Expanded scale loaded voltmeters have become the accepted means of field checking a receiver battery pack to see if it has the capacity for another flight. The meters that are available for $10 to $15 are all of the analog variety and of limited use other than for what they were developed. With the dramatic drop in price of digital multimeters, many R/C modelers consider them to be "necessary" support equipment. These meters have an extremely high impedance (resistance) and therefore display what is essentially the open circuit voltage of the battery pack. The open circuit voltage of a Ni-Cd battery pack reveals very little regarding the state of charge of the pack. For this reason, the expanded scale voltmeter with a built in load is the recommended way of checking the status of the pack. Even this is not ideal since the discharge curve of Ni-Cd batteries is very flat. A better method of checking a Ni-Cd battery pack is with a "loaded" digital voltmeter with an inherent higher resolution.

The problem with using a loaded digital voltmeter is that no one makes such a device. This should not be a deterrent to modeler. The construction starts with two pin jacks to match the leads or probes on the digital multimeter; one red for the positive lead and a black for the negative lead. A resistor of the appropriate value could be used but that is kind of boring since it just lies there and resists. A brighter idea is a light bulb, which clearly shows it is providing a load for the battery. After careful experimentation, a # 44 bulb (6.3V-250mA) was found to load the pack with 220 mA. This is fairly close to the load imposed by the R/C system. If a higher load is needed, either a PR12 or PR13 bulb provides a drain of about a 500 mA. A normal battery pack should be able to sustain 500mA for an hour. The bulb can be used to roughly check the battery pack capacity. When the bulb dims significantly, the battery pack is near the end of the charge. A connector will be needed that will mate with the charge plug on the model. Finally, some type of enclosure will be needed to house the components and assembly may begin. A small plastic box about the size of a small matchbox will make a neat compact unit. It is nice if the box is clear or at least translucent so the bulb can be seen as it lights when the battery is loaded. Otherwise, the integrity of the box must be compromised by adding another hole in addition to the three needed for the jacks and battery lead. One of the leads can be soldered directly to the tip of the light bulb base. Attaching to the side of the bulb base may cause a problem because the material may not accept solder. A small brass clamp can be made from 1/32" thick stock by 3/16" wide and secured to the bulb with #2 hardware while leaving a small section to which the lead is soldered. The clamp can be made so that it can be soldered directly to one of the jacks providing a secure mount for the bulb. All that is required now is to plug the meter into the red and black jacks on the load box and the battery lead into the battery connector on the plane.

If the loaded digital meter indicates 4.8 volts or greater, it has sufficient charge for additional usage. If it indicates less than 4.5 volts, it is dangerously low and should not be used. The use of a battery pack with a reading below 4.8 volts is marginal and is dependent on the equipment, number of servos, activity of the servos, and other factors. Before being used at this level, the capacity should be checked. On a day when the equipment is not being used, it should be allowed to discharge until the meter reads 4.6 volts. Then the system should be turned on and ground checked every 5 to 10 minutes. The servos should be actuated to determine how long it takes before they start to twitch. This will give a rough calibration as to the meter reading that should indicate the bottom limit for use.