Lithium Battery Breakthrough
Jason Shulman's history-making electric powered world championship plane.
At this year's F3A Aerobatic World Championships held in Poland, history was made. For the first time in its over-30-year existence, an electric powered model was competitive against the most powerful O.S. and Y.S. glow-powered pattern airplanes in an FAI world championship format. At the start of the event, contestants and spectators alike were skeptical, believing that competing with an electric model was more of a publicity stunt than a serious competition attempt. But after the first flight, all skeptics were silenced and by the end of the preliminary rounds, a 4th place position transferring into the finals proved that an electric-powered model had the right stuff to compete against the most powerful glow-powered aircraft, as flown by the best pilots in the world.
The technology that's made this possible is also changing the face of electric modeling at local fields and indoor flying sights across the country. Lithium Polymer batteries are the latest technology and offer several times the capacity/run time of previous generation Nickel Cadmium and Nickel Metal Hydride batteries, with reduced weight and size. Plus the latest generation LiPo (short for Lithium Polymer) cells have a low internal resistance, so they're able to provide high amp draw for powering high-performance and even large-scale models.
Thunder Power is the current performance leader in LiPo batteries. Imagine flying a GWS Tiger Moth or FD stick for over an hour on a single charge. Or imagine an indoor 3-D model that you can hover in a small office for over 15 minutes. With LiPo batteries you no longer have to imagine, as this type of performance is now a reality.
Understanding LiPos
Lipo batteries are very different from previous generation batteries and understanding how they work, and especially how to charge them, is the key to getting the best performance.
Series
Individual Lithium Polymer cells have a nominal voltage of 3.7 volts (vs. 1.2volts per cell for Ni-Cds). Cells are wired in series to give the following pack voltages:
1 cell = 3.7 volts
2 cells in series = 7.4 volts
3 cells in series = 11.1 volts
Parallel
Unlike Ni-Cd and Ni-MH cells that self-discharge when wired in parallel, LiPo cells can be hooked up, charged and discharged in parallel with no detrimental effect. Wiring two LiPo cells in parallel doubles the capacity (more run time), plus an important advantage of wiring in parallel is that each cell only sees half the total current.
3S 2P
A battery pack that has three cells in series (giving 11.1 volts) and 2 of these 3-cell packs are wired in parallel is commonly referred to as a 3S, 2P (3 series, 2 parallel).
C Rating
LiPo cells are also commonly given a C or current rating. This is the maximum average recommended discharge current for the cell. For example, the Thunder Power 1900mAh packs have a 6C rating. To determine the maximum recommended discharge rate multiply the capacity times the C rating. 1900mAh x 6C = 11,400. So the maximum recommended discharge rate would be 11,400mA or 11.4 amps. If your application has a higher amp draw, remember that LiPo cells can be wired in parallel, and with 2 cells in parallel each cell sees half the total current. With 3 cells in parallel, each cell see one third the current.
By wiring packs in a combination of series to get the voltage and parallel to achieve the capacity and individual cell current to an acceptable level, LiPo cells can be used to power nearly every type and size of model.
Charging
Charging LiPo batteries requires a very different charge method than other types of cells. It's imperative to use a charger designed specifically for LiPo batteries, and with most LiPo-specific chargers it's necessary to correctly select the cell count (1, 2 or 3 cells) manually. Fortunately, several good chargers are available through FMA. Astro Flight, plus many other companies including E-Flite and Thunder Power, will be releasing inexpensive LiPo-specific chargers within the upcoming months. These chargers give a constant current charge rate at 75% the cell capacity until the pack voltage reaches 3.6 volts per cell. This charges the pack to about 80% of total capacity. At this point the charger switches to a constant voltage charge rate of 3.6 volts per cell to top off the battery. To charge a fully depleted pack typically takes about one and a half hours.
Serious Safety Issues
If LiPo batteries are improperly charged they can cause an explosive fireball. Our staff recently tested the effect of improperly charging LiPo cells and the results were dramatic! We cannot over-emphasize the importance of using a charger specifically designed to charge LiPo batteries, and to be positive that the manual cell count (if the charger has one) is correctly set for the specific battery being charged. If you use a good quality LiPo charger, these batteries are totally safe.
LiPo Do's and Don'ts
Do: Only use a charger that is specifically designed to charge LiPo batteries. Using other types of chargers can cause an extreme fire hazard!
Do: LiPos don't develop memory or voltage depression characteristics like Ni-Cds. Do charge them without the worry of cycling or discharging them.
Do: Store LiPos at least partially charged. LiPos will maintain their performance levels over time, even during non-use, much better than Ni-Cds and there's no need to cycle them.
Don't: Use any type of charger that is not specifically designed to charge LiPo batteries. Using non-LiPo-specific chargers can cause an extreme fire hazard!
Don't: Fully discharge your LiPo battery pack. Most speed controller have a voltage cut off that will prevent over-discharging the battery. Discharging a LiPo beyond it's critical minimum voltage will cause damage to the battery.
Don't: If you have a crash and the battery is damaged, don't put the battery in your car or house immediately after a crash. It's possible that a chemical reaction can take place in the damaged battery that could cause a fire. Put the battery in a safe place for a least one hour.