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Old 03-28-2017, 02:53 PM   #1
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battery discharge

I have a two year old 4d agm batt. in my mainship 1 , I charged it about 1 mo. ago, boat is in water year round, no water in blige, I went to check it last week, I turned the key on to get a voltage reading. Showed 11 volts, I put it back on charge for 2 more days, with charger on I read almost 15 v. took charger off read 12.70 v at batt. with cables on, ( surface charge) all cables are less than 2 yrs. old, all clean, all heavy & thight. Only thing on batt ,when switch is off,is high & dry bilge pump, pos. side of batt. goes directly to shut off switch.( took pump wire off) .When I put the pos. ca. back on the batt. I get a VERY small spark, I've seen this many times before on other boats.I always attributed this to small diff. in ground potential. To test for voltage leak, I would use a VOM on amps. correct ? in series , pos. side, correct? what scale, what reading? Also, what should I get for an ohm reading across battery cables, with switch off ?should be the same as VOM not connected to anything, correct? Everything looks ok to me, Hope I don't have a bad batt. very heavy & not cheap ! Eng. cranked ok, maybe just bad meter on dash ? Thanks Rich
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Old 03-28-2017, 03:07 PM   #2
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Ok, if the draw is less than 10 amps, you should be fine with the VOM on that scale. It will read maybe 2 decimal places, so you should be able to see 1/100th of an amp. Most single wire engine alternators have a small draw when you hook the battery up. That may be your spark. I know it's not fun, but I would pull the battery fully charged and have it checked at a reputable battery store.

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Old 03-28-2017, 03:33 PM   #3
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It sounds like your battery and charging system may be fine (except for letting the battery sit mostly discharged for a month which isn't good for them) and your problem is a small DC drain which slowly discharges your battery.

Your current drain with all known loads off is small, maybe an amp or so. All VOMs are different and I wouldn't try this if you don't have at least a 5 amp scale. Check the current draw from the battery and then look at each circuit and figure out which one is drawing the current and find out why.

This sort of testing is much, much easier with a clamp on DC ammeter as you don't have to physically disconnect each circuit. But you can just put the VOM on the heavy positive lead and disconnect each branch and watch for the one that makes it go to zero.

One common device that pulls a fair amount of current is a CO detector, so check that first.

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Old 03-28-2017, 04:28 PM   #4
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If there's a spark, there is current flowing. A small amount perhaps but current.


Many people overlook the CO detector when considering battery discharge and current flow. Older ones were pretty big current hogs and could discharge a battery in a few weeks. Newer ones are much more frugal with current.


Other hidden current draws are memory circuits for stereos and amplifiers for TV antennas and propane or gasoline detectors.


Unlike some folks, as a former electronic technician, I believe a current meter is the best way to measure current. A current flow of over ten amps (which could damage your meter) would drain the battery very quickly and the spark would not be "VERY small".
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Old 03-28-2017, 06:25 PM   #5
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Agree with WesK, additional devices that can have parasitic current drains are ACR relays, automatic bilge pumps which "test" for water by running every few minutes and solar charge controllers. It's also possible but less likely that you have a battery charger that has a slight drain on the battery when it is not in charge mode.

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Old 03-28-2017, 06:30 PM   #6
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Thanks for the input, but I don,t have a co detector ,and when I turn the battery switch off, EVERYTHING is out of the system .( only bilge pump, which I took off the batt. ca. )Don't think it could be the alternator either, all power ( dash panel, starter,ect.) comes off field side of battery switch.If I look into the battery cables, with an ohm meter I should only see the radio memory, if everything else is ok, what should I see (ohms) on meter? what scale ? Also, should I put a load tester across the batt. ? I have only used them on smaller battery's . would it work the same on a 4d ? Thanks
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Old 03-28-2017, 06:37 PM   #7
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Well, you see a spark when you connect the cable, therefore you have something drawing current. Using an ohmmeter won't be as telling as using an ammeter right at the battery. Disconnect the positive lead, set the meter up to read amps, put the positive meter lead on the battery and the negative meter lead on the positive cable. Read the current. If it's anything more than zero you have a current drain someplace. Then start really disconnecting things till you find what is causing it.

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Old 03-28-2017, 08:13 PM   #8
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Some devices have capacitors, so you get a spark when you connect power. But current flow is minimal after cap charges. Just throwing out possibilities. Chargers often have caps.
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