About Lithium Polymer (LiPo) Battery

 

A Guide to Lithium Polymer Batteries

Lithium batteries 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 but none is more important than safety.

1. Charging/Safety

Do not push the limits of lipo products, or be haphazard about following safety requirements.

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 spew violent flames and result in fires.

Guidelines for charging/using LiPos (Lithium Polymer Batteries):

1. Use only a charger designed/approved for lithium batteries. 

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.

3. Use the voltage reader. Before you charge a new Lithium pack, check the voltage of each cell individually.Then do this after every tenth cycle thereafter. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen.

If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced.

Voltage Reader give you the ability to check individual cell voltages and charge one cell at a time. Make sure and get the appropriate connector to go in to your taps.

 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.Vented fire safes, Pyrex dishes with sand in the bottom, plant pots, are all good options.

6. Do not charge at more than 1C unless specifically authorized by the pack vendor. Today’s highest discharge batteries can supposedly be safely charged at greater than 1C,however so far in all cases doing so shortens the life of the pack.

 7. Never puncture the cell, ever. If a cell bulge/balloons quickly place it in a fire safe place, especially if you were charging it when it bulged/ballooned. After you have let the cell sit in the fire safe place for at least 2 hours. Discharge the cell/pack slowly then throw the battery away.

8. If you crashed with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crashed in any way carefully remove the battery pack from the chassis and watch it carefully for at least the next 20 min.

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

10. Keep a bucket of sand nearby when you are charging batteries. This is a cost effective way to extinguish fires.This is very cheap and absolutely necessary.

2. Voltage and Cell Count:

LiPo acts differently than NiCad or NiMH batteries do when charging and discharging. Lithium batteries are fully charged when each cell has a voltage of 4.2 volts. They are fully discharged when each cell has a voltage of 3.0 volts.

 It is important not to exceed both the high voltage of 4.2 volts and the low voltage of 3.0 volts. Exceeding these limits can harm the battery.

The way to ensure that you do not go below 3.0 volts while RC’ing is to set the low voltage cutoff (LVC) of your electronic speed control (ESC) or use a Lipo Buzzer.

It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC's programming mode to set the LVC to 3.0 volts per cell with a hard cutoff.

3. C Rating, Naming conventions explained.

How fast a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour.

For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour.

All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mAx 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x3).

All batteries have limitations on how fast they can discharge. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have2 x 2000mAh cells and you wire them in parallel then the result is the same as1 x 4000mAh cell. This 4000mAh cell has the same C rating as the original2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x5) 20 amps. This method of battery pack building allows us to use LiPoly batteries at higher currents than single cells could produce.

The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 2S means it's a 2cell pack. The number in front of P means the number of cells in parallel. So a2S4P pack of 2000mAh cells has a total of 4 cells inside. It will have the voltage of any other 2S pack since the number of cells in series determines the voltage. It will have the current handling of 2 times the maximum C rating of the individual cells. So say our 2S4P pack had a maximum discharge of 4C. That means that it has a nominal voltage of 7.2 volts (2x3.6) and a maximum discharge rate of 32 amps (2000mAh x 4Cx4P ). 

4. Which battery should you buy?

One great way to find out what the best battery is, is to look at graphs of the batteries performance. Looking at how low the voltage of the cell drops at various amperage's will give you a metric to compare that battery to similar size/weight batteries. 

If graphs aren't your thing then simply look at what other people are using in successful setups that are similar to your application. If a lot of people are reporting long flight times and lots of power from airplane X, with power system Y, and battery Z and you do the same, then if your setup is similar the same battery will probably work well for you. 

It pays to learn something about Watts, Volts, and Amps. Understanding these concepts is beyond the scope of this document, but can serve you well in not only figuring out what battery is best but also in your electric RC hobby.

Do note that a 30C battery is not really any better than a 10 or 20C battery.Sure a higher C rating means it can discharge faster. But at the same time a battery discharged at 20C continuously will be empty in 3 minutes. Do you really only want to use the battery for 3 minutes?

A final note on choosing a battery. Don't cheap out. Confirm that your batteries are capable of running that the amperage level you plan to use thematic. Running a cell at a higher C rating than the battery can handle can not only damage your batteries, but it can also damage your speed control. Using a weak battery can destroy a perfectly good speed control of any brand. Better to buy a bit better battery than you need than to destroy your electronics.

5. Dealing with temperature.

Lithium batteries like heat, but not too much. In the winter time, try to keep your batteries from the cold as much as possible. Leave them in the car while your RC’ing,or keep them in your cargo pants... etc. At the same time don't let them heat up too much. Try to keep your batteries from reaching 160F(70C) after use. This will prolong the life of the cells. A good way to measure temperature is a hand held IR meter which can be found for around at most hobby shops.

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