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Old 11-13-2011, 11:58 AM   #1
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Confused about battery drain checks

I have an 1986 Sovereign and have questions about testing for battery drain. When I disconnect the neg battery cable and check for voltage between the neg battery post and the removed neg cable I get 12 volts. When checking for amps using my multi tester at the same location I don't get a reading even if I turn on a 12v light. I am somewhat electrically challenged and don't understand but am eager to learn more.

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Old 11-13-2011, 12:59 PM   #2
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Originally Posted by bikeguy View Post
I have an 1986 Sovereign and have questions about testing for battery drain. When I disconnect the neg battery cable and check for voltage between the neg battery post and the removed neg cable I get 12 volts. When checking for amps using my multi tester at the same location I don't get a reading even if I turn on a 12v light. I am somewhat electrically challenged and don't understand but am eager to learn more.

Thanks,
The voltage test you are doing indicates there is indeed, something in the trailer that is turned on, or completing the circuit, to give you the voltage reading.

The current test you are doing is evidently not being done right because you should see the current flow, especially when there is a light turned on. I would say double check the way you are using the meter. On Amps, and always start out on the highest scale of the meter to be safe, insert the meter in series with the load. i.e. take a cable off the battery, and put the meter between the battery and the cable.

Current flows from negative to positive outside of the battery, but I almost always get the meter in the circuit backwars...reverse the leads if you do the same.
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Old 11-13-2011, 01:07 PM   #3
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To test for voltage, you apply the meter leads to the (+) and (-) posts of the battery. On the meter you need to 1st select the mode that will display DC volts, then the range that will cover 15 volts. A lead acid battery that reads 12 volts is dead and needs to be charged. On some meters you may have to plug the red lead into a different hub on the meter to make these choices, most use a switch or button. If the battery has been charged, disconnect the charger or as you did, a cable (negative is recommended as you won't draw an arc if you get the wench between negative and the frame). It is best to wait an hour or two to let the surface charge dissipate to get an accurate reading.

To read current you need to put the meter leads in series between the neg post and the neg cable as you were trying to do, red lead to neg post, black to cable The meter must first be set to DC amps and select the highest range. Again, you may have to select a hub on the meter for amps that is different than the one used for volts. Turn on a load and you should see a reading, if very low on the scale switch down a scale or two to bring the reading up to mid scale if yours uses a pointer. Most meters have a fuse that blows to protect the meter from over current on ammeter mode. A few meters require an external shunt resistor to read current, or high current values.
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Old 11-13-2011, 01:53 PM   #4
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Originally Posted by bikeguy
I have an 1986 Sovereign and have questions about testing for battery drain...
I am assuming you suspect something is draining your battery and SteveH is correct. Since you are reading battery voltage across the open circuit you created, it indicates current flow through the meter that is represented by the voltage and I'll skip the chapter that explains why that happens.

Once you get the meter set up where you are reading amps, don't be surprised if the load you are seeking is drawing a fraction of an amp (1 amp = 1,000 milliamperes ). Even a small load can drag a battery down over the course of a couple weeks. Your next step will be to watch the meter and start pulling fuses until the current stops, that will tell you which circuit the load is on.

On newer coaches the culprit is the always on LPG alarm or the radio that is on standby and not truly off.

If you really aren't interested so much in how much but if there is a load or not, the 12v you noted across the open can be used. Some use a simple 12v test light in place of the meter. When you pull the fuse on the circuit with the load the light goes out. The ammeter is the best way as you can better see smaller loads.

A battery that appears to be taking a charge but drains quicker than expected may have a bad cell. A way to test for that is with a battery hygrometer if you have a battery with removable caps.
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Old 11-13-2011, 02:59 PM   #5
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Another thought, if you have batteries in parallel, disconnect the other(s) else you won't be sure of what you're reading.
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Old 11-14-2011, 11:27 AM   #6
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re: "A battery that appears to be taking a charge but drains quicker than expected may have a bad cell. A way to test for that is with a battery hygrometer if you have a battery with removable caps."

Hydrometry is HazMat territory, has the potential to contaminate your batteries, and it typically not needed in RV battery use.

Both specific gravity and voltage are direct measures of battery state of charge. Yes, hydrometry will allow you to find a specific bad cell but, if a battery is bad overall, it should be replaced anyway.

As with any measure, you need to use the proper technique and know how to interpret the readings you get. This applies to both voltage and specific gravity.

As silvergoose notes, voltage is measured in parallel with the battery bank and current in series.

A fully charged battery will start out at 14v or so and settle down to 12.6v after a few hours. The residual load of about 20 watts for alarms and control boards plus the 1% per week to month self discharge rate of lead acid batteries means that the voltage will drop down to 12.0 volts if left for a week or two.
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Old 11-14-2011, 11:30 AM   #7
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Thanks so much for you interest and help. So far I have set my multi meter to dc 10 amps put the neg lead on the neg battery terminal and the red lead on the disconnected cable and I get no reading. When I turn on a light it reads 1.34. With the meter set on dcv I get 12.6v and removing the incomming wire from each terminal on the fuse box behind the front couch does not change the voltage reading. Do you have any further suggestions.

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Old 11-14-2011, 04:22 PM   #8
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I would only suggest you recheck the current (amps) with the meter set on a lower scale, say 3 amps, and if it still reads 0, you might try going even lower on the scale.

A current drain of a few 100 milliamps will drain a battery over a period of a week, and you wouldn't even see that on a 10 amp meter.
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Old 11-14-2011, 06:19 PM   #9
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I agree with SteveH on his suggestion. I think you will find a load of an amp or less, maybe just a few hundred milliamps.

You may or may not have them, but some of the newer devices impart a parasitic load on the battery. The common ones are a hard wired LPG detector or a radio that keeps its memory alive with battery voltage. If running on gas, the fridge electronics and the DSI water heater electronics are sources of battery drain many of us don't consider. If you have an LPG detector, it may not be connected though the main fuse panel but rather has an inline fuse holder, probably close to where the line connects to the battery bus.
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