Modern Lithium Polymer batteries (LiPo, Li-Poly) are able to store and deliver large amounts of energy from light-weight packs. Think of and treat LiPo batteries as fuel. Lithium Polymer cells, as with any high energy source (petrol, electricity, gunpowder etc) must be handled with appropriate precautions and care. Lithium Polymer batteries have been proven world-wide to be a practical and enjoyable power source for model aircraft.LiPo batteries may take a wide variety of shapes due to the gelling agent separating the cells. Machines and tools with strange sizes and shapes often use LiPo batteries for this reason. However, because they are stronger and potentially more dangerous than other rechargeable batteries like NiCd or NiMH batteries, special care must be taken.
What is the composition of a Lipo Battery?
LiPo battery consists of one cell or 2 or more cells connected to provide a specific voltage and/ or current capacity. The battery is interfaced into an AV using a basic connector, such as a Deans. The connector consists of a red (+) and black (-) lead as well as a Cell Balancer Connection.
It is not uncommon for individual cells in a battery pack to discharge at different rates over the duration of a flight. Cells may vary in voltage by a couple tenths of a Volt during discharge. It is important to program your transmitter or Autopilot to alert you before a cell drops below 3 volts. A LiPo cell has a nominal voltage of 3.7V. For the 2s lipo 5000mah above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.
Special Considerations when Using LiPo Batteries
The main concern that people using LiPo batteries have is that they have a tendency to explode. This is an obvious problem and means that extra care must be taken to prevent this from occurring.
General usage tips:
1.Lithium batteries don't work well in cold air. If you are flying in the winter keep the batteries in your car for best performance.
2.Don't let the batteries overheat. Try and keep them under 140-160 degrees F. This will prolong your battery life.
3.Don't push the batteries past their rated maximum C rating. This will damage the battery and the apparent capacity of the batteries will drop. If when you recharge you are only putting ½ to ¾ of the rated capacity back into the batteries you are probably pushing them too hard.
4.If your building your own cells then put spacing between each cell in the pack to help cooling of the pack. This is most important when building packs larger than 2 cells.
5.Some LiPoly cells use aluminum tabs that you must solder to. Normal soldering procedures will not work on aluminum. You'll need to purchase aluminum soldering paste. The vendor where you purchased your aluminum tab cells should stock this paste.
Lipo battery Voltage and Capacity
When talking about a LiPo, the primary characteristics to understand are the battery’s voltage and capacity. This is typically noted in a shorthand such as “4S-2200”. In this example, “4S” denotes that the battery has four cells in series. The nominal voltage of each cell is 3.7 volts (4.2v fully-charged), so the total pack voltage is:
4 cells x 3.7v = 14.8v.
The second number denotes the capacity of the battery in milliamp-hours (mAh). A fully charged 2200mah is rated to provide a current of 2200 milliamps (2.2 amps) for one hour before it is fully discharged. This capacity value is completely independent of how many cells are in series. In simple terms, the capacity value allows you to estimate how long a battery will provide useful power in a given application. In practical terms for RC use, the capacity rating is typically only helpful for rough comparisons of different batteries. i.e. a lipo 3s 5000mah battery will provide about double the run time of a 2S-2500 lipo in the same RC car.
A 4S2P-2200 battery would consist of two 4S-1100 batteries wired in parallel to provide a total 2200mAh capacity. All other things being equal, you would care for and use this battery the same as you would the previous 4S-2200 example (which is really a 4S1P-2200, but we ignore the 1P). There may be a difference in physical size, but a 4S-2200 and a 4S2P-2200 are functionally equivalent. The differences will really only matter to the guy at the factory who has to assemble the battery.
Check for Lithium Cell Balance
Before charging a new pack, check the voltage of each cell in the pack individually by using the taps. Keep a record and check every tenth use cycle. An unbalanced pack, or a pack with voltage levels not within 0.1 volts of each other, may catch fire. If the pack becomes unbalanced after every discharge, it should be replaced.
Do Not Charge Batteries Unattended
LiPo batteries don't take too long to charge, but there should always be someone near the battery and charger to make sure nothing bad happens. This is very different from other rechargeable batteries, which are more set-it-and-forget-it products. Charge in an open, ventilated space on a safe, fire-retardant surface such as a fire-safe Pyrex dish with sand or a fireplace. Keep a bucket of sand nearby in case a fire starts. If it does, dump the sand on the fire to put it out.
Life of Lithium Polymer Batteries
A Lithium Polymer battery needs to be replaced when it holds 80% or less of its capacity. If used at the maximum continuous discharge rate on every cycle and every charge is at 1C or greater and it’s used down to 3.00V per cell under load (when your LiPo compatible ESC cuts power to the motor) then don’t expect to get more then 40 or 50 cycles from your LiPo Pack. Please see my guide to prolonging the life of your LiPo’s.
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