Lithium Polymer batteries are the latest technology and offer several times the capacity/run times of the previous generation Nickel Cadmium (NiCD) and Nickel Metal Hydride batteries (NiMH), 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.
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.
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
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.
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).
LiPo cells are also commonly given a C or current rating. This is the maximum average recommended discharge current for the cell. For example, a 1900mAh pack wth 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 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 some LiPo-specific chargers it's necessary to correctly select the cell count (1, 2 or 3 cells) manually or more commanly this is done automatically by the charger. 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.
If you are storing the LiPo for a long period of time it is recommended the the cells are fully charged and then discharged to about 50%-60% of their capacity.
LiPo's Do and Donts
|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. Some speed controllers 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.