Torque RC Guide to Lithium Polymer “LiPo” batteries...

More information on LiPo batteries...
 

What is a Lithium Polymer Battery?

In a nutshell, a battery consists of two metals immersed in an electrolyte solution. A chemical reaction takes place that causes electrons to flow from one metal to the other. This produces a voltage difference between the positive and negative poles. When a load (such as a motor) is placed between the two poles, electrons will flow through the circuit until the metals are chemically neutralized (battery dead).

The voltage produced between the two metals is called the electrochemical potential. This energy is directly related to the types of metal used in the battery.

Lithium is the lightest metal in the universe and has the highest electrochemical potential. Lithium would be perfect for making lightweight powerful batteries right?

Well, almost... Because lithium metal is very unstable, battery manufacturers began using non-metallic lithium-ions.

Voltage and Capacity of LiPo Battery Packs

Each individual lithium polymer cell has a nominal voltage of 3.7 volts. It is extremely important that a LiPo battery cell is not discharged below 3.0 volts or charged beyond 4.2 volts. Doing so will damage the battery.

Electronic speed controllers (ESC's) designed for LiPo battery packs have a Low Voltage Cutoff (LVC) that will automatically cut the power to the motor when the battery voltage (of an individual cell) reaches 3.0 volts.

To understand how LiPo battery packs are put together, you need to have a basic understanding of voltage and capacity. If you are not familiar with batteries, think of voltage as the "muscle" and capacity as the "energy" of a battery.

LiPo Balancers

As stated above, lithium polymer cells must stay between 3.0V and 4.2V for optimal performance. A standard LiPo charger looks at voltage of the entire LiPo battery pack when charging. Some times the individual LiPo cells become unbalanced over time. For example, one cell may have a voltage of 3.8V while the rest are at 4.2V.

A LiPo balancer monitors and controls how much each cell in the LiPo pack is charged or discharged. Balancing the pack will allow for the fullest possible safe voltage which results in more power while in flight. A balanced pack also tends to last longer as no individual LiPo cells become over charged.

Storing LiPo Battery Packs

LiPo batteries should always be stored in a cool environment. In other words, don't leave them in your car with the windows up on a hot summer's day. Keep them in a cool location out of the direct sunlight. It is highly recommended that Lipo batteries be kept in a charging bag while be charged and while not in used. A charging bag is a very cheap form of insurance!

Advantages of LiPo Batteries

• Have nearly double the capacity with half the weight of NiCad or NiMH battery packs
   
• Retain charge significantly longer than NiCad or NiMH batteries when not being used
   
• Do not develop a "memory" as NiCad batteries do
   
• With LiPo cells in combination of series and parallel, LiPo packs can be built to power most all RC aircraft

 Lithium batteries are becoming the preferred power source for most electric modelers today. They offer high discharge rates and a high energy storage/weight ratio. However, using them properly and charging them correctly is no trivial task. There are many things to consider before using lithium cells in your RC vehicle. But none is more important than safety.

Until you are willing to follow all saftey precautions, DO NOT use lithium batteries. If your a type of person that prefers to push the limits of products, or be haphazard about following saftey requirements. Lithium technology is not for you. Read on to find out why.

Lithium cells must be charged very differently than NiCad or NiMH. They require a special charger specifically designed to charge lithium cells. In general any charger that can charge lithium ion can charge lithium polymer, assuming that the cell count is correct. You must NEVER charge lithium cells with a NiCad or NiMH only battery charger. This is dangerous. Charging cells is the most hazardous part of using lithium batteries. EXTREME care must be taken when charging them. It is important to set your charger to the correct voltage or cell count. Failure to do this can cause the battery to catch fire or explode! PLEASE BE RESPONSIBLE when charging lithium batteries.

Here are a few essential guidelines for charging/using LiPos (Lithium Polymer Batteries).

1. Use only a charger approved for lithium batteries. The charger may be designed for Li-Ion or Li-Poly. Both batteries are charged in exactly the same. Remember a NiCad or NiMH charger cannot be used to charge LiPo packs.
2. Make certain that the correct cell count is set on your charger. Watch the charger very closely for the first few minutes to ensure that the correct cell count continues to be displayed. If you don't know how to do that, get a charger that you do know how or don't charge the batteries.
3. The current should never exceed a 1C charge rate. For example, a 5000mAh 11.1V pack should be charged at 5 Amps or less. Charging at lower rates down to .77C could increase the life of the battery, although there is not conclusive evidence that I am aware of to support this. For example, a .77C charge rate of a 5000mAh pack would be 3.85 Amps.

Most LiPo chargers automatically detect the voltage (or cell count) of the LiPo battery pack. It is always a good idea to double check, or confirm that the charger detects correct the voltage pack.

4. NEVER charge the batteries unattended. This is the number one reason for houses and cars being burned to a crisp by lithium fires.
5. Use a safe surface to charge your batteries on so that if they burst into flame no damage will occur. Specialist fireproof pouches are now available for LiPo packs and are highly recommended.
6. DO NOT puncture the cell, ever. If a cell balloons quickly place it in a fire safe place, especially if you were charging it when it ballooned. After you have let the cell sit in the fire safe place for at least 2 hours. Discharge the cell/pack slowly and dispose of it safely.
7. If you crash with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crash in ANY way carefully remove the battery pack from the model and inspect and test it in a safe environment. If in any doubt, dispose of the pack.

8. Charge your batteries in a open ventilated area. If a battery does rupture or explode hazardous fumes and material will spew from the battery.

9. It can happen to you, do not think to yourself that “it won't happen to me” as soon as you do that something will go wrong.
 

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Disadvantages of LiPo Batteries

• Have very short shelf life of two to three years
   
• Very sensitive to high temperatures
   
• Very sensitive to high and low voltages. Exceeding either will damage the battery
   
• Prone to explode if exposed to heat, or charged/discharged improperly

Remember

Safety FIRST

When using LiPo Batteries