by Paul Gentile

The popularity of electric- powered aircraft has soared (pun intended) over the past few years.  Part of the reason behind the recent popularity has been the advent of Lithium Polymer or Li-Poly batteries.

Li-Poly batteries pack a high energy-to-weight ratio when compared to their Ni-Cad and NiMH battery cousins.  This stored energy has good and bad potential, and we will touch on both here.

Li-Poly battery cells are 3.7 volts, as compared to Ni-Cad and NiMH batteries that are 1.5 volts per cell.

When Li-Poly batteries are wired in parallel, they do not discharge like other batteries.  In addition, when you wire cells in parallel, each cell only sees half the total current, or amp draw.

Total current is very important for Li-Poly batteries and is identified with a C rating.  You may see Li- Poly batteries advertised as 3C, 6C, 8C, 10C.  This means that a 3C 1500 mAh (1.5 amp) Li-Poly battery pack should never be discharged at a rate higher than 3 x1500 rnAh or 4500 rnAh (4.5 amps).

Discharging a Li-Poly beyond this rating could cause damage to the cells or even fire.  This is a very serious concern.

Changing a propeller on your airplane can change the current draw and cause higher than expected discharge rates, so it is beneficial to have a current meter on hand.  The manufacturer's specifications for the motor, speed control, and propeller combination you are running also come in very handy.

The other letters on Li-Poly packs refer to S for series wiring of cells and P for parallel wiring of cells.  A 3S pack would be 3.7 volts x 3 cells = 11.1 volts.  A 3P pack would mean three parallel cells, or 3.7 volts and a higher C rating.  A 3S 3P pack would have 3 cells in series (11.1 volts) and 3 sets of cells in parallel.

Li-Poly batteries also do not require cycling, or discharging like other batteries.  In fact, you never want to cycle down Li-Poly batteries.  You should always leave a partial charge to avoid damage.

Chargers and speed controls should always be rated for Li-Poly use.  Do not attempt to use your Ni-Cad or

NiMH equipment.  An improper charge rate could cause a Li-Poly pack to explode and burn at over 2000 degrees.  A non-Li- Poly rated speed control could cause over discharge and cell damage.

Here is a list of do's and don'ts for your Li-Poly packs:

Never put your Li-Poly packs in water and never put water on the packs.

Don't leave your Li-Poly batteries unattended while charging.  See www.modelaircraft.org for this year's list of people whose cars and houses have burned down while leaving packs unattended during charging.

Don't puncture or short out Li-Poly batteries.

Don't fully discharge your Li-Poly packs, as this will damage the cells.

Don't put the Li-Poly battery in your car, or leave it in your airplane after a crash.  If the battery is damaged internally, you may not notice.  According to the AMA, several members' cars have already burned up this year due to this scenario.

Do use common sense and respect the energy that is stored in that little package.

Do follow all manufacturer ratings and specifications for use and storage.

Do store your Li-Poly packs in a fireproof container.

Li-Poly batteries are used everyday safely in cell phones, laptops, consumer electronics, and iPods.  In our hobby, we are pushing these batteries to their limits, charging and discharging them at high rates and sometimes smashing them into the ground at high speeds.  We need to respect their potential and keep it safe.

Enjoy the power and convenience of electric flight with Li-Poly batteries; I do.  Just respect the energy stored in that little Li-Poly package and it will reward you with some of the fastest, 3-Dst (if that is a word), most fun flying you will have.

(Reprinted from the newsletter of the Monmouth Model Airplane Club, Inc., Keansburg, New Jersey.)