12v positive and 24v negative possible?

[heintje23]

Is it possible to take the positive of my 12v bank and use the negative of my 24v bank? The max amp is 30.

[atoll]

you mean to get 12v supply from your 24v bank?

[nathanp]

I don't fully understand what you are asking, but you have to complete the circuit back to the battery that the positive is coming from. If the 12v and 24v share their ground circuits, then it would work, but if not, no.

[hellosailor]

I suspect the answer is 'yes' since damn near anything is possible, but without some specifics on how the batteries interconnect and what you are trying to accomplish, you may either get 36 volts out of the kludge, or nohting at all, or burn out the batteries, or set the boat on fire.

So, some specifics might be needed here. But literally, "yes".

[heintje23]

I have replaced my electro hydraulic autopilot pumps with two new ones each drawing 27amp maximum. The old ones were running on 24v and the new ones run on 12v. I have put in new wires for the 12v positive supply. I was wondering if I could use the old 24v negative wire as the new 12v negative for the new pumps. I think the boat has a common ground so then it should be possible.

Is there any way to check if the boat has a common ground or not other then visually following the negative cables?

[atoll]

check the resistance between a 12vneg and 24v neg.

though on an alumilium boat i would think the circuits would be isolated

[Steve Bean]

Ground is ground, no?

[nathanp]

I would just put a voltmeter on the new 12v+ wire that you ran and the ground wire you want to use, if there's 12v on the meter it'll work.

[fairbank56]

I would not rely on just "having continuity" between the 12v negative and the 24v negative. Where does your 12v negative connect in relation to the 24v system? The grounding bus (if you have one) and it's conductors should not normally carry any current. If your 12v negative connects to the same point on the engine as your 24v negative or to the 24v main negative bus, your ok.

Eric

[goboatingnow]

Originally Posted by fairbank56

I would not rely on just "having continuity" between the 12v negative and the 24v negative. Where does your 12v negative connect in relation to the 24v system? The grounding bus (if you have one) and it's conductors should not normally carry any current. If your 12v negative connects to the same point on the engine as your 24v negative or to the 24v main negative bus, your ok.

Eric

The term " ground " is frequently used in DC circuits to mean negative return ( hence the symbol)

" grounding " wires , protective earth etc are all different concepts

Dave

[rw58ph]

You also have to make sure the wire is sized correctly. Wire size for 24 volt is usually smaller than 12 volts. You can run 24 volt on wire sized for 12 volt but you can not run 12 volt sized for 24 volts.

Regardless I would have a 12 volt and a 24 volt ground wired. Keep them seaparate.

[hellosailor]

IF the negative lines (which ma or may not be isolated from ground and from each other) all make the same run to a common negative bus or ground point, you would be able to use the old 24v negative line.

But if the wiring was rated for less (half? the) amperage that your new 12v equipment draws, then it would be undersized and it might cause excessive voltage drop in the lines. Check the wire size and make sure that it is of sufficient capacity, otherwise, run a new negative line as well.

When you are sizing the wires, you need to measure the ground-trip length of the wiring run. That's positive and negative, both sides of the circuit, to get the total wire length to use with the current capacity and voltage drop.

[David M]

On DC circuits it is called positive and negative. On AC circuits it is called hot, neutral and ground.

[hellosailor]

"On DC circuits it is called positive and negative."
Ah, it is called by many names, whether they're technically correct or not. For instance in autos, there's hot and ground--and a ground cable attached to the negative post of the battery. No negatives. Boat systems being so similar to autos...The negative might or might not be the same. When one of the engine mounts has been used as the common point for the negative battery cables, panel distriution, bonding and everything else, does the engine block now count as "negative" or "ground" ?

Conventional versus electron flow : Basic Concepts Of Electricity

In grade school they still taught us that electricity flowed from the + to the - but these days, that is known to be incorrect, it actually flows from the - to the + so no matter which nomenclature you use, in some context it is wrong.

[goboatingnow]

Originally Posted by David M

On DC circuits it is called positive and negative. On AC circuits it is called hot, neutral and ground.

Technically correct , but the term ground is used commonly to refer to tbe negative . Especially on split power supplies for example the negative may not be the " ground" hence the common term GND in circuit design. gND may be earth referenced or may not be

Dave