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Portable Power

Written by Tim Dees

A practical limit exists to just how small electronic gadgets can get, but we haven’t reached it yet. Ten years ago, most cellular phones were the size of small bricks. There are now models that can get lost in your pocket with your change and keys. What limits the size and weight of many of the devices that we use every day is the power supply.

Battery technology has not kept up with the advancement of the electronics that they run, so that half the size and weight of many portable electronic components is consumed by the battery. Portable radios used by public safety officers are an excellent example. Most of the radios in common use incorporate a custom battery into the shape and design of the radio, and removing it reduces the weight considerably.

Although the chemistry involved differs considerably from one battery type to another, the principle is the same. Two dissimilar metals are immersed in an electrolytic mixture. When a circuit is established between the two metals, a chemical reaction takes place that results in free electrons being released. The reaction often produces heat as a byproduct (for example the reaction is exothermic), which is why batteries sometimes get warm when being discharged or charged.

The electrons flow off of the positive terminal of the battery and power the device connected to it. The electrons are then returned to the negative electrode of the battery to complete the circuit. When the reactive chemicals in the battery have been used up, the current stops. With a rechargeable battery, a current is applied to the negative electrode, which reverses the chemical reaction, in essence storing up electrons in the battery, and the battery is ready to be discharged again.

Since the chemical reaction and reversal process is not 100% efficient, each battery will hold slightly fewer electrons with each charge/discharge cycle. Most of the batteries in common use in public safety radios and other devices use either Nickel-Metal Hydride (NiMI) or Lithium Ion (Li-Ion) chemistry, and are good for around 1,000 service cycles.

Most users of battery-powered devices are familiar with the dreaded “memory effect.” This is most common with Nickel-Cadmium (NiCad) batteries, still used in some rechargeable flashlights. An officer would use his flashlight for 20 minutes or so during a shift, then throw it into the charger when he got home. After a few days of this, the flashlight would go dead after 15 to 20 minutes of use, even though the battery was rated to last 60 to 90 minutes. NiCad chemistry is not tolerant of being “topped off,” and the portion of the battery that was going unused would eventually grow stale. NiMI and Li-Ion batteries do not suffer from the memory effect, and can be topped off as much as desired.

NiCad batteries are also hard on the environment. Cadmium is a heavy metal that will poison ground water if the batteries are not disposed of properly. Other batteries have their own problems, but the NiCad chemistry is particularly nasty. Many Radio Shack stores will accept used batteries for recycling at no charge.

Lithium Ion battery technology may still have some legs before it’s supplanted by something else. A Nevada company called Altair Technologies has developed a lithium-titanium oxide “nanomaterial” for Li-Ion electrodes that greatly reduces material fatigue.

The new electrodes have the potential to result in a Li-Ion battery with three times the power-to-weight ration of present models, a recharge time measured in minutes instead of hours, and a service life extending to thousands of charge-recharge cycles. The new batteries may be energetic enough to power equipment that is presently tied to a wall socket because of high current requirements.
Another new battery technology, also still in development and not yet available commercially, is based on Lithium-Sulfur chemistry. Li-S batteries may have the potential to deliver 50 to 400% more power per unit weight as compared to a similar Li-Ion battery. In one test, a laptop computer with a typical battery life of two to three hours ran for more than eight hours using a Li-S cell. The main drawback with the new battery chemistry may be in service life.

Current Li-S technology is good for only about 150 charge-discharge cycles. Sion Power, the company developing the Li-S technology, hopes to get that up to 300 cycles soon. Cost-conscious police administrators may overlook the Li-S batteries if this limitation can’t be overcome. Batteries are already expensive line items in the budget, and the limit in available money may overrule an increase in available power.

Another portable power technology that should be on the market by the end of this year is the portable fuel cell. Fuel cells have been in use by NASA since the early days of the manned space program to generate electricity from stored hydrogen and oxygen, producing water as a byproduct. These are great when you’ve got a moon shot budget, but are too expensive for most earthbound applications. The fuel cells in development for personal electronics use methanol (wood alcohol) for fuel.

A fuel cell under development by Samsung can run a laptop for ten hours on 100 ml (a bit less than half a cup) of methanol. Methanol is cheap, environmentally friendly, and easy to store. The catalytic agent in the new fuel cell is a platinum/ruthenium alloy, which degrades very little with use. This is a good thing, because platinum and ruthenium are not especially cheap or plentiful as metals go. The prototype fuel cell is larger than a standard notebook battery, but is expected to shrink as the technology is perfected.

Still another portable power solution under development is a microgenerator developed at Georgia Tech in Atlanta. The microgenerator is about the size of a dime, and spins a magnet above a mesh of coils embedded in a silicon chip. When mated with a like-sized microturbine, the magnet spins at 10,000 RPM (compare that to an automobile engine that typically cranks out 3,000 RPM) and generates electricity. At this point in the project, the microturbine power is supplied by an air-powered drill similar to a dental drill.

One of the key problems in making the microgenerator work was in producing a magnet small enough to fit inside the tiny housing and still sturdy enough to withstand the incredible G-forces produced at 10,000 RPM. The problem was resolved through optimal design and encasing the magnet in a titanium alloy.

So far, current models produce about 1.1 watts, just about enough to drive a cell phone. The projected goal of the project is to generate 20 to 50 watts, putting the microgenerator on a scale with standard batteries. The research is being funded by the Army Research Laboratory, which is looking for new and lighter power sources for battlefield gear like radios, laptops, GPS systems, and night-vision equipment. It’s not uncommon to have products of defense research spill over to civilian public safety.

Tim Dees is a former police officer who writes and consults about applications of technology in law enforcement. He can be reached at (509) 585-6704 or by e-mail at tim@timdees.com.

Published in Law and Order, Apr 2005

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