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Old 18-12-2017, 22:15   #1
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Overloaded 12v circuit with 24v

Firstly let me admit I'm an idiot but explain to you why and seek some thoughts of the consequences.
So we bought a new set of deep cell batteries today and I installed them... but I advertently put one in backwards. When I turned on the switch to test they worked ( just turning on battery power not starting engine) there was a smoke in the engine bay. The battery switch was on for less than 30 seconds but it was enough to melt all the plastic from one wire between the alternator and starter.
I can replace that line tomorrow and possibly get the alternator checked... what is likely to be the unseen damage I've created?
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Old 18-12-2017, 22:53   #2
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Re: Overloaded 12v circuit with 24v

I don't think anyone can answer that. It depends on many varribles. If it were me, I'd replace the cooked wire, sort the batteries and give it another go.

But wait for other comments.
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Old 18-12-2017, 23:23   #3
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Re: Overloaded 12v circuit with 24v

Your heading sounds wrong. Your explanation indicates you shorted a battery.

If you put double the voltage in a circuit then things will spin twice as fast and your lights will burn twice as bright. (In the 20th century.) With LEDs and most solid state devices you'd probably not notice any difference or even do any damage.

My advice to students was always "if you can draw the circuit and explain it and it's built accordingly then it will work first time. Ask questions like "is it fused?" and "is the wiring sized appropriately?"
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Old 19-12-2017, 00:09   #4
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Re: Overloaded 12v circuit with 24v

Thanks Leftbrainstuff.
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Old 19-12-2017, 04:56   #5
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Re: Overloaded 12v circuit with 24v

Great case for putting a fuse on all batteries.
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Old 19-12-2017, 06:06   #6
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Re: Overloaded 12v circuit with 24v

Quote:
Originally Posted by leftbrainstuff View Post
Your heading sounds wrong. Your explanation indicates you shorted a battery.
Exactly correct. Installing one battery backward was unlikely to result in 24 volts. The cable between the alternator and starter cooked because it was the smallest wire in the circuit, with a short created by the diodes in the alternator facing the wrong direction for the backward current flow.

It's possible you damaged the alternator diodes, and it's probable that you overheated some connections along the way. Unfortunately you may not see the results of overheated connections for a few months as the overheat will promote corrosion that doesn't happen right away.
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Old 19-12-2017, 06:37   #7
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Re: Overloaded 12v circuit with 24v

The good news is that in 30 seconds it's unlikely that you would have done any damage to the expensive batteries...they were just putting out like they're supposed to. However, although the information is limited, it sounds like there might be something wrong with your system. When you turn the battery switch on, there should not be any current running until you try to start the engine or run something. It doesn't matter how much voltage you've inadvertently created with your battery wiring mistake.

For a wire to burn, it must have excessive current running through it. If nothing is turned on, then it sounds like you have that wire running directly to a ground so that the battery is trying to drain through it.

Good luck.
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Old 19-12-2017, 07:04   #8
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Re: Overloaded 12v circuit with 24v

Not enough information to give a definitive answer. A few things that would help get more accurate suggestions.

1. Were these 12V batteries connected in parallel or pairs of 6V batteries connected series then parallel or the one backwards battery in a separate bank not connected to other batteries?

2. Can you describe what you mean exactly when you say you hooked one up backwards.

3. How many batteries?

4. What were they connected to and what was turned on when you turned on the switch?

A couple of issues.

- If you had several 12V batteries connected together in one bank and you connected one backwards, that is the backwards battery - to the other batteries + and the + to the - then you would have had a short between the batteries, big sparks and possibly a fire. Since you didn't mention this then something else was happening.

- If the battery you connected backwards was isolated from any others and that battery alone was switch on then you put voltage into the system backwards, IE negative voltage to the positive wires. Depending on what was attached to that circuit it could have fried electronics. That's why you need to know what else if anything was connected to that backwards battery.
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Old 19-12-2017, 07:06   #9
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Re: Overloaded 12v circuit with 24v

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Originally Posted by leftbrainstuff View Post
Your heading sounds wrong. Your explanation indicates you shorted a battery.
Exactly. To get 24 V two 12 V batteries have to be connected in series. "Backwards" does not indicate that but that term as used by the OP is very ambiguous and we really can't tell exactly what happened.
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Old 19-12-2017, 07:27   #10
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Re: Overloaded 12v circuit with 24v

Quote:
Originally Posted by Privilege View Post
The good news is that in 30 seconds it's unlikely that you would have done any damage to the expensive batteries...they were just putting out like they're supposed to. However, although the information is limited, it sounds like there might be something wrong with your system. When you turn the battery switch on, there should not be any current running until you try to start the engine or run something. It doesn't matter how much voltage you've inadvertently created with your battery wiring mistake.

For a wire to burn, it must have excessive current running through it. If nothing is turned on, then it sounds like you have that wire running directly to a ground so that the battery is trying to drain through it.

Good luck.
The alternator diodes are the short. With a battery installed backward, the diodes act as a short because they're designed to work in the opposite direction. Being in the auto repair biz, I've seen this a hundred times.
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Old 19-12-2017, 10:53   #11
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Re: Overloaded 12v circuit with 24v

Quote:
Originally Posted by skipmac View Post
Not enough information to give a definitive answer. A few things that would help get more accurate suggestions.

1. Were these 12V batteries connected in parallel or pairs of 6V batteries connected series then parallel or the one backwards battery in a separate bank not connected to other batteries?

They are 3 x 12v 155Amphr batteries in series.

2. Can you describe what you mean exactly when you say you hooked one up backwards.

I hooked positive to negative and negative to positive for one of them when installing. Other batteries were isolated so no sparks and brimstone...

3. How many batteries?
3

4. What were they connected to and what was turned on when you turned on the switch?

Nothing. Everything turned off.

A couple of issues.

- If you had several 12V batteries connected together in one bank and you connected one backwards, that is the backwards battery - to the other batteries + and the + to the - then you would have had a short between the batteries, big sparks and possibly a fire. Since you didn't mention this then something else was happening.

- If the battery you connected backwards was isolated from any others and that battery alone was switch on then you put voltage into the system backwards, IE negative voltage to the positive wires. Depending on what was attached to that circuit it could have fried electronics. That's why you need to know what else if anything was connected to that backwards battery.
Nothing else connected and batteries isolated.
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Old 19-12-2017, 11:11   #12
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Re: Overloaded 12v circuit with 24v

Quote:
Originally Posted by nzmal View Post
Firstly let me admit I'm an idiot but explain to you why and seek some thoughts of the consequences.
So we bought a new set of deep cell batteries today and I installed them... but I advertently put one in backwards. When I turned on the switch to test they worked ( just turning on battery power not starting engine) there was a smoke in the engine bay. The battery switch was on for less than 30 seconds but it was enough to melt all the plastic from one wire between the alternator and starter.
I can replace that line tomorrow and possibly get the alternator checked... what is likely to be the unseen damage I've created?
If you connected them (2) wrong and you fried a cable, I would connect them in parallel to get 12 volts. Then I would replace the cable you fried and try it again. If nothing was operating chances are you are good to go. You might have fried a fusible link (near the starter) you will only know this if the engine does not turn over. If the engine does not turn over check your fuses and give us more feedback of your situation...
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Old 19-12-2017, 11:22   #13
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Re: Overloaded 12v circuit with 24v

I did that once, fried the battery selector switch but everything else was fine.
Since then, I have clearly labelled battery terminals before I start. Coloured paint or similar adjacent to terminal makes it clear in dim light.
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Old 19-12-2017, 11:30   #14
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Re: Overloaded 12v circuit with 24v

Thanks Djarraluda. Nice to know I'm not the first..
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Old 19-12-2017, 11:51   #15
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Re: Overloaded 12v circuit with 24v

Quote:
Originally Posted by nzmal View Post
Nothing else connected and batteries isolated.
Hi Nzmal,

I am a bit confused by some of your information. I highlighted my original questions in blue, your answers in red and my new comments in green.

1. Were these 12V batteries connected in parallel or pairs of 6V batteries connected series then parallel or the one backwards battery in a separate bank not connected to other batteries?

They are 3 x 12v 155Amphr batteries in series.

This is the bit that really confuses me. Perhaps you mean in parallel? Three 12 V batteries in series would give you a 36 V system which is extremely rare. Serise means batteries end to end in a line,
parallel means side by side. So series for three batteries:
B1 - goes to the boat ground
B1 + goes to the - of B2
B2 + goes to - of B3
B3 + goes to the boat electrical panel or starter or whatever you are powering.

Parallel All the battery positives are connected to each other. All the battery negatives are connected to each other.

See the diagram below. It only shows two batteries but it works the same way with three.



2. Can you describe what you mean exactly when you say you hooked one up backwards.

I hooked positive to negative and negative to positive for one of them when installing. Other batteries were isolated so no sparks and brimstone...

What exactly did you connect that battery positive and negative to? Was it a battery switch, the starter, an electrical panel or what? I think from what you say it was not another battery.



3. How many batteries?


3

4. What were they connected to and what was turned on when you turned on the switch?

Nothing. Everything turned off.

Very good that everything was turned off. Did you turn anything on? Turn on any battery switch? When did the wires melt?

If you could draw a diagram of the batteries, the main cables and battery switch or switches it might help a lot.
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