Test Charging System After Reversing Battery Polarity

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Author Topic:   Test Charging System After Reversing Battery Polarity

jd550 posted 05-16-2009 11:46 PM ET (US)
Like the handful of others here who have made the mistake of [connecting the battery to the outboard motor with the polarity reversed], it finally happened to me. No need to make excuses, it is a done deal. I have Suzuki DF115 four-cycle motor.
After fixing the polarity, here is where it is a little strange - everything still appears to work fine, I get 14.8 DC volts on my multimeter when the motor is running. No blown fuses, no melted batteries, no melted cables. From reading past posts, it seems I am guaranteed that I at least fried my diodes. Does this mean my alternator is now producing AC current and will drain my battery? Can I confirm this with a multimeter?
Thanks. And yes, this will be the only time in my life this happens. I am not too pleased with myself at the moment.
Jerry Townsend posted 05-17-2009 12:18 AM ET (US)
Having 14.5 VDC at the battery with the engine running is good. Now, switch your multimeter to VAC and see what you get - if the ac voltage is more than a few tenths of a volt you probably have fried diodes. I have not tried this on an outboard engine - but maybe someone can run a check and report the ac voltage from a known working system. --- Jerry/Idaho
jimh posted 05-17-2009 09:16 AM ET (US)
I can think of two situations in which the battery polarity could be reversed and no harm would occur.
It is possible that no harm could be done if the charging system has a steering diode in its output. . A steering diode is a large diode in the output circuit that protects the other diodes from a polarity reversal. It only allows current to flow out of the charging circuit and into the battery. The typical outboard charging circuit typically does not have such a steering diode. That's because there is a voltage drop across the diode, and it just wastes power.
If you connected the battery with reversed polarity but the primary battery distribution switch was in the OFF position, then no harm would be done, except to something connected directly to the battery. Typically that might be a sump pump motor, and generally motors don't mind a polarity reversal, as they just run backwards from their normal direction of rotation.
But in the absence of these two situations, connecting the battery with reversed polarity usually causes a lot of current to flow for a very short time, and something usually fails as a result. If you find no damage, consider yourself quite lucky.
jd550 posted 05-17-2009 09:55 AM ET (US)
Thanks gentlemen.
I wonder if my 'luck' can be attributed the presence of a thyristor in the rectifier/regulator assembly. Does this serve a similar purpose to a steering diode? My service manual notes the presence of such a component.
Obviously at this point, the discussion is merely academic, but curiosity is getting the better of me.
jimh posted 05-17-2009 10:17 AM ET (US)
A thyristor (also called an SCR) could explain your luck. It sounds like the charging circuit in the Suzuki DF115 is rather different than the typical old-fashioned outboard motor. If the charging current is delivered through an SCR as the output device, no harm would occur with a polarity reversal because the SCR would be reverse biased; no current would flow.

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Author	Topic: Test Charging System After Reversing Battery Polarity
jd550	posted 05-16-2009 11:46 PM ET (US) Like the handful of others here who have made the mistake of [connecting the battery to the outboard motor with the polarity reversed], it finally happened to me. No need to make excuses, it is a done deal. I have Suzuki DF115 four-cycle motor. After fixing the polarity, here is where it is a little strange - everything still appears to work fine, I get 14.8 DC volts on my multimeter when the motor is running. No blown fuses, no melted batteries, no melted cables. From reading past posts, it seems I am guaranteed that I at least fried my diodes. Does this mean my alternator is now producing AC current and will drain my battery? Can I confirm this with a multimeter? Thanks. And yes, this will be the only time in my life this happens. I am not too pleased with myself at the moment.
Jerry Townsend	posted 05-17-2009 12:18 AM ET (US) Having 14.5 VDC at the battery with the engine running is good. Now, switch your multimeter to VAC and see what you get - if the ac voltage is more than a few tenths of a volt you probably have fried diodes. I have not tried this on an outboard engine - but maybe someone can run a check and report the ac voltage from a known working system. --- Jerry/Idaho
jimh	posted 05-17-2009 09:16 AM ET (US) I can think of two situations in which the battery polarity could be reversed and no harm would occur. It is possible that no harm could be done if the charging system has a steering diode in its output. . A steering diode is a large diode in the output circuit that protects the other diodes from a polarity reversal. It only allows current to flow out of the charging circuit and into the battery. The typical outboard charging circuit typically does not have such a steering diode. That's because there is a voltage drop across the diode, and it just wastes power. If you connected the battery with reversed polarity but the primary battery distribution switch was in the OFF position, then no harm would be done, except to something connected directly to the battery. Typically that might be a sump pump motor, and generally motors don't mind a polarity reversal, as they just run backwards from their normal direction of rotation. But in the absence of these two situations, connecting the battery with reversed polarity usually causes a lot of current to flow for a very short time, and something usually fails as a result. If you find no damage, consider yourself quite lucky.
jd550	posted 05-17-2009 09:55 AM ET (US) Thanks gentlemen. I wonder if my 'luck' can be attributed the presence of a thyristor in the rectifier/regulator assembly. Does this serve a similar purpose to a steering diode? My service manual notes the presence of such a component. Obviously at this point, the discussion is merely academic, but curiosity is getting the better of me.
jimh	posted 05-17-2009 10:17 AM ET (US) A thyristor (also called an SCR) could explain your luck. It sounds like the charging circuit in the Suzuki DF115 is rather different than the typical old-fashioned outboard motor. If the charging current is delivered through an SCR as the output device, no harm would occur with a polarity reversal because the SCR would be reverse biased; no current would flow.