Voltwatch and Remaining capacity

Voltwatch and Remaining capacity
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Voltwatch and Remaining capacity - 12/13/2004 7:45:36 AM

DrV

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I have a 1200mAh NiCd 4 cell battery in my 40 size spitfire which uses total of 5 standard servos including a retract servo. This is a new airplane and I have the Voltwatch on it. I fly the airplane for approximately 7-8 min each flight. I do a C/10 charge the night before and the battery reads 5.7 volts before I leave home to go to the airfield. After three 7-8 min flights, the voltwatch displays the third green light which I believe is between 5- 4.96V while not moving all the servos. When I do move all the servos I can see the red LED flashing. I believe this means I have great resistance in the wires. I am running no more than 20" of wire in each wing to the servo. At home the voltage unloaded read 5.07v and discharge capacity at C/5 on the Triton read 960mAh. This does not make sense. What does this mean? Is the Voltwatch incorrect? Would someone be able to explain these results? I have checked all the servos, nothing is binding to show such current draw as voltwatch is showing. According to the capacity left in the battery I only used 340mAh but according to the voltwatch it flashes the red LED under load which should be 4.68V or less.

I would appreciate any input

Mike

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RE: Voltwatch and Remaining capacity - 12/13/2004 5:07:19 PM

JNorton

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A red light means the voltage is dropping on the buss. The most common cause is a binding servo at the extremes of travel. You are seeing it more with the batteries slightly discharged. Move the sticks one axis at a time to determine what servo is binding.

To use a voltwatch effectively means reading it just after landing, before the voltage of the batteries rise to give a false sense of security.

I use them on all my planes.

John

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RE: Voltwatch and Remaining capacity - 12/13/2004 5:13:18 PM

JNorton

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I know you said the servos were not binding but I think you will find they are. To give you a sense of security plug in a voltmeter to the servo buss (red and black wires of a servo lead) and you will see the same result. Voltwatches are accurate voltmeters they are not a battery fuel gage however.
John

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RE: Voltwatch and Remaining capacity - 12/13/2004 6:46:08 PM

DrV

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The red light is not flashing if I run the servos slowly in one axis such as the ailerons. Moving the stick right slowly and holding it there does not cause the red to flash. Same applies to the left. If I cycle each channel individually this does not happen. This only happens if I turn all 5 servos at the same time quickly. I have limited the endpoint of each servo from the radio for this specific reason. However I will recheck again. In addition I have found that one of the aileron servos seems not to function correctly. When I move it in one of its endpoints and hold it there, then apply light pressure to the servo arm, it does not hold and seems to slip the gear and comes back up again. All the other servos don't do this. Maybe this is a bad servo and draws a lot of current from the battery. I will replace this servo and see if the problem recurs.

Thank you for the advice.

Mike

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RE: Voltwatch and Remaining capacity - 12/13/2004 10:05:20 PM

JNorton

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DrV,

Good idea to suspect the servo. I like the Voltwatch because it gives me an eyeball as to what the receiver buss is doing.

Unplug a servo one at a time. What are the symptoms. Unplug the current battery from the receiver, temporarily plug in another battery without the switch. Do you have the same symptoms? If it seems fixed I would suspect the switch or harness.

Another check would be replacing the receiver to make sure there hasn't been any connection problems internally like a bad solder connection. If you have the same results it could be there is nothing wrong, simply that the servos are drawing that much current for the size wire.

A little prevention now is worth it.

John

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RE: Voltwatch and Remaining capacity - 12/13/2004 10:33:39 PM

Rodney

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Sounds more like a high impedance between the battery and the reciever, probably switch wiring. I'd venture to say that there is some corrosion on the connectors at the switch or battery or too small gage wires on the switch assembly. Try plugging the battery right into the receiver (bypassing the switch and switch wiring) and see if the same thing happens. If it appears better, replace the switch.

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RE: Voltwatch and Remaining capacity - 12/13/2004 11:21:58 PM

JPMacG

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I see the same sort of thing. I think it's a combination of the internal resistance of the battery and harness wiring, connectors, and switch. In particular, the connectors and switch provide relatively high contact resistance. If you have a hard-to-move control surface that would make the peak current draw high as well.

With more time and experience you will learn to interpret the VoltWatch. To see what it looks like, discharge your battery most of the way on your cycler and then see how the Voltwatch responds as you move the servos.

I have Voltwatches on all my planes and they have already saved me from flying with a bad battery.

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RE: Voltwatch and Remaining capacity - 12/14/2004 12:06:18 AM

JNorton

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quote:

Unplug the current battery from the receiver, temporarily plug in another battery without the switch. Do you have the same symptoms? If it seems fixed I would suspect the switch or harness
quote:

Don't yah just love it when everyone agrees!

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RE: Voltwatch and Remaining capacity - 12/14/2004 6:01:31 AM

DrV

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Thank you all for the recommendations. I will first replace the bad servo on the aileron and see if the problem recurs. If it is still there, I will try all other suggestions to isolate the problem. Everything is new on the airplane except the servos. I don't think there is any corrosion between the connectors or the switch. The airplane is kept indoors and I live in Los Angeles, away from the ocean, where the relative humidity is not very high. Again it is still possible and I will recheck. Another thing that creates difficulty is that this only can be duplicated after the third flight. Once the batteries rest for about 10-15min I do not see the red light flashing. I have to do these things at the airfield after the third flight or discharge the batteries partially when I get home.

Thank you again for all the input

Mike

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RE: Voltwatch and Remaining capacity - 12/14/2004 7:40:05 AM

Mike01

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Get a digital voltmeter, like a multimeter (borrow one from a friend - somebody you know has one). Have a friend hold the terminals directly to the battery (if you have no naked metal there, do something creative to gain access to the battery).

Take a reading there while moving your servos around. If you get the same reading, you know you have a problem. If you don't, you know the true votlage of your battery under load and you know that you have a resistance problem.

Voltmeters are basically amp (current) meters with high value resistors that are connected in parrallel with the battery (as opposed to in series like an amp meter). The value of the resistor is used to calibrate the voltmeter. Therefore, a high resistance between your battery and your voltmeter will throw that calibration way off and result in false readings.

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RE: Voltwatch and Remaining capacity - 12/14/2004 7:58:24 AM

DrV

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Mike01, I have already tested the Voltwatch which seems to be fairly accurate with true voltages on a multimeter. I don't think this is a voltwatch problem. This is like you said a resistance problem somewhere in the link and I have to find the exact location.

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RE: Voltwatch and Remaining capacity - 12/14/2004 4:51:05 PM

JNorton

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Mike01

quote:

Voltmeters are basically amp (current) meters with high value resistors that are connected in parallel with the battery (as opposed to in series like an amp meter). The value of the resistor is used to calibrate the voltmeter. Therefore, a high resistance between your battery and your voltmeter will throw that calibration way off and result in false readings

Voltmeters are basically a "milliamp" or "microamp" not an amp meter. The multiplier high value resistors are connected in series to the meter. The voltmeter is then connected in parallel to whatever you wish to measure. In older VOM's 20K ohm per volt loading was pretty standard, newer DVMs have a 10 Meg or 100 Meg ohm per volt circuit loading. All this means is that a high resistance (1k) between the battery and the meter will NOT throw the reading off.

A fairly low resistance between the battery and the receiver will definitely affect operation. I.E. switch resistance or resistance between the connectors of the battery or the connectors to the receiver.

Voltwatches are a high impedance voltmeter that does not affect circuit operation and I have found them to be very accurate.

I would just like the concept of voltmeter loading verses a fairly low resistance in series with the battery causing trouble to be understood.

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RE: Voltwatch and Remaining capacity - 12/14/2004 4:57:15 PM

Mike01

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quote:

Voltmeters are basically a "milliamp" or a or a "microamp" not an amp meter. The multiplier high value resistors are connected in series to the meter. The voltmeter is then connected in parallel to whatever you wish to measure. In older VOM's 20K ohm per volt loading was pretty standard, newer DVMs have a 10 Meg or 100 Meg ohm per volt circuit loading. All this means is that a high resistance (1k) between the battery and the meter will NOT throw the reading off.

A milliamp meter or a microamp meter is still an amp meter, ie a device that measures current. You're nitpicking.

As for throwing it off...even attaching a voltmeter to a deans plug on the end of battery pack vs the battery bars of cells themselves gives you a slightly different reading, and that is a very low resistance. So how can you say a high resistance will not throw it off even more?

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