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Old 19-03-2014, 17:24   #1
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AC voltage in a DC system?

Hi All,

As you'll determine from this query, an electrical engineer I'm not, despite many years afloat. Anyway, here it goes.

I've burned through a couple of handheld vhfs in the last year, and 1 Iridium satphone. It was the Iridium, of course, that really got my attention. (I bought some seriously cheap VHFs off Amazon and have small kids, and so attributed those deaths to one of those primary causes.) Anyway, with both the VHFs and the Iridium, the behavior was the same: the batteries (lithium ion) stopped charging. Both were plugged in continuously to our 12-volt DC system through a "cigarette lighter" adaptor.

A fellow cruiser here in the Galapagos advised me to check out where I might have some stray AC voltage in my system. And, indeed, according to the multimeter, when I'm charging (engine alternators and solar) I'm producing around 3 volts AC (as measured directly on my batteries). It was his thought that this voltage would mess with the sophisticated electronics in those lithium ion batteries and fry them—and, potentially, damage other electronics on the boat.


Is this possible? AC in a DC system? Is my inverter to blame? How do I eliminate or contain that AC voltage?

I plan to run some more tests tonight and tomorrow morning: turn off the solar, measure without engines running, etc. But, at first glance are there any electrical geniuses out there having an "ah ha" moment that they can share with an electrical neophyte?

s/v Dafne

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Old 19-03-2014, 17:36   #2
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Re: AC voltage in a DC system?


Until one of the electrical grownups arrive, I'll just say this:

Before you get carried away, you may need to verify that you actually have the problem you think you have:

Check that your multimeter, when switched to AC, does not measure ANY voltage across a source of "known pure" DC (like a disconnected torch battery)

Otherwise the reading cannot be trusted

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Old 19-03-2014, 18:00   #3
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Re: AC voltage in a DC system?

Batteries prefer to be fully charged and then fully discharged. Generally the term is deep cycling. Not sure if having them permanently on charge is good for them (batteries or chargers) The AC thing is a new one on me.
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Old 19-03-2014, 18:02   #4
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Re: AC voltage in a DC system?

I think your next step is exactly what you plan: Turn things on/off one at a time and try to see what it's related to. You engine alternator will create some pulsing as it charges, but I wouldn't expect it to show up on a multimeter. Similarly, your inverter will create a pulsing load, but again I wouldn't expect it to show up on your meter.

Oh, one simple thing. Are you sure the meter is reading volts and not 3 milivolts (mV)? That's worth double checking. 3mV could be ignored.

Based on what you find, we can sort out next steps....
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Old 19-03-2014, 18:26   #5
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Re: AC voltage in a DC system?

First, those li-ion batteries are probably replaceable, saving the radios.

Second, if you're sure your meter is reading properly, as suggested above by others, are there any fluorescent lamps around?
Fluorescents will induce all kinds of funny EMF into various circuitry, which can be more or less sensitive to it. It really shows up on an oscilloscope trace, but a VOM/multimeter will see it too (don't know if it's from the tube or the ballast, never bothered to root it).
Nearby active radio transmitters (and even a microwave oven) can do the same, inducing minor AC currents into various objects, wiring, and circuitry.
You may have a squirrely grounding problem somewhere too.
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Old 19-03-2014, 18:39   #6
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Re: AC voltage in a DC system?

They call them alternators because they produce ac current,/ somewhere around 115-120 volts depending on the rpm it is being turned. The voltage is then reduced and flows through a diode plate that changes ac into dc. There is a small amount of ac current that "leaks" back through the diodes into the dc output circuit. This can be corrected by adding a suppressor resistor to the circuit. There is usually one installed either on the alternator or voltage regulator. Is this ac measured only when running engine? Is it there when you are connected to shore power with engine off? Is it there when your inverter is running with engine on or engine off? Go through these steps and then you will know how to proceed
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Old 19-03-2014, 20:04   #7
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Re: AC voltage in a DC system?

[QUOTE=paulbennett;1496959]Hi All,

"Both were plugged in continuously to our 12-volt DC system through a "cigarette lighter" adaptor."

Lion batteries cannot be left on continuously unless the charger has some way of turning off the charge to the battery when full. Especially if you have cut off the wall wart or made up your own 12 v leads to plug in the cig lighter.

regards Bill
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Old 20-03-2014, 06:08   #8
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Re: AC voltage in a DC system?

What you are describing is “AC Ripple”.
See Figure 4, here ➥

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 22-03-2014, 05:42   #9
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Re: AC voltage in a DC system?

If you want to get the dead battery packs back into service, dig out the cells and measure them with the multimeter, one will be deeply discharged. As long as one hasn't been over charged, just charging up the low cell will bring the pack back into balance and everything will work again. As has been mentioned, you can buy replacement cells with the tabs on them, a hot soldering iron is all you need to replace the dud cell.
to stop it hapning again, watch the max charging voltage, this is the number 1 killer of all types of lithium ion cells.

T1 Terry

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