I'm trying to get all my battery stuff straightend out so I was wondering if you can use a regular voltmeter/multi-meter to get your readings on your Rx/Tx packs.

Yes but they won't tell you much mor then there is voltage. If you have info in the pack that tells you what voltage represents what amount of charge you will be OK! But for true condition I would think you would want a tester for the battery pack, just like the garage uses to check out your cars battery under load! But smaller of coarse!
Charlie

Go to this site http://www.rcbatteryclinic.com/ and go to "Loaded digital voltmeter, better than an ESV".

You can, But. What you are interested is not the static voltage, IE unloaded voltage but the loaded voltage. What you want to measure is the voltage under load of the receiver and servos moving, or the transmitter in operation.

Almost all transmitters have a voltage reading, either a digital voltage or on the older transmitters an analog meter showing "output". If your transmitter has the digital readout, you have a good measurement of the actual voltage of the pack under the load of the transmitter. THe "output" meters indicate the condition of the pack in that if the voltage drops, it will be indicated on the meter. Not an absolute measurement.

For the receiver, you have two options. First is to use an expanded range voltmeter. These come in two types, a loaded and unloaded. You wan't the loaded type because you are usually taking a reading of the battery from the charging jack. A loaded meter will give a truer reading of the pack. The second option is a device like the VoltWatch. It functions as an expanded range voltmeter but it reads the voltage and traslates them to LEDs. One led will light indicating the voltage at that point. The great thing about the voltwatch is that it is reading the voltage from the distribution bus in the receiver, IE where the servos plug into. It eliminates all variables like switches and shows just what the receiver is seeing.

You can, with a little Ohms Law put a resistor across the leads of a standard volt meter and get a loaded reading. You would want a resistor that would cause a 500ma draw and would handel a 3w or better load.

campgems, in your last paragraph, you mention putting a reasistor in the positive line form the volt meter . I have been using just that system for years. Wire in a 10 W 10 ohm resistor, in the positive line and have a plug fitted on the end to suit your charging plug in the model. This will place a load equivalent to 4 standard servos all working at once, and will give an accurate reading of the battery under load. Tandy's or any electrical supplies store will supply the resistor .
Bill

The madgenius
I hope that you just misspoke when you said that you put the 10 ohm resistor in the positive line when you measured the battery voltage.

What Campgems said was that he put the resistor across the leads of the battery, which is the correct way to do it. This way the 10 ohms is in parallel with the meter and represents the load to the battery. The meter reads the drop across the load.

What happens when you put the resistor in series with the positive lead is that you don’t put hardly any load across the battery at all. The input impedance of typical good quality meter is about 1,000,000 ohms. If you put that in series with a 10 ohm resistor you get a total resistance of 1,000,010 ohms. That does not represent a load to the battery. What you will read on the voltmeter is basically the no load terminal voltage.

A simple test will demonstrate what I am saying. Read the no load voltage on the battery under test. Then put a 10 ohm resistor in series with the positive lead and read the voltage again. The voltage will be the same. Next put the resistor across the battery positive and negative leads and read the drop across the resistor and you will get a lower voltage which is the loaded voltage of the battery.

I think that a 10 ohm resistor is to low, for the tx it will draw 1 amp. I myself put a 33 ohm resistor on both the red and black connector of the meter. The current is Voltage divided by the resistor.

Platinum,
Yes, what you say is true. Or you could just set up your meter to read the current directly and not have to do the math.

If I understand you correctly, that setup still will not tell you what the voltage is under load because you still have the resistors in series with the battery and meter.

Ya'll just way to serious bout them there battries. Plug it in and when it gets to low to fly the plane anymore charge it back up. That is the correct way. J/K guys.

right.

Of course you guys are right. After 55 years of being a technician and engineer it is hard to get out of the habit. Sorry, I didn’t mean to hijack the thread.

Mr. Pruitt, Campgems, CG R

I find this interesting, do you have any pics that
would help me to understand?

Thanks,

Bob

I just use one of these guys. I plug it into my charge jack after 2-3 flights, and I quit flying when it gets below 70%. On a 700mAh pack, that's usually 5-6 flights, on a 1500 mAh pack, I have not gotten down that far.

http://www.horizonhobby.com/Products...ProdID=EXRA502

I thought about his and almost posted wrongly because for some reason I am stuck on checking the receiver pack and you are reffering to the TX pack.
10 ohm would be about right for the receiver.
But when it comes to the transmitter, why? I have numerous transmitters several brand name and more cheap ones from cheap RTFs and every single one of them has some type of indicator built in that lets you know the batteries condition and since the indicators only work when the TX is on they are under load as well.
OK on to the question of using a regular voltmeter. You can and best results are with a load applied but why bother with getting a resister and wiring it up correctly when an expanded scale voltmeter works just fine and doesn't cost much.
Here is the one I have for $10
http://www3.towerhobbies.com/cgi-bin...?&I=LXNK81&P=7

And PLEASE note that it includes a TX battery connection too if you insist on having one.

Another way to check the receiver battery, aid if you only have a digital voltmeter (not ESV.. just a standard digital voltmeter) then you can plug a servo extension into a vacant slot on the RX, attach the voltmeter leads to the wires of the servo extension (black and red wires) and turn on the TX then the RX.

The load is there because this is what the battery "sees" on a typical flight. Wiggle the TX sticks around a bit and you will see the voltage vary as you adjust the load by moving servos. Then, follow the guidelines for what voltage is safe to fly with.

Thank you CG R,

Bob


dignlivn

You are very welcome, Bob.

CGr.

Not all transmitters can use the jack for a voltage check, or for cycling the battery. They have a diode in the +line which prevents a draw being placed on the jack. THere are hacks on some of the boards for jumpering out this diode so you can cycle the batery. I know that a lot of the Futaba transmitters have the diode. THe 9C series doesn't. Anyway, like said a few times before, most every transmitter has a voltage indicator or digital readout.

Don

I never said he had to test it while installed.

i ment 33 ohm betwen the red and black connector

ok, Im having a hard time wrapping my head around this one.... I went to the battery clinic page about the loaded digital voltmeter, and the idea looked great. Im all set to build one, when it suddenly occours to me.... how is the Voltwatch NOT performing this function already?

I have heard people say that the Voltwatch cant give you a good read of your battery under load... but if its performing the function of a voltmeter, and measuring when the servos are active, isnt that under load? How is loading the battery with a resistor or a lightbulb different than having the radio powered up. In fact, Id kind of think that it would be better, since it would be the exact load of your current radio setup....

so, am I brilliant or am I totally missing something here? [8D]

Voltwatch does perform the same function, sort of.
The load applied to a Voltwatch is not consistant but it is good enough.
I have a Voltwatch installed in all of my airplanes and I trust it.

Missileman hit it on the nose. CONSISTANCY. The voltwatch measures whatever the voltage is at the instant depending on the load which is constantly varying. A constant repeatable load is the BEST way to know what is happening.

The voltwatch is actually more dependable than a loaded voltmeter if (a big if) you take the readings right after landing befor turning off the receiver and transmitter as you have been using the actual load from flying. If you wait awhile to read it though, the battery will partially recover so might read slightly higher than right after a flight. Not only that, if you cycle the stick while reading the bar graph and you see the lights dancing up and down the scale, you know you have other problems like dirty contacts, stuck servo or high impedance somewhere between the battery and the reciever.