Checking battery voltages is one of the most, if not the most, important pre-flight checks that a modeler should perform. This has become particularly important with the more frequent use of digital servos. Because of the design and resolution, they are always hunting for their center, the draw on the battery is more than analog servos under the same conditions. Choose a good voltmeter to check, you should choose one that has the ability to apply a load to the battery being tested. Pictured in photo 1 below are two popular units that I own and use. The data below is mostly for receiver batteries as most transmitters have volt meters or can not be checked through the charge jack. The charge jack on most modern transmitters has a diode so electrons can only flow into the battery.
One is the Hangar 9 ESV. It is a high quality unit that has the feature of being able to check batteries under three different loads, .5 amp, 1 amp and 2 amp. This is an outstanding feature for battery condition monitoring. The .5 amp setting is a good load for standard radio systems, while the higher amp draws are good for high performance and lithium ion and lithium polymer batteries. Our Fromeco Li-ions instruct you to re-charge the battery when it reaches 7.0 volts at a 1 amp load. When first hooking up the battery, the meter reads actually battery voltage. There is a button to depress to apply load.
The other one is a Hobbico ESV, very economically priced. It has settings for both 4.8 volt and 6 volt as well as a differentiation between receiver and transmitter. With the select switch in the off position the meter will show actual voltage. By selecting either rec or trans, a load of 250 mah. You use the small button on the side to toggle between 4.8 volt and 6 volt settings.
Now for the meat of this article. You should always check the battery with and without a load. In the photos below you will notice that the first voltage is higher with no load and you are seeing the surface charge only. With a load, this number will drop, the amount depends on the load applied. With a 250 - 500 mah load on a good battery, you will typically see approximately a voltage drop of .2 to .4 volts. You should hold the load for 10 - 15 seconds to verify the decrease in voltage stabilizes. A weak or uncharged battery will generally continue to decrease in voltage.
Here is a true story of the importance of this practice. Joe was preparing to fly his Dawg House RC Yak 55, he hooked up his voltmeter for a quick look. Looked good, he started the engine and took off. Very quickly into flight he lost radio as the plane appeared to go into it's failsafe mode. Plane crashed and upon post crash investigation, the receiver battery showed about 3.5 volts after holding the voltmeter with a load on it for a few seconds. A 2.4 GHz radio goes to failsafe at approximately 3.5 volts and a FM radio at approximately 3.3 volts. Expensive lesson to learn. He believes that he simply forgot to charge the battery and the voltage he seen for a brief moment was the surface charge.
If you really want to learn about your batteries, start a trending program. Check voltages before a flight, with and without load, record this number. Now after the flight, check the voltages again and record them and the time the radio was on. If you really want to get good info, try to determine how much of the radio on time was in flight. You can then start to see trends of the condition of your battery. This trending has become a very good practice for our helicopters with all the high performance servos. Also, on some of my larger aircraft, I get a good idea of how well the batteries are holding up. Having this info may help you decide which battery to buy next time or save an airplane by knowing when it is time to quit flying and charge the battery. An example is once you start checking batteries on a regular basis and notice before one flight that at about 10 seconds the battery voltage has dropped .7 or .8 volts and you are used to seeing only .2 or .3 volt drop, you might make it a short flight or investigate a little further. Maybe batteries are just getting old, maybe they are telling you that one of the cells is weak. Again, trending will help you determine if this is a gradual change or sudden. I worry more about sudden changes rather than gradual, as gradual is usually the battery simply aging.
Click on the photos for a larger view:
1) Two popular voltmeters, Hangar 9 and Hobbico. Hangar 9 street price is $39.99 and the Hobbico street price is $24.99. There are many more out there that will perform this function.
2) Hangar 9 voltmeter showing voltage with no load.
3) Hangar 9 showing voltage with .5 amp load (500 mah) after 10 seconds.
4) Hobbico voltmeter showing voltage with no load.
5) Hobbico voltmeter showing voltage with 250 mah load after 10 seconds.