Rod Bender

Hey Guys,

I purchased a hobbyco expanded volt meter from tower, but the tx side is dead. The rx side works, and seems to be pretty closed comparing to my large digital volt meter. I now want to just use my digital volt meter for both tx and rx, question is: At what voltage to you consider your day done? Any digital specs on rx and tx nicad pacs would be greatly appreciated ... Thanks in advance, Greg

Red B.

If you are to use your DVM for battery measurements you must make sure you put a load on the battery, otherwise the voltage reading means very little. The load doesn't have to be fancy, a simple light bulb (say 6V 2.4W) will do, or use a fixed valy resistor (approx. 20-25 ohm that can dissipate more than 2 W power.)

In order to determine a safe "low voltage" you really need to make a discharge curve as different batteries tend to behave quite differently. This is how to do it manually:

- Starting with freshly charged battery connect it to the DVM and the load in order to discharge it.
- Take a voltage reading every 15-20 minutes.
- Plot the voltage in a diagram (voltage versus time elapsed)
- Near the end of the cycle take readings more frequently as the voltage tends to drop fairly quickly.

By studying the resulting graph it is fairly simple to determine a safe low voltage.

When at the field check the battery by connecting it to the DVM (with load) and wait for approx. 20 s before reading the voltage.

Below you will find an intrinsic discharge curve that shows what you could expect (this discharge curve was measured using a data logger).

/Red B.

KW_Counter

Rod,
About 1 volt per cell under load.
I quit when the receiver is at 4 volts or the transmitter at 8.
Check out the Battery Forum for more on this.
Good Luck,
KW_Counter

Rod Bender

Red,

Thanks for the comeback. Ok, understood on the load, but per your graph, what was the time elapsed for this curve? there is no value for the time in minutes. Also, per your graph, at what point would you consider your rx pack unsafe? 4.9?........4.8? ,,,,,,,Thanks, Greg

BillS

Be careful. Four volts is well below the knee on the battery curve.

Most everyone I know stops flying at 4.8 volts.

Bill

RCPAUL

My Feedback: (1)

The transmitter side of the ESV probably does not work because of the diode in the transmitter charger circuit. You'll have to remove the transmitter battery to check it. I do not recommend you bypassing the diode.

Paul

Rod Bender

Thanks Bill,
In comparison, the expanded volt meter has the green/white/red voltage area's. 4.8-9 volts looks to be a quiting point before it even gets into the red area. Even with running it down to 4.8, I've had run times of 60 minutes with a 4.8v 800mah nicad. BUT, this was in the shop doing servo exercises, I'd imagine actual flying would yeild less with the stress and torque on the servo's. Safe is better than sorry ..... Five 10 minute flights per plane in a single day is plenty for me .... Greg

Paul, Your right on the tx, it is the diode on the Futaba that is preventing a reading. It works on my Airtronics, but its an older version. I had contacted Tower on the rx issue, and they sent me another evm yeilding me the same results. Now I have 2 evm's that I do not want to use .... Greg

mustangman177

Rod:

If you want to fly continuously, get a quick field charger from Hobbico. They can charge different types of battery packs at a fraction of the time of a standard charger. Easy to use if you want to fly longer at the field.

Cary

MustangFan

I use 1.175 or 1.188 (little safer) volts per cell (Nicd) as a stopping point.

I have considered 1.1 volts per cell as absolutely dead.

I use a voltmeter with a 20 ohm 5 or 10 watt resistor lor load ... available at Radio Shack.

Look at the curve supplied by Red B and see if you agree.

BillS

My batteries will usually go 60 minutes. My transmitters have a built in voltmeter, which is consider safer because of continuous monitoring.

I use a digital meter. Seemed foolish to purchase an inferior meter when a good Fluke was already available. A load resistor was put in a film canister with a momentary switch on the top. The setup was cheap and allows use of a better meter. In addition it facilitates reading both unloaded and loaded voltage. The meter has a home in the flight case.

Bill

Red B.

The time scale doesn't matter much (unless you want to calculate the capacity of the battery). For a 600 mAh battery I would expect approx 3 hours with a 200 mA load. Using this battery pack I would stop flying at approx 4.7 V (erring on the safe side). You dont't want to end up too close to the shoulder where things start to go downhill fast.

/Red B.

Rod Bender

Bill,

Very nice idea, I'll clone that one and thanks Red for you input. These hobbico's are going back. Flying Thursday, 2 new planes, maidens on both...not even bringing my fun planes....its time ..... Greg