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ContinuousWave: Small Boat Electrical
Reverse Polarity to Battery
|Author||Topic: Reverse Polarity to Battery|
posted 04-01-2013 12:47 PM ET (US)
[The battery cables to a 1996 Johnson 225-HP outboard engine were] reversed, [some switch, perhaps the main power switch or perhaps the ignition key was] turned to ON, and all gauges cycled through, but no [response came from the] System Check gauge. No attempt was made to start the outboard engine. [The polarity of the battery cable connection was] corrected.
Now the motor turns, the power tilt works, the [Voltmeter] works, but no System Check cycle occurs, no horn sounds, and no lights illuminate [presumably on the System Check annunciator panel]. [A] fuse [was] not blown, [there was] no smoke. Continuity on [conductors with insulation color] brown and purple was checked, [and] all seems fine. What most likely has blown? The rectifier? [The] boat [is] still in storage. [I am] planning my part acquisition.
posted 04-01-2013 09:32 PM ET (US)
It is hard to predict exactly what damage will occur if a fully-charged storage battery that can deliver several hundred amperes of current is connected to the primary power distribution on a small boat with the polarity reversed. Usually the diodes in the battery charger circuit will be forward biased and an extremely high current will flow, and this typically results in those diodes failing. However, some regulating circuits have protection against this sort of current flow, and there may be no harm to the rectifier diodes.
This is a good example of the importance of properly color coding the primary power distribution cables and the battery terminals so that there cannot be any confusion about the polarity of the circuit.
If the OMC System Check gauge does not initialize and run through its self check cycle when the ignition key is move to ON from OFF, that is an indication of a problem. Is there power to the gauge?
Does the engine start and run? Does the engine charge the battery?
The polarity reversal creates a situation in which current can flow along paths that are not normally there when the polarity is normal. The moment you connect the battery to the engine with reverse polarity, before you turn the key, the battery charging circuit will be connected and perhaps other engine electrical circuits that are not downstream of the ignition key. All of those circuits are places where damage can occur. Once you turn the ignition key to ON, more devices are connected to the reversed polarity circuit. These added devices are now also places where damage can occur. Some of the circuits are protected by fuses, which, if they act very rapidly, can stop current flow before damage occurs. Other paths, usually high-current paths, are not protected by fuses, and a lot of current can flow in a reverse polarity situation, causing damage.
posted 04-02-2013 10:53 AM ET (US)
[I] haven't had a chance to start the engine, but, when I turn the key to ON no lights or horn activate on the System Check tachometer guage. The engine does crank over fine and I will attempt to start this weekend.
Under the electric cover on the port side of the engine are three connectors, each with a three-prong outlet and a three-prong plug on top. I I understand the schematic, one of those plugs contain all of the system check wires. What is that plug for? What does it do? Perhaps that is faulty or burnt out?
posted 04-02-2013 11:23 AM ET (US)
If you have a schematic diagram that shows the electrical circuit of the engine, check the fuse that is under the cowling and usually in the feed of power to the ignition panel.
Usually the System Check gauge is powered by its eight-pin connection to the wiring harness.
|L H G||
posted 04-03-2013 07:59 PM ET (US)
Reversal of batery cables to an outboard will result in damage to the engine's charging system. This almost always will destroy the rectifier, and possibly the voltage regulator and stator as well.
posted 04-03-2013 11:14 PM ET (US)
Larry's comments are appropriate for 1970's outboard engine electrical technology. Modern regulator circuits may be completely unaffected by a polarity reversal. It really depends on the design of the circuit. I don't know how a System Check tachometer reacts to reversed polarity; I prefer to not experiment on my own System Check gauge to find out. If your System Check gauge is rendered non-functional, we can make a reasonable assumption the polarity reversal caused the damage. That would be useful to know.
posted 04-04-2013 12:06 PM ET (US)
The battery charging circuits that will be immediately damaged by a battery connection with reversed polarity are those circuits in which the main power diodes are directly exposed to the battery. There are two general types of outboard engine charging circuits that have that feature:
--circuits that use an automotive-type belt-driven alternator; these alternators use a field current regulation method, and their diodes are part of the output circuit; these are common in outboard engines, particularly recent models; and
--permanent magnet alternators on outboard engines with no regulation circuitry; these alternators don't have regulators and their diodes are directly connected to the battery; these are common in older outboards, where the charging current output was low and the battery itself was the regulating mechanism.
When you connect the battery with a polarity reversal, those diodes become foward-biased and a very high current flows. The current flows until something gives. Usually it is the diodes, unless they are really extraordinarily high-current diodes and the battery is not very good, or there is a lot of resistance in the cables.
More modern outboard engine battery charging circuits typically have some sort of regulator device in the circuit between the alternator raw output circuit (where the diodes are connected) and the battery. It is really not possible to characterize all of these devices with a blanket statement about the extent to which they can survive a polarity reversal from a battery connected incorrectly. Some might be destroyed and some might not be affected at all. It really depends on their circuitry.
In the 1996 OMC V6 engine under discussion here, it really does not matter if the regulator is damaged or the rectifier is damage. The two circuits are contained in a single unrepairable assembly. If either is damaged you have to replace the circuit assembly.
It may be possible for the engine to start and run, even with the rectifier-regulator assembly damaged. That is why I asked about the charging circuit output current and voltage. If the engine starts and runs, and it has the normal battery charging output current, you may be able to assume there was no damage to the rectifier-regulator assembly. I would check with a good OMC mechanic. They probably have a lot of experience with engines that have had the battery connected with the polarity reversed, and they can probably recite chapter and verse on what will likely need to be done.
posted 04-05-2013 10:00 AM ET (US)
Whether or not the stator coil in the alternator will be damaged by a battery polarity reversal is extremely difficult to predict. Current flow through the stator when the battery polarity is reversed depends on the state of the four diodes in the rectifier.
At the instant the battery is connected with reversed polarity, the current will flow mainly through the diodes, and little current will flow in the stator coil. If any of the diodes fail, the circuit becomes less balanced, and current can flow in the stator coil, but this depends on how the diodes fail, which diode fails, and how much time elapses until the next diode fails.
In the most favorable circumstances, the two diodes whose cathodes connect directly to the battery will fail into an open state simultaneously. If that were to occur, no current would flow into the stator.
In the worse circumstances, the two diodes whose cathoded connect directly to the battery fail so that one becomes an open circuit and the other becomes a very good, low resistance short circuit. This forces current to flow in the stator. To make the situation even worse, one of the diodes whose anode is connected to ground fails open. This forces all the fault current to flow through the stator.
However, it is impossible to predict the mode of failure of the diodes and the sequence in which they will fail, so it is impossible to predict if current will flow in the stator during the fault.
In general, the current carrying capacity of the wire in the stator in terms of its ultimate ability before failure is much higher than a diode's capacity to tolerate overcurrent. The diodes will typically fail much faster. The only unknown factor in the diode failure is the mode. The diode may fail to toward either an open circuit or a closed circuit, and have varying amount of resistance.
For all of these reasons, it is difficult to make a general statement about the effect of a battery polarity reversal on the stator coil. However, in general, we can say with a high degree of certainty, that reversal of the battery polarity is not a good event and is likely to cause damage. Exactly what will be damaged depends to a large extent on the particulars of the circuit and the particulars of the components.
posted 04-05-2013 10:27 AM ET (US)
To put some numbers to the question of the probability that current will flow through the stator in an alternator with four diodes, I have modeled the possible outcomes as having 16 states. The 16 states comes from having four diodes and each can either fail into an open circuit or fail into a closed circuit. Of the 16-states, only four states cause current to flow through the stator. On this basis, we could say that in general there will be a probability of 0.25 that current will flow through the stator at some point in the failure process.
The failure is a dynamic process. Once the first circuit component fails we have a different circumstance for the outcome. Remember that the wire in the stator is likely to outlast the relatively small semiconductor junction in the diode, and this tends to contribute to the diodes failing more than the wire in the stator. As a general consideration that probably reduces the probability for the stator to fail.
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