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Author Topic:   Three-bank Charger
stubbleduck posted 05-21-2007 12:06 PM ET (US)   Profile for stubbleduck   Send Email to stubbleduck  
First- thank you in advance for any direction provided.

Background information-
I’ve recently moved up to a 24v Minn kota and installed a 3-blank onboard charger. The 24 volt is 2 12v-batteries connected (+) to (-) with the trolling motor hooked to the respected vacant battery posts for the 24volt system.

The onboard charger is a minnkota 3-blank 15amp (5amp to each battery if three batteries were connected) mounted near the counsel. Cords that connect to the batteries are fused on the positive ends. These cords need to be extended to reach both the starting battery and the 12/24 volt near located in the back of the boat.

I’ve ran my wiring (insulated) and match the wire gauge as recommended from the battery posts to a terminal block and installed an additional 30amp fuse on the positives connectors. The terminal block is a 6-gang seachoice (, identical to this but with 6 on each side instead of 4. I mounted the terminal block onto a wood block prior to mounting on the boat.

Connecting to the terminal block is where I need help and I've been able to find diagrams.

At the terminal block, I’ve hooked all the positives to one side and the negatives to the other side . I’ve labeled the batteries (1-starter, 2&3-trolling motor) along with the wires respectively. According to the Minnkota charger guide book, I don’t have to unhook the 24 volt system to charge.

Last night I started to hook up the batteries to the post and started with the 24volt connections, the first battery on the 24volt hooked up fine…but when I hooked up the second battery (-) neg. connector it caught on fire and toasted the wire for about 6 inches.

If anyone can offer up some assistance on getting this connected correctly, I would be greatly appreciated as I don’t want my boat to catch on fire.

I've also posted this on the main page in addtion to the small boat electrical, just incase someone may have some input from that area also.

Thanks Scott

2manyboats posted 05-21-2007 12:51 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
Did you wire all the batteries in parallel to the buss bar and then try to connect two batteries in series to get your 24 volts ?
Plotman posted 05-21-2007 01:26 PM ET (US)     Profile for Plotman  Send Email to Plotman     
Whoa - you are saying that you connected the negatives from each battery to one side of the bus, then the positives to the other, while you had two of the batteries connected in series?

Dude, you created a dead short on each of the two batteries for your 24v trolling motor. Draw yourself a diagram on paper, and it is real obvious. You NEVER want to directly wire the positive and negative terminals of a battery together, which is exactly what I think you did.

Minnkota does indeed say that you can hook the batteries in a 24v series up to their chargers without breaking the 24v connection, but that is because the circuits are isolated inside the charger. By using a bus instead of a terminal block, you un-isolated them.

You want to use the kind of terminal block where there are six screws on each side, but one where each terminal screw along each of the long sides of the block is connected to one (and only one) screw on the other long side (see . What you used is more akin to a bus, that ties all of the pos and all of the neg together. This, as you discovered, is a big no-no.

stubbleduck posted 05-21-2007 04:06 PM ET (US)     Profile for stubbleduck  Send Email to stubbleduck     
Thank you. And yes....a very big no-no.

Ok, so I need to get a different kind of Terminal block, like the one you've shown. Will do at lunch.

After it sparked and caught flame, I did disconnect everything. Prior to the spark I had the trolling motor wires connected but the trolling motor was not plugged in.

Could I have ruined anything like the charger? Or the trolling motor wires? Batteries? Is there anything I should look at because of the wiring [mistake]

And again Thank you, obviously my speciality is not electrical.

Also, I bought that terminal block from a marine store and told them exactly what I was doing and that's what they sold me. Needless to say I'm disappointed.

jimh posted 05-22-2007 12:00 AM ET (US)     Profile for jimh  Send Email to jimh     
I have been giving some thought to what probably happened. Let me try to guess at where things stand now, after the fire:

If you did not connect the charger wiring to the buss bars, then the charger is probably OK.

The 24-volt battery consists of two 12-volt batteries connected in series. When you began to connect the batteries in parallel by connecting them to your bus bars, you directly shorted the plus and minus terminals of one of the batteries together. The short circuit current of the battery is very substantial. It immediately turned the connecting wiring into a very effective heating element and started melting the wire.

If that is all that happened then you just have melted some wiring and insulation, and probably put a healthy dent in the battery's charge. You can replace the wiring. The battery can be recharged. So far the damage is not too bad.

If you had the charger connected, there should be fuses in the charger leads. If there were excessive current, the fuses should have blown. It is typical on chargers which are intended to be used with 24-volt series-connected battery banks that BOTH leads have fuses. The fuses are there to protect against just this sort of mistake in wiring or other current short circuits. Check the fuses and replace any that are blow. Check the charger operation again, before re-wiring the system. See if it produces the proper voltage output from all three sets of outputs.

The buss bar terminal block might even have survived intact. If so, take it back to where you bought it. And let the guy who sold it to you know he is not of much value when it comes to handing out electrical advice.

jimh posted 05-22-2007 10:20 PM ET (US)     Profile for jimh  Send Email to jimh     
Was the charger connected when the short circuit was made?
stubbleduck posted 05-30-2007 07:30 PM ET (US)     Profile for stubbleduck  Send Email to stubbleduck     

You hit it on the money. The charger was connected on the short circut. I've retested it and it works fine. The one thing I will need to do is replace the wiring. It looks good after about a foot but I think I'll just replace it for good measure.

Thank you very much,

jimh posted 05-30-2007 11:49 PM ET (US)     Profile for jimh  Send Email to jimh     
If the charger were connected but not power ON, it is probably fine. If it were powered ON then the fuses should have protected it.

Many people are complacent when working with low voltage circuits because they do feel there is much risk of electrocution from 12-volts of direct current. They are right about that, but, as this discussion has shown, the short circuit current can be very high in low voltage circuitry. The instantaneous heating effects from very high short circuit currents can be as dangerous as the electro-shock effects from higher voltage circuits.

If you wear any metal jewelry such as a watch band or a wedding band, it is very prudent to remove it when working around a storage battery.

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