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 by Bob Aberle |
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Years ago we used an analog-type meter with an expanded scale for more resolution (and accuracy). We called these test units "expanded scale voltmeters," and the term "ESV" has stuck. But today we have modern digital-readout voltmeters, and expanding the scale is no longer necessary. All of these field-testing devices generally impose a load of 250-500 mA current. I like using the Hobbico Digital Voltmeter Mk III (stock number HCAP0356) from Tower Hobbies. The special price was $21.99 at the time of this writing. When you purchase this voltmeter, the 9-volt supplied battery is wrapped in plastic inside its compartment. You must remove the battery, unwrap it, and plug the connector into it, then the LCD screen will light up. The best way to use this field tester is to have a charging jack located on the side of the fuselage. That way you won't have to remove the wing to gain access to the airborne battery pack. After every flight, plug in the tester and select it for the correct number of battery cells, with the load on. Wait approximately 10 seconds, then look at the LCD screen. The Hobbico unit has been thoughtfully set up to tell you when it is time to recharge. It will clue you in on a four-cell pack when the voltage gets to roughly 4.8 volts under load. That is somewhat conservative, but I support this choice. There are more sophisticated field-type battery testers on the market. Rod Johnson of i4C Products has a Loaded Battery Tester that will let you select load currents of 500-, 1000-, and 1500 mA. This would be appropriate for larger-capacity batteries, as are used in larger model aircraft. The i4C unit is powered from the battery under test and needs no extra battery, so it is always available. Rod has another interesting battery monitor called the C-Volt. It is a small LCD-readout digital voltmeter that you mount on the side of your model's fuselage. It goes on when you power up your airborne RC system. It doesn't contain a load since that would draw too much extra power. You observe the C-Volt meter while you operate a few aircraft controls simultaneously, such as the rudder, elevator, and ailerons. By doing this you will load down the battery, providing a meaningful check of the remaining power. At 4.7 or 4.8 volts for a four-cell battery, it is time to recharge or replace it. Testing batteries used for electric power is a slightly different story. If you are using a BEC and sharing power, the radio is using only a fraction of the battery's power (such as a few hundred mA), and the motor itself might be drawing a few amps or more power (much more significant). You are recharging this battery after each flight, so you know you start every flight at a full charge. That being the case, there is no reason to really test the battery. When your model goes to take off, if it appears sluggish or wants to land after only a minute or two when it's in-flight, look into the possibility of a bad cell in the pack. In this instance, the way the model flies reflects on the battery's condition. Replacing Batteries: I've written my opinions about the minimum capacity at which point it is time to replace your batteries. Then the choice becomes whether to buy the same battery pack direct from the RC manufacturer or from a reputable aftermarket battery-pack supplier. Since most RC manufacturers supply only 500-700 mAh-capacity packs with their systems, your first choice for replacing batteries is to go to higher-capacity cells of the same physical size. I hate to suggest that you not go back to the manufacturer, but the aftermarket suppliers have some great stuff these days. I'll list as many as I can at the end of this article. Keep in mind what I wrote about when you go from a 500 mAh-capacity battery to one that has 1600 mAh capacity; you won't be able to use that same charger that came with your system. You must get something such as the ACE DDVC or Digipulse Multi-Charger. When you get some experience under your belt, and especially if you pursue electric-powered flight, you may want to make your own battery packs from individual cells. The aftermarket battery suppliers sell individual cells with solder tabs for your convenience. When you get into the higher-capacity cells which draw much more current, you will have to resort to copper bars or copper braid to make the intercell connections. But that's for a later article. Charge Retention: If I'm unable to fly on a given day and several days or a week goes by before I get another chance, I recharge the night before that next flight attempt. Ni-Cd batteries do lose a certain amount of charge each day after they are fully charged. Specifications indicate that charge retention depends on battery temperature. Ni-Cd cells stored at 32 degrees Fahrenheit can be expected to lose approximately 10% of their charge in 30 days. Those same cells stored at 68 degrees Fahrenheit will lose 30% of their charge in 30 days. At 104 degrees Fahrenheit, the loss will be almost 70% in a 30-day period! NiMH cells tend to lose charge at an even faster rate. From this information you can probably understand why I instruct you to be safe and recharge at the C/10 overnight rate if a week goes by. The new Li-Poly cells exhibit excellent charge retention, losing only 1% or 2% of their charge in a six-month period. Battery Storage: There are two schools of thought on this topic: to store the Ni-Cd or NiMH cells fully charged and to store them when fully discharged. Either way is okay, but experts tell us not to store cells at partial charge. I find it easier to return from the flying field and put the cells on overnight charge. That way they go back on my battery shelf at full charge. If I don't use the battery in a month or two, I recharge it again. I have many battery packs, so some will be forgotten. However, most survive and provide three to five years (and more) of service.
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