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Old 13-05-2016, 09:20   #1
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24v negative and 12v negative

hi
I have 12v and 24v on my boat is it normal to have all the negatives connected?
thanks
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Old 13-05-2016, 09:21   #2
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Re: 24v negative and 12v negative

Yes, it's normal.
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Old 14-05-2016, 05:34   #3
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Re: 24v negative and 12v negative

It is normal unless you have a shunt connected to the negative of one battery, in which case you treat the other side of the shunt as the battery negative. Think of the battery and shunt combination as a black box.
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Old 14-05-2016, 07:12   #4
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Re: 24v negative and 12v negative

hi I didn't even think of that I do have a shunt on the 24v so the batteries are on the left of the shunt do I have to connect it there to the 12v negative.
I also have a 70 amp 24v to 12v step down converter which has a negative.
I'm getting a bit confused how will it affect my battery monitor.
thanks great you thought of that!
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Old 14-05-2016, 07:44   #5
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Re: 24v negative and 12v negative

OK and actually preferred to have all battery negatives on same bus. You can have a shunt on any or all batteries, regardless of voltage, so long as the battery side of each shunt goes to the battery bank being monitored by that shunt for amps in and out. The main negative side of each shunt goes to the main negative bus which has all battery negatives to it. Having a shunt on the main negative bus is no problem.

In practice you wouldn't use the bus side of a shunt as a bus itself as you will have many devices which will have negatives going to the main negative bus - engine, main panel, chargers, etc. etc. etc. You should never put more than three cables to any one connector.
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Old 14-05-2016, 07:58   #6
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Re: 24v negative and 12v negative

Shunts keep confusing people. I think it's mostly the rotten terminology used. They call one side the LOAD side.

It's actually pretty simple. The other side is the BATTERY side, so, connect ONLY the battery to that side.

The LOAD side then goes to the engine ground, which usually runs thru a bus or a Power Post.

You want and need everything that's going through or from the battery being monitored to go through the shunt, since ammeters are wired in series in circuits, like a flow meter in a pipe.

Battery (& side) >>> shunt >>> ground to engine (load side - bus with all other negative connections)

I call that negative bus my NDP - negative distribution post (I used a Power Post, not a bus bar since I had limited space).

Good luck.
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Old 14-05-2016, 08:23   #7
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Re: 24v negative and 12v negative

hi thanks for the info but I'm still a little confused because I have 24v batteries and 12v. I am interested in monitoring the 24v batts but I also have my 24v to 12v converter that I use because my solar panels are 24v.this device has a negative that's connected to the 24v batts. so do I connect the 12v negative to the battery side of the shunt?
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Old 14-05-2016, 08:30   #8
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Re: 24v negative and 12v negative

so I just connect everything up on the load side of the shunt yes. I don't understand how does the shunt differentiate between 12v and 24v ?
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Old 14-05-2016, 08:32   #9
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Re: 24v negative and 12v negative

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Originally Posted by totalrefitdan View Post
so I just connect everything up on the load side of the shunt yes. I don't understand how does the shunt differentiate between 12v and 24v ?
Based on which battery is connected to the battery side of the shunt.

Try this:

Ammeters & Shunts 101 Ammeters & Shunts 101
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Old 14-05-2016, 08:45   #10
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Re: 24v negative and 12v negative

should have read your post more carefuly it answers my question ,thanks
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Old 14-05-2016, 22:06   #11
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Re: 24v negative and 12v negative

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so I just connect everything up on the load side of the shunt yes. I don't understand how does the shunt differentiate between 12v and 24v ?
It doesn't It will monitor Amps of the 24v bank, if it's in the right spot.

If you also want to monitor the 12v you'd need a 2nd battery monitor.
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Old 15-05-2016, 10:24   #12
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Re: 24v negative and 12v negative

Stu and smacc are right on. Note: A higher voltage system will deliver twice the power as a lower volt system for the same number of amps displayed. Ten amps of AC power at 120volts is ten times more power than ten amps at 12v.

Same for 24 vs 12. 10A on a 24v circuit will be twice the power of 10A on a 12v circuit. Most meters will figure it out. You do have to size shunts for the amps they can carry though. I suspect you will be fine with 250A and above. If you already have shunts they are most likely that big already.

I am just making a guess that you probably won't go over 100A on your boat but that may not be true if you have a big system with big loads and alternators/chargers. Most shunts I have seen are rated 250A or 500A. Use the smallest one that fits your system requirements.

You have to use a DC shunt for DC circuits. There are other ways to measure AC amps and they can't use a DC shunt.
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