Originally Posted by spudboy

Because the solar panels (when split between 12v & 24v charging) will share an earth. This means that the 12v and 24v circuits are no longer independent from each other.

Also - in the long run I'd like to have a 24v > 12v converter, so the house batteries get charged from the 24v alternator when the engine is running. Same issue as the solar panels, in that the 2 systems will need to interact.

Just earth the negative side of the 24v batteries, earth the negative side of the 12v battery and earth the negative side of the solar panel charging system. As long as the positive side of each is separate then there is no issue.

The negative sides can all be connected via the chassis, it is only the positive sides of the respective systems that need to be separated.

When I solar charge - the negative is either earthed on the negative battery terminal or on the body of the vehicle (has earth straps between body and chassis) and if charging 12v just ensure the positive side of the solar is only connected to the positive side of the 12v battery. My solar reg can do either 12 or 24v. For 12v I just plug in the solar to the rear Anderson plug on the car.

Also - you cannot charge (well at least to do it properly) you 12v battery from the 24v system via a 24v - 12v converter. To do this you really need a 24v-24v Dc battery charger.

Garry