Digital Volt Meter

[mccd07]

Can anyone suggest a reliable and economical volt meter.

I have looked at many hobby stores on-line but am not sure what to by.

All I want to do is check my Lipo's before flight. 3s 2100 MAh.

Also, do I need a volt meter that give a llive load reading? Not really sure what that means but have read about it.

They say you can put a .5, 1 or 2 amp load. What does a 3s 2100 MAh put out for load?


Thank you all.

[Dood]

For about $20 at RadioShack, you can pick up a multimeter.
Thats all you really need for checking pack voltages and stuff.

On an electric helicopter, voltage drops under load are so negligible, you're not going to see dips in voltage under load without something like a flight analyzer such as the Eagle Tree.

Batteries themselves don't have a load, but they have a rating that states what loads they are capable of powering.
That load rating is what is known as the "C Rating" (I think this might be what you were wondering about)

You're 2100mAh battery may have a C Rating of 25C.
If it were a 1C battery, that pack would be capable of powering a 2.1A load.
2C would be 4.2A, 3C is 6.3A, and so on.

On a 2200mAh pack, 1C= 2.2A.

25C's of a 2100mAh (or 2.1Ah) is 52A.
On a 450 size helicopter, typically ran with a 35A speed controller, will draw UP TO 35A's continuously, probably about 45A's PEAK (depending on the ESC)
The servo's, gyro, receiver combined woking hard, won't be pulling much more (if at all) 2A's.

Needless to say, a 25C pack on something like a 450 size helicopter, is WAY MORE than sufficient.
BTW, the C Rating is what that battery is capable of supplying CONTINUOUSLY, much, much more than that in short bursts.
For stuff like hovering and forward flight, I wouldn't expect the motor to pull more than about 12 to 15A's.

If you really feel you need to know exactly how much current you're pulling during flight (which you probably don't) the only way you're going to be able to test that is, like I said, a Flight Data log such as the Eagle Tree.


They say you can put a .5, 1 or 2 amp load. What does a 3s 2100 MAh put out for load? No need for one on a Lipo flight battery.
Them load checkers are really for the guys running a NiMH or NiCD battery for rx packs.

A 4s NiMH pack may read 5.0V with no load.
Under a 2A load, it may read 4.6V or lower.
Lipo batteries, by nature, don't change much with a load.

A fully charged Lipo cell will usually read about 4.3V.
At the very beginng of the flight, they take a big hit, and after that, they VERY GRADUALLY drop in voltage. The voltage, when graphed, is almost linear.
At the the very end of the flight, the voltage will drop FAST, Very fast.
It could go from 3.4v per cell to below 3.2v per cell in a matter of seconds and continue to drop at that same rate.
Anything less than 3.1 volts per cell, Is dangerously low. If it drops lower than that, consider your Lipo ruined.

I'll see If I can find a graph that shows the discharge rate of a Lipo battery under load.

[Dood]

Here is a graph of a Thunder Power Xtreme V2 Lipo Pack.

http://i175.photobucket.com/albums/w155/DJB78_bucket/thunder-power-extreme-v2-lipo-disch.jpg

Take a look at it. You can see that the cell voltage stays at or near it's nominal voltage (about 3.5 to 3.6V) during the duration of the flight.

Notice that between a 2C load and a 25C load (which is a HUGE difference, btw) the voltage is only about 2 tenths difference.

Like I said, It's very negligible.

Let me (or anyone) know if you have any questions.

I hope I explained this pretty good.

[mccd07]

Thank you very much for this information.

Very helpful.