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ContinuousWave: Small Boat Electrical
Battery Leads Reversed
|Author||Topic: Battery Leads Reversed|
posted 04-21-2008 01:53 PM ET (US)
Out of a completely brain dead moment I reversed the battery cables on my Johnzuki DF70. There was a brief spark and nothing. I did not attempt to start the motor as I only needed to adjust the tilt of the engine in the garage. What could I have fried? I believe there are some fuses but I don’t know where they are located. If the diodes in the alternator toasted would I need to replace the alternator or can just the diodes be replaced?
I am out of town for a few days ARG!
posted 04-21-2008 08:55 PM ET (US)
When you connect the battery with the polarity reversed you get very high current flow through diodes which are normally reversed biased but are now forward biased. The high current flow occurs instantaneously, and it is only limited by the series resistance of the wiring, the connectors, and the diodes themselves. In simpler terms, it is like connecting the positive terminal of the battery directly to the negative terminal of the battery, and making the circuit through the diodes of the alternator.
When a high current like that flows there is immediate heating of the conductors in the current path, and in the case of the diodes, whose current rating is probably only one-twentieth of the current that is flowing, the internal semiconductor material in the diode will probably very rapidly heat up and melt.
It is hard to predict exactly how the diode will fail. It can fail into an open circuit; it can fail into a short circuit; it can fail into a moderately resistive circuit. Any of these failures can occur. What you can bet on, however, is that one or more of the diodes will fail. About the only mechanism that will prevent that is a combination of really high current ratings in the diodes and some unusually high resistance in the circuit elsewhere.
If you are lucky you will have a failure where:
--a diode fails into a dead short to ground and shunts all the current away from everything else
--something upstream fails before the diodes
But in general nothing really good comes out of that momentary polarity reversal. A lot of bad stuff happens in an instant.
posted 04-21-2008 08:59 PM ET (US)
By the way, in many electronic devices which are operated by DC voltage, it is a common design feature to include a really big diode wired across the power input terminals and set to conduct if the wrong polarity is applied. The logic behind this design is that in the event of a polarity reversal, this diode will take all the current and protect everything else in the device until the fuse blows.
This is more or less what happens with the alternator, but the problem is there typically is no fuse between the motor and the battery to blow. The diode just conducts until it melts down (internally).
There may have been some protection effect from the alternator diodes, and, if things are going your way, you may find that other devices downstream have been saved by the sacrifice of the alternator diodes.
posted 04-22-2008 12:47 AM ET (US)
Hook the battery up THE CORRECT WAY and try it. You may luck out. Besides, if something is broken and burned out, you have already done it. Good luck to you
posted 04-22-2008 09:16 AM ET (US)
When re-connecting the battery with the proper polarity, if there are diodes in the circuit which have failed into a short circuit, they will conduct and draw current. This may cause additional damage. When initially re-connecting the battery, I suggest using a fuse in the positive lead. If the fuse blows immediately you know there are more problems. If the fuse does not blow, then remove the fuse and see what happens.
posted 04-22-2008 11:11 AM ET (US)
I am hoping to get home tomorrow to take a look. With the tilt function not working I am hoping a fuse blew somewhere. Worst thing about it I was getting ready to take the boat to the dealer to have the install (done by an independent guy)certified under warranty.
posted 04-22-2008 08:07 PM ET (US)
Let's look at the possibilities:
A. Diodes failed open. They functioned as very expensive
B. Diodes failed closed. They will, unless very beefy and
Brief spark then nothing points to A. And I've never heard
I'd make sure the electronics were disconnected (just turn
As to whether you can get the diodes separately, check out
posted 04-22-2008 08:38 PM ET (US)
The problem with the diode-as-fuse theory is they are not in series with the current source, they are shunted across it.
posted 04-22-2008 08:52 PM ET (US)
Usually the charging current source is connected directly to the battery cables, without any fuse protection. The rest of the motor's electrical system is usually downstream of a fuse. Here is a diagram:
Referring to the diagram's nomenclature, if you are lucky, CR2 and CR4 or CR1 and CR3 both fail in the same way at the same time. That keeps current from flowing through the stator coil. If only one of the pairs fails, then some current will flow through the stator coil, which could cause damage to it.
I have seen diodes fail in various ways, and I have seen cases where only one of the four diodes failed.
If the downstream electronics had enough guts to hold up long enough until the fuse blows, they could be unharmed.
posted 04-23-2008 02:50 PM ET (US)
Well I hope I am lucky when I get home. Combing the web I found two others who brain cramped as I did and only blew a fuse. I am keeping my fingers crossed for simple blow fuse. Now the bummer, I will be out of town a couple of extra days.
posted 04-23-2008 02:55 PM ET (US)
I did this on a seadoo once or twice (its dark down there). Only blew a fuse. Good Luck
posted 04-23-2008 05:02 PM ET (US)
One critical detail is missing. How long was the battery connected in the incorrect way?
posted 04-24-2008 12:37 AM ET (US)
The cables were not reversed for more than a minute or so.
After reading more stuff on the web I am feeling better about the chance of it just being a fuse.
posted 04-24-2008 08:36 AM ET (US)
Another factor: the condition of the battery and associated connections. If the battery were not fully charged and in good condition, and it there was some resistance in the connections, the current flow will be limited by those factors.
But most diodes connected across a large 12-volt battery with forward bias don't survive for minutes. Everything happens in the first few seconds.
posted 04-24-2008 09:17 AM ET (US)
The diodes are in series with the current flow, so they
end up protecting the wiring.
A minute would be more than enough -- a car battery shorted
Most cars have a fusable link between the battery+starter and
posted 04-27-2008 12:16 AM ET (US)
Well I got a call from the dealer and they said everything was fine. I replaced a 30amp fuse that blew otherwise everything was good.
posted 04-27-2008 12:27 PM ET (US)
Next step: buy a Power-Ball Lotto ticket! You're on a run of good luck.
Good luck, that is, unless the dealer charged $100 to replace the fuse.
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