Lithium Polymer Battery FAQ(2)

Q3.) How fast can I charge my batteries?

Different cells can withstand different charging rates. Check with the manufacturer to make certain you don't damage your pack.
For fast charging, most packs can be safely charged in 15 minutes, which requires a charging current of 4 times the capacity of the pack.
Trickle charging is usually done at a rate of 1/10th the capacity, or C/10. Cell manufacturers list this as the charge rate in which the cells will not vent (release gases that build up from overcharging). However, even this charge rate can reduce the life expectancy of a cell if left on after the cell is fully charged.

Q4.) What is Cell reversal?
In a battery, not all cells are created equal. One will be weaker than the others. So, as the battery is discharged, the weakest cell will use up all its active material. Now, as discharge continues, the current through the dead cell becomes a charging current, except that it is reversed. So, now reduction is occurring at the positive terminal. As there is no more nickelic hydroxide, it reduces the water, and produces hydrogen. Cell pressure builds, and it vents. The cell has lost water and the life of the cell has been shortened
This is the big danger of battery cycling to prevent memory. Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells do have a finite life. Each cycle is a bit of life.

www.rcecho.com

Lithium Polymer Battery FAQ(1)

Q1.) Does Temperature Affect Lithium Polymer Batteries?

Yes, lithium polymer batteries are affected by temperature. All batteries use a chemical reaction to provide electricity, and a low temperature slows this reaction down significantly. You cannot harm a lithium battery by running it at a low temperature, but you will notice a significant decrease in performance. Allowing lithium polymer batteries to freeze is generally not recommended, so be sure to store the lithium battery in a cool, dry location over the winter months.

Q2.) How can I calculate theoretical flight time?

In order to calculate the maximum theoretical flight time of your models, you will need to know some information about your battery and motor equipment:
#Motor Amperage
#Pack capacity in Mah (milliamp hours)

After you have that information, you can use the following formula to compute the maximum theoretical flight time of your model:
Time (hours) = (battery capacity (amps) / (motor current (amps)

Notice how the battery capacity must be given in amps. To find your battery capacity in amps, divide the capacity in milliamps by 1000. You will probably want the result expressed in minutes. so multiply the time you just calculated by 60.

By this formula, a 33.5 amp motor powered by a 5000 Mah battery would have a run time of a run time of 8.96 minutes. This is a theoretical maximum, if you actually ran the above set up for the calculated maximum time you would destroy the battery. Most lipo batteries shouldn’t be discharged below 80%, so multiply the calculated time by .8 to account for this.

www.rcecho.com

How To Safely Use, Charge, and Discharge Lithium Polymer (Lipo) Batteries

Lithium polymer batteries are great for RC aircraft, but they have an explosive chemistry that must be treated with caution.

Prevent this from happening to your batteries by following these tips:

1. Store lithium polymer batteries in a flame proof LipoSack while charging. - Charging your lithium polymer batteries in a flame proof LipoSack can contain a fire should it occur. It could mean the difference between a minor clean up and the loss of your house or car. Also make sure that the storage area is well ventilated.
   
2. Read the manual – The importance of reading your battery and chargers manual cannot be emphasized enough. The battery manual will state the proper charging rates and times.
   
3. Use common sense – Don’t charge batteries unsupervised. Even when you do everything right, incidents can occur. Also, do not charge lithium polymer batteries in your car. A flame out can be disasterous if it occurs inside a vehicle.
   
4. Use the right battery charger – Charging a lipo battery with a charger designed for other batteries will cause problems, and will probably result in a fire.
   
5. Charge lithium polymer batteries on a fire proof surface – It’s really important to charge lithium polymer batteries on a flame proof surface such as concrete. In the event of a fire, a fire proof charging surface will stop it from spreading, or at least slow it down significantly.
   
6. Keep a fire extinguisher, or bucket of sand near the charging area – If a fire does occur, you don’t want to be running around looking for something to put it out with. Water will not help put out a lipo fire. Being a conductor, it will cause a short circuit and could even make the fire worse.
   
7. Don’t charge lithium polymer batteries near flammable substances – lithium polymer batteries are flammable enough as it is. Don’t make the problem worse by storing flammable substances near charging batteries.
   
8. Check lithium polymer batteries for swelling prior to charging and each use – A puffed battery is unstable, and can be in danger of exploding. If you see a puffed battery, immediately disconnect it from the charger or aircraft and put it in a bucket of water. Dissolve a few tablespoons of salt in the water to aid conductivity, and leave the battery in the bucked for about 4 days. The salt water depletes any power remaining in the battery by creating a short, and it can’t catch fire while underwater. After the four days are up, take the battery out and cut off the connectors (which may come in handy for other projects). You can then dispose of the battery in the trash. The battery no longer contains toxic metals, won’t harm the environment, and by using the salt water you’ve guaranteed that it won’t catch fire. This should be done as soon as you see a puffed battery. You can’t salvage a puffed battery, the best you can do is to dispose of it safely.
   
9. Never charge a lithium polymer battery in a model – If you charge a lipo battery in your RC airplane or helicopter, you are risking the total loss of your model. Only charge lithium polymer batteries on a flame proof surface, in a LipoSack.
   
10. Make sure the charging leads are connected properly – Connecting positive to negative and negative to positive can cause a major fire.
    
11. Don’t overcharge batteries – By their very chemistry, lithium polymer batteries cannot be discharged to a potential of less than 3 volts without damage. For the same reason, don’t charge them to over 4.2 volts. This means that you have to land your rc aircraft before the motors stop turning. Some aircraft come equipped with a voltage cut-off, others do not. If you don’t have a voltage cut-off, then land as soon as you sense the propeller or rotors slowing down.
    
12. Double check that the charger settings are correct – Lithium polymer battery chargers require you to set the battery configuration. Ensure that this configuration matches the battery you’re charging, or else your lipo could get overcharged and explode. Some chargers automatically sense the battery configuration, but make sure that the setting is correct regardless. They have been known to be wrong on occasion.
    
13. Balance lipo batteries – Lithium polymer batteries have balance connectors, designed to make sure that each cell in the pack has the same charge. If this isn’t the case, some cells can become overcharged and explode.
    
14. Never let the battery leads touch – If the battery terminals touch each other, the battery will short circuit and, in most cases, be destroyed. If this happens and you get a puffed battery, dispose of it by following tip 9 above.
    
15. Don’t ever store / charge lithium polymer batteries in your car – Unless you hate your car. Batteries can and do explode, and if this happens inside a vehicle the result is usually catastrophic. On a hot day, temperatures can rise inside the car and cause stored packs to rupture.
    
16. In the event of a crash, remove the battery and supervise it for at least 4 hours – A crashed plane’s battery can appear fine, but can have an internal short circuit. This short circuit can cause an explosion, even hours after the crash occurred. A LipoSack is a great place to keep a battery that’s been in a crash. If enough time elapses and nothing happens, then your battery is probably fine. If you see puffing, dispose of it immediately following the instructions in tip 9 above.

Always use common sense, read the manual, and know the risks associated with lithium polymer batteries. Handled properly, the risk of a fire is relatively small. Store lithium polymer batteries in a LipoSack for additional saftey.

www.rcecho.com

The difference between Ni-Cd, Ni-MH, and LiPo

As you know ,there are three main types of batteries currently in use for electric airplane motors. These are Nickel Cadmium (NiCad), Nickel Metal Hydride (NiMH) and Lithium Polymer (LiPo).

Dou know the difference between Ni-Cd, Ni-MH, and LiPo this three type of batteries?


Li-polymer Battery
LiPo (Lithium Polymer). This is one of the newest cell types available. It is also the lightest battery type currently available on a commercial basis and can provide more power than the other main cell types. There are no known problems of memory effect with this battery type and it is the easiest battery type to care for. The downside of this battery is that it has the highest engineering costs and therefore the price is usually considerably higher than other cell types.


Ni-MH Battery
Ni-MH (Nickel Metal Hydride). This cell type is the most common cell type currently available for laptop computers, (although Li-ion is rapidly becoming the most popular) This battery type is relatively cheap to manufacture and therefore tends to be cheaper than Li-ion. This cell type is prone to 'memory effect' so it is important to take good care of your Ni-MH battery to ensure that you obtain the best runtimes.


Ni-Cd Battery
Ni-Cd (Nickel Cadmium). This is one of the oldest cell types and is generally only found in older laptops. The main pro for this cell type is its ability to handle higher loads, and therefore is more commonly found in portable power tools or devices that need a lot of power to work efficiently. The main downside of this cell type is that it is notorious for suffering with memory effect so good care must be taken with this battery to ensure most effective use of battery.

www.rcecho.com