Guide to Li-Po Batteries

What are LiPo batteries and why are they so popular in the RC world?
LiPo or Lithium Polymer batteries have a much more even delivery of power during use, giving more consistent speed and punch throughout each cycle. They also have little or none of the memory effect that NiMH and NiCd battery packs suffer from. In short, LiPo’s provide high energy storage to weight ratios in an endless variety of shapes and sizes. For the past few years, NiMH stick and saddle packs have dominated the RC world, but now LiPo’s are fast becoming the norm for many RC enthusiasts.

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

1. Use only a charger approved for lithium batteries.
2. Make certain that the correct cell count is set on your charger.
3. Before you charge a new Lithium pack, check the voltage of each cell individually.
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, fireplaces, plant pots, are all good options.
6. DO NOT CHARGE AT MORE THAN 1C unless specifically authorized by the pack vendor.
7. 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.
8. If you crash with your lithium cells they may be damaged such that they are shorted inside.
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 flying or charging batteries. This is a cost effective way to extinguish fires. This is very cheap and absolutely necessary.
Voltage and Cell Count:

LiPolys act 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, but you should not go below 3.7 volts. It is important not to exceed the high voltage of 4.2 volts and the low voltage of 3.7 volts. Exceeding these limits can harm the battery.
The way to ensure that you do not go below 3.0 volts while flying is to set the low voltage cutoff (LVC) of your electronic speed control (ESC). 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 3.3 volts per cell with a hard cutoff, or 3.7 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.7 volts per cell. You will know when flying that it is time to land when you experience a sudden drop in power caused by the LVC.

If your ESC has an automatic lithium mode. Use it, it will correctly sense the number of cells and set the auto cutoff appropriately.

If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will result in a different number of cells being used. If you had 6 to 7 round cells then 2 lithium polymer cells will correctly duplicate the voltage of those cells. If you had 10-11 cells then 3 lithium polymer cells would be right for you.
Here is a list of LiPo RC battery pack voltages with cell counts. If you are wondering what the 2-12S in parenthesis means; it is a way the battery manufacturers indicate how my cells hooked in series (S) the battery pack contains.

• 3.7 volt battery = 1 cell x 3.7 volts (1S)
• 7.4 volt battery = 2 cells x 3.7 volts (2S)
• 11.1 volt battery = 3 cells x 3.7 volts (3S)
• 14.8 volt battery = 4 cells x 3.7 volts (4S)
• 18.5 volt battery = 5 cells x 3.7 volts (5S)
• 22.2 volt battery = 6 cells x 3.7 volts (6S)
• 29.6 volt battery = 8 cells x 3.7 volts (8S)
• 37.0 volt battery = 10 cells x 3.7 volts (10S)
• 44.4 volt battery = 12 cells x 3.7 volts (12S)

It should pointed out that you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two celled series packs hooked up in parallel to double the capacity (2S2P is actually a popular configuration in high capacity LiPo receiver packs).

CAPACITY

Capacity indicates how much power the battery pack can hold and is indicated in milliamp hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps) can be put on the battery for 1 hour at which time the battery will be fully discharged.

For example a RC LiPo battery that is rated at 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 amps) a very common current drain in a 400 sized RC helicopter while hovering, the time to drain the battery would be only about 4 minutes.

As you can see, for a RC model with that kind of current draw, it would be very advantageous to use a larger capacity battery pack such as a 2000 mAh pack. This larger pack used with a 15 amp draw would double the time to about 8 minutes till the pack was discharged.

The main thing to get out of this is if you want more flight time; increase the capacity of your battery pack. Unlike voltage, capacity can be changed around to give you more or less flight time. Naturally because of size & weight restrictions, you have to stay within a certain battery capacity range seeing that the more capacity a battery pack has, the larger and heavier it will be.

DISCHARGE RATE

Discharge rate is simply how fast a battery can be discharged safely. A battery with a discharge rating of 10C would mean you could safely discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.

Let's use our 1000 mAh battery as an example; if it was rated at 10C that would mean you could pull a maximum sustained load up to 10,000 milliamps or 10 amps off that battery (10 x 1000 milliamps = 10,000 milliamps or 10 amps). From a time stand point, this equals 166 mAh of draw a minute so the 1000 mAh pack would be exhausted in about 6 minutes.

This is calculated by first determining the mAh per minute of the pack. 1000 mAh divided by 60 minutes = 16.6 mAh's per minute. You then multiply that number by the C rating (10 in this case) = 166 mAh of draw per minute divided into the packs capacity (1000 mA) = 6.02 minutes.

How about a 20C rating on a 2000 mAh battery? 20 x 2000 = 40,000 milliamps or 40 amps. Time wise, a 40 amp draw on this pack would exhaust it in about 3 minutes (2000/60= 33.3 multiplied by 20c = 666 mAh per minute - divided into the packs capacity of 2000 mA = 3 minutes). As you can see, that is a pretty short flight and unless you are drawing the maximum power for the entire flight, it is unlikely you would ever come close to those numbers.

Most RC LiPo Battery packs will show the continuous C rating and some are now indicating a burst rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power. An example might be something like “Discharge rate = 20C Continuous/40C Bursts”

OVER DISCHARGING - THE NUMBER ONE KILLER OF LIPO'S!!!

A very good rule to follow here is the "80% rule". This simply means that you should never discharge a LiPo pack down past 80% of it's capacity to be safe. For example, if you have a 2000 mAh LiPo pack, you should never draw more than 1600 mAh out of the pack (80% x 2000). This is assuming a healthy pack as well that has the full 2000 mAh capacity (as packs age, their capacity drops).

An 80% discharged LiPo cell, will give an approximate open circuit voltage of 3.73 to 3.75 volts. A 3S LiPo pack therefore would show about 11.19 to 11.25 volts after a flight when it's about 80% discharged, a 6S pack would be about 22.44 volts.

Charging RC LiPo Batteries
Charging RC LiPo Batteries is a topic in itself. LiPo, LiIon, and LiFe batteries obviously have some very different characteristics from conventional RC rechargeable battery types. Therefore, charging them correctly with a charger specifically designed for lithium chemistry batteries is critical to both the life span of the battery pack, and your safety.

Maximum Charge Voltage and Current
A 3.7 volt RC LiPo battery cell is 100% charged when it reaches 4.2 volts. Charging it past that will ruin the battery cell and possibly cause it to catch fire. This is important to understand once I start talking about Balancing RC LiPo batteries, so keep that in the back of your head for right now.

It is critical that you use a charger specified for LiPo batteries and select the correct voltage or cell count when charging your RC LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 11.1V (a 3S pack) by mistake and tried to charge your 2S pack, the pack will be destroyed and most likely catch fire. Luckily, all the better computerized chargers out there these days will warn you if you selected the wrong cell count.

All LiPo battery chargers will use the constant current / constant voltage charging method (cc/cv). All this means is that a constant current is applied to the battery during the first part of the charge cycle. As the battery voltage closes in on the 100% charge voltage, the charger will automatically start reducing the charge current and then apply a constant voltage for the remaining phase of the charge cycle. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with chargers constant voltage setting (4.2 volts per cell) at this time, the charge cycle is completed. Going past that, even to 4.21 volts will shorten battery life.

RC LiPo Battery Charging Current
Selecting the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here use to be "never charge a LiPo or LiIon pack greater than 1 times its capacity (1C)."

For example a 2000 mAh pack, would be charged at a maximum charge current of 2000 mA or 2.0 amps. Never higher or the life of the pack would be greatly reduced. If you choose a charge rate significantly higher than the 1C value, the battery will heat up and could swell, vent, or catch fire.

Most LiPo experts now feel however you can safely charge at a 2C or even 3C rate on quality packs that have a discharge rating of at least 20C or more safely and low internal resistances, with little effect on the overall life expectancy of the pack as long as you have a good charger with a good balancing system. There are more and more LiPo packs showing up stating 2C and 3C charge rates, with even a couple manufactures indicating 5C rates.

RC LiPo Battery Balancing
Balancing is required on any RC LiPo battery pack that has more than one cell since the charger can’t identify from different cells and know if one might be overcharged even though the total voltage of the pack indicates otherwise. For example let’s look at a 3 cell LiPo battery pack (three LiPo cells hooked in series or 3S).

This would be an 11.1 volt battery pack (3.7 volts per cell x 3 = 11.1 volts). The 100% charge voltage of this LiPo pack = 12.6 volts (4.2 volts x 3 = 12.6 volts). Our trusty charger set up for a 11.1 volt RC LiPo battery pack will then stop charging at 12.6 volts – simple right.

Well what would happen if one of those three cells is charging a bit faster than the other two? There could be two cells at only 4.1 volts and the one that is charging at bit faster could be getting overcharged up to 4.4 volts before the charger stops charging at 12.6 volts. That would certainly cause damage to that one cell, perhaps even a fire.
Balancing ensures all cells are always within about 0.01-0.03 volts per cell so over charging or discharging of one or more cells won’t ruin your battery pack, or worse become a safety issue from overcharging a cell.

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