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Old 11-22-2020, 01:26 AM   #1
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Dead flat house bank

My new to me steel ex prawn trawler has been on a pole mooring essentially unattended for 2 weeks. Yesterday, I went to check on on it to find the 12v SLA 300ah house bank dead flat at 0.3v. The 400w solar panels and no frills mppt charger were not charging the batteries and were switched off.

My guess is that I have left a small 12v draw turned on and after 4 overcast days the solar has not kept up and finally the system has stopped charging when it turned itself off due to low voltage cutoff. The drain has then continued, pulling the bank down to near zero volts.

As the solar was not recharging the system, I made up some long jumper leads and used my dive boat to "jump start" the system. At 6v the solar reconnected and started recharging at 15 to 20 amps.

The only concern I do have is that when connecting the "jumper" cables I would get sparks when connecting the first (negative) lead BEFORE the positive was even connected to the house bank. Note both leads were connected to the applicable terminals at the dive boat end.

Why was there sparking with only one alligator clamp being connected? How was the circuit being completed?

Is "jump starting" from one boat to another's starter or house battery a good practice?

Thanks
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Old 11-22-2020, 02:30 AM   #2
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Assuming your dive boat has a bonding system that is connected to earth then the circuit can complete via the salt water. But just why your steel boat's house negative terminal would have a non-zero voltage is something you will need to resolve. Its possible that you did NOT in fact leave a small 12V draw turned on, but instead have other issues with the house bank wiring. The joys of a new boat and coming to grips with PO's DIY!
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Old 11-22-2020, 06:25 AM   #3
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I start at the dead battery end. The positives are connected first, then the negatives.
Disconnect the negatives first and then the positives.
No spark.
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Old 11-22-2020, 07:59 AM   #4
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Hello all! I am new to this form and just bought a trawler myself.

But to get back to the point or post. To me, no matter which wire you disconnect first and there is a load on that battery. You will or could have a spark.

As to why the OP is getting a spark. Was the other end of the jumper cables connected to anything?
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Old 11-22-2020, 08:42 AM   #5
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A long shot explanation for the negative sparking is that you have reversed polarity on your dive boat and connecting the negative (which was actually the positive) provided a ground path (through the water) that caused the spark.

Another issue is that SLA (sealed lead acid) batteries are definitely not deep cycle types. When you replace them (and you almost assuredly will have to after sitting dead for days) use golf cart 6V batteries wired in series. That is one of the only sure deep cycle batteries. Otherwise replace with AGMs which eliminate some of the problems with non deep cycle batteries.

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Old 11-22-2020, 09:41 AM   #6
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It indicates you have a dead short somewhere on your boat.

Another test is to place a 12 volt lightbulb between the positive lead wire and the battery post. If it glows it also indicates a short.

Searching out can be a challenge but there are now digital tools on the market which will help you find it.

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Old 11-22-2020, 10:28 AM   #7
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Quote:
Originally Posted by Iggy View Post
As to why the OP is getting a spark. Was the other end of the jumper cables connected to anything?

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Originally Posted by DavidM View Post
A long shot explanation for the negative sparking is that you have reversed polarity on your dive boat and connecting the negative (which was actually the positive) provided a ground path (through the water) that caused the spark.

David
The other ends were connected to the correct terminals on my dive boat batteries first. When I got the sparks I thought the same thing ie that I had reversed the terminals at that end but rechecked and they were correct

When I say "sparks" it wasn't a dead short arc flash but I was a definite sparking, which I thought strange given that I only had one lead connected.


Thanks.
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Old 11-22-2020, 10:33 AM   #8
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It indicates you have a dead short somewhere on your boat.

Another test is to place a 12 volt lightbulb between the positive lead wire and the battery post. If it glows it also indicates a short.

Searching out can be a challenge but there are now digital tools on the market which will help you find it.

pete
Thanks. I'll definitely be following up to see of I did indeed leave something on or if there is a parasitic draw somewhere.

FWIW, either way it must be a slow draw as when the boat is used or in full sun it still has full battery capacity in the morning.
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Old 11-22-2020, 11:43 AM   #9
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The other ends were connected to the correct terminals on my dive boat batteries first. When I got the sparks I thought the same thing ie that I had reversed the terminals at that end but rechecked and they were correct

When I say "sparks" it wasn't a dead short arc flash but I was a definite sparking, which I thought strange given that I only had one lead connected.


Thanks.
I am just wondering. So the other end of the cables were connected. Red to pos. and black to neg. What others wires were connected to the battery that sparked? You may have a short? I am thinking that your powering something.
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Old 11-22-2020, 11:50 AM   #10
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I am just wondering. So the other end of the cables were connected. Red to pos. and black to neg. What others wires were connected to the battery that sparked? You may have a short? I am thinking that your powering something.
All the normal 12v feed lines were still connected to the battery. It was the alligator clamp from the first (negative) jumper lead that sparked. This surprised me as the positive jumper lead wasn't even connected yet.

Thanks
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Old 11-23-2020, 09:36 AM   #11
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All the normal 12v feed lines were still connected to the battery. It was the alligator clamp from the first (negative) jumper lead that sparked. This surprised me as the positive jumper lead wasn't even connected yet.

Thanks
To me and who am I. It sounds like you have a bad ground cable or connection on that battery.

You completed the path to ground with the jumper cables.
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Old 11-23-2020, 06:37 PM   #12
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Perhaps there is some current flow through the two boats' ground and bonding system, through the water?


Either way, it would like your boat is in need of a thorough inspection to see if you have any current leaks or unexpected loads.
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