Electrics and Electronics Park Pilot: Greg Gimlick, Fall 2008

Left: LiPo packs come in all shapes and sizes, ranging from the 10S pack on top to the 2S pack at the bottom. Some indoor RC airplanes fly with a single cell that’s the size of a postage stamp. Right: Greg Gimlick stores his LiPo batteries in a fireproof safe. He charges them inside the Battery Bunkers on the right or the LipoSack at the bottom. Article and photos by the author. Featured in the Fall 2008 issue of Park Pilot.

Park Pilots are a unique breed; we have the equipment and ability to fly almost anywhere we find that’s big enough and safe enough. We’ve got cool gear that looks good and flies great, and we have the power to motor our models around the sky with authority. Much of that power comes from those magical Lithium Polymer (LiPo) battery packs we charge so carefully. Here’s a closer look at these packs and how you should treat them.

Safety First

You’ve seen the horror stories online if you’ve cruised the forums. They can be very impressive, especially some of the videos. Anything can be made dangerous if you try hard enough, and I don’t buy into the idea that LiPos are “the most dangerous batteries in the world,” as claimed by one online video. I’ve used them since they came out and I’ve never had a problem, although I do take a cautious tack when using them. Common sense is the name of the game here, and if you follow the manufacturers’ instructions and chargers like those I’ve previously shown you, you’ll have a great time — with none of the doom and gloom you hear about.

What Makes LiPo Special?

The short answer is energy density. The longer answer gets into all sorts of engineering data that would bore you to tears. I’m not a chemical or electrical engineer, but let’s just say the chemical composition of these cells allows more power to come from a small package than anything previously used in electric power. Unlike the batteries we used in the past that were built inside of small cans, the new LiPo cells are small envelopes made up of gelled electrolyte sheets and separator sheets that are laminated to each other. This makes them much lighter, and they provide a much higher nominal voltage than the traditional NiCd or NiMH cell. LiPo cells charge to a full voltage level of 4.2V per cell with a nominal voltage of 3.7V per cell. They hold their voltage well under load because of the low internal resistance. Most park model speed controls will cut the power to the motor during the discharge when the voltage reaches 3 volts per cell. Discharging below this level can cause damage to the cell and drastically shorten its usable life. New technology is coming, and there are variants of the LiPo cell that are advertising discharge levels below 3 volts. Be sure to follow the manufacturer’s directions.

Current and Resistance

You hear these terms all the time and you probably already have some idea of what they mean, but here’s how they affect what we do with our airplanes. Current is the flow of power from our batteries through the speed control and to the motor. Think of it as the ability of the voltage to become usable. It’s like water flowing through a hose, and we want a lot of it to flow easily. To get it to flow easily, we need to ensure that there are no kinks in the hose. Think of a kink in the hose as internal resistance. If there is an impediment to the flow, we lose power. LiPo cells pack a lot of current capacity into a small envelope, and their low internal resistance gets it out quickly.

Energy Density/Power Density

Marketers love to use these terms when comparing their products to others on the market, but what do they mean? Energy density is simply the amount of power a cell can hold, and it’s expressed in terms of watt-hours per kilogram. For instance, if a LiPo cell has a capacity of 800mAh and the nominal voltage of the cell is 3.7, the energy density of the cell is 2.96 watt-hrs (3.7V x .8 Ah = 2.96) By comparison, a NiCd cell might show 1.2V times .8Ah, or .96 watt-hrs. You can see why the manufacturers like to use this number to show their advantage over heavier cells. Power density is the level of power a cell can provide on demand expressed in watts per kilogram. This is an indicator of how easily the energy from the cell can be converted to useful energy in a power tool, or in our case, a model. To further simplify the relationship, think of energy density as a big jar of water and power density as the opening at the top. We want a lot of water in the container and we want to be able to dump it out very quickly, so a big container with a big opening is best. In our case, we want a battery cell with a lot of capacity (milliampe-hours or amp-hours) and the ability to get it out quickly (low resistance).

Here are the contents of a LiPo pack as the plates and separators are unwrapped. Red Scholefield provided these photos that show the inside of a crash-damaged pack he disassembled in a lab. Do not try this yourself.

Completely disassembled pack and cells have lots of plates and toxic contents. Dispose of your damaged packs and do not ever take them apart.

Common Sense Usage

Make life simple and safe. Follow these simple rules for LiPo usage. * Never use a damaged battery. If it shows signs of being creased or dented, replace it. It may not look bad from the outside, but may be damaged internally. * Always charge LiPo packs in a safe container. There are several commercially available containers, like the Battery Bunker or the LipoSack, which both work extremely well. You can also fashion your own out of fireproof safes, prepared ammunition cases or even Pyrex baking dishes. * Keep a fire extinguisher nearby. * Never charge a LiPo pack in your airplane. Use your container. * Never disassemble a LiPo pack in hopes of repairing it or replacing a cell. Most LiPo cells use terminal straps that require special equipment to solder. * Buy from a reputable LiPo dealer/manufacturer. Cost shouldn’t be your number one priority, although quality of construction should. * Never discharge LiPo cells below their recommended levels. If no discharge level is provided, you can plan safely on 80 percent of capacity. * Never overheat a LiPo cell. Temperatures higher than 140 degrees Fahrenheit can be dangerous. * Do not store LiPo cells in the freezer. * Only use chargers designed for LiPo cells, and use a balancer if the charger doesn’t balance packs automatically.

Charging Out of Here

Please visit Red Scholefield’s Battery Clinic at http://www.hangtimes.com/redsbatteryclinic.html. You should also visit www.rcgroups.com and www.rcuniverse.com. Forums can sometimes be a bit overwhelming with information, truths and half-truths, so go carefully. Most vendors and manufacturers have pages of information about using their products safely.