BASICS / CAMERA CRADLES /

Special Battery Strategies

David Hunt          March 26, 2002

INTRODUCTION

Peter van Erkel's article in The Aerial Eye, spring 1997 (3:2) addressed KAP battery systems from the standpoint of a possible cell failure. Cell failure is not as great a concern to the KAPer as it is to the RC airplane buff, but weight is of great concern.

We will examine several ideas to save weight, each depending upon the goals of the KAPer. In conjunction with this idea van Erkel included a schematic of a circuit that could step up a voltage as small as 0.8 volts to 5 volts, proposing that a 2 or 3 cell battery pack (2.4V - 3.6V) could be used to power the receiver and servos. The schematic in the article confused some of the values, hopefully that can be better addressed here.

PUSHING TO THE LIMITS
The KAP battery system can be an excellent place to trim some extra weight from your cradle. Lets look at three approaches to save weight with the battery system.

Different Battery Chemistry
DC-DC Step-Up Converters
Subminiaturizing

Strategy #1 - Different Battery Chemistry

On the Battery Fundamentals page we discovered that some types of batteries are lighter, yet have the same power. Referring to the Energy Density chart on the Fundamentals page, if you are using NiCd batteries you could use NiMH batteries with the same mAh rating and save weight. The chart below compares NiCd and NiMH battery packs with the same voltage and close to the same mAh rating.

Comparison of NiCd and NiMH Battery Packs

If you are using NiMH battery packs already then you may want to experiment with the Lithium-ion battery packs. This will mean investing in a different charging system because Li-ion batteries require a constant voltage charging system where NiCd and NiMH require a constant current charging system. If you have a digital camera or camcorder you may already have battery packs and chargers that you can use.

My digital camera uses a 3.4 volt Li-ion battery pack rated at 1100 mAh and weighs 36 grams (1.3 oz.). It is slightly larger than a double-A battery. 3.4V 1100 mAh Li-ion Battery Pack

My digital camcorder has a 7.2 volt Li-ion battery pack rated at 1500 mAh and weighs 97 grams (3.4 oz.). The larger Li-ion battery pack does not provide any real weight savings. A 4AA-cell NiMH battery pack, 4.8V, 1550 mAh, weighs 115g (4.0 oz.). The weight savings using the 7.2 volt Li-ion battery is used up to provide 2.4 extra volts, which are not needed in most KAP applications, so this battery would not be a good candidate for a KAP rig.

Some ingenuity would be required to build connectors to attach these Li-ion batteries to a KAP cradle. A voltage converter or regulator would be needed for each battery pack to supply the 5 volts needed for most R/C receivers and servos, or the 12 volts needed for most video cam/transmitters. Voltage converters figure into the next two approaches.

Strategy #2 - DC-DC Step-Up Converters

This strategy involves using a lower voltage battery pack with a step-up DC-DC converter, or using a single 5 volt battery pack to supply 5 volts to the R/C components and 9 or 12 volts to a video aiming system through a 5V to 12V step-up converter.

Weight is saved in both scenarios because fewer cells are used. Using a step-up converter it is possible to supply 5 volts using only 2 or 3, 1.2 volt cells. A 12 volt battery pack would require 10-1.2 volt cells so using only 4 cells (4.8V) would save the weight of 6 cells. The weight savings could be as much as 132g (4.7 oz.) based on NiCd AA cells. The battery pack for the Xcam2 uses a step-up converter to supply 12 volts to the camera/transmitter using 4 AA batteries.

We will look at the Maxim and the C&D Technologies DC-DC step-up converters. The Maxim converters require a few external components to work properly (5 or more). The technical specifications for the C&D Technologies converters state that no external components are necessary, however I have seen applications where a couple of capacitors were added. The Maxim devices are a little cheaper than the C&D devices but the extra components needed by the Maxim devices cancel out most of the differences.

Both companies offer a very large line of DC converters, if you decide to try devices other than those mentioned below, be sure that the output current is sufficient for your application. A video system can use 150 mA or more, and some of these devices supply 100 mA or less.

The following is a list of DC-DC converters that KAPers may find of use. There is a link to the manufactures technical data page for each device and three of the devices have links to a KAPER page that has a wiring diagram and other information for KAP use.

Strategy #3 - Subminiaturizing

If you have an interest in building the lightest possible rig that can be lifted in the lightest possible wind, and still have some R/C control, the following ideas might be of interest. I haven't been able to test this concept, so what I am about to say is strictly an informed guess.

Using the NKE0305S, mentioned above, you could power a special ultra light camera cradle which had only a shutter servo, maybe a pan servo. For a power source use a battery such as the 3 Volt "123" pictured below. This is a non-rechargeable battery used in many cameras (i.e. Olympus Stylus Epic) and it weighs only 1/2 ounce (15.5g) so the total weight of battery and step-up converter would be under 1 ounce. Add an ultra light RC receiver like the GWS R-4P (4.8g) and a Hitec 81s mini-servo (16.6g) and all your RC components combined will weigh less than 2 ounces.


3 Volt 123 Photo Battery There are smaller batteries and some may work in this application. Be aware that many of these small batteries have a very limited useful ampere output and may not be able to supply the current demands of a receiver and servo. I haven't been able to find ampere related technical information for disposable batteries.

You may have noticed that I did not suggest using the MAX756. The Maxim chip requires external components and two of the capacitors are rather large. The weight breakdown is close to 1 ounce, and that does not include solder and the input and output leads and connectors. Add 1/2 oz. for a battery and you are not significantly lighter than the 4-AAA NiMH battery pack mentioned above.