LiPo Battery Info “101” Primer

LiPo Battery Basics

So you want to know what all those numbers mean?

Lipo batteries have revolutionized our hobby, and there is a wealth of information you can find online about Lipo batteries, but there is still a lot of confusion about them.  There are some safety precautions that you need to follow, which prompts some people to stay away from Lipos.  Hopefully the information here will shed some light on Lipo batteries and the basic terminology you need to know.

LIPO BASICS

Historically, the batteries used in RC were big, heavy, low on power, and didn’t last very long.  “Lipo” is short for “Lithium Polymer” which is the technology used to make Lipo batteries.  Lipos are very similar to a more common technology that you have probably seen in countless applications called “Lithium Ion”.  They are similar in that they both have nominal cell voltages of 3.7 volts, but dissimilar in that Lipos do not have a hard metal casing like Li-Ion batteries but rather have cells which are encased in a foil based flexible “pouch”.  Additionally, while Li-Ion batteries use an organic liquid solvent as the electrolyte, Lipo batteries use a dry electrolyte polymer that resembles a thin plastic film.  If you were to cut open a Lipo battery you would be able to unfold the film and it would be several feet long (depending on the size of the battery).  This construction allows for a very thin battery along with a wide range of sizes and shapes of batteries.

Lipo batteries (along with significant advances in brushless motor technology) have finally been able to provide the types of power previously only attainable with nitro engines.  The run times are substantially better than older battery technology, and not to mention electric is much cleaner, easier to use and has none of the endless tuning associated with nitro.  These features of Lipo technology have propelled more and more people to get involved in RC which has been great for our hobby.

TERMINOLOGY

Voltage and “S” rating

Lipo batteries consist of one or more “cells”, which you can think of as an individual battery.  When a Lipo has more than one cell, they are wired in series so that the voltages add up.  The nominal voltage for each cell is 3.7 volts.  You may have seen Lipos referred to as “1S”, “2S”, “3S”, and so on.  The “S” represents a “Cell”, and the number before it is simply the number of cells the battery has. You may wonder why this is not called a “C” versus an “S”, but it is referring to the fact that the cells are a certain number of cells wired in “Series”.  This, by default determines the nominal voltage of each battery.  A 1S battery is a 3.7v battery, a 2S is a 7.4v battery, a 3S is an 11.1v battery and so on.  The more volts you have the faster your motor will turn, but in turn this also creates more heat.  It is also important to know how many volts or how many “S” your ESC is rated for so that you do not destroy it with too much voltage.  You also need to take into account how many KV your motor is rated for so that you do not spin it too fast.  If you multiply the KV rating of a motor by the voltage of a specific pack it will tell you how fast that motor will turn with that battery (as long as the ESC and the motor itself can handle it).  For example, a 4000kv motor with a 2S/7.4v Lipo will spin to 29,600 RPM (4000×7.4).

“C” Rating

A Lipo’s “C” rating is probably the most misunderstood designation of Lipo batteries.  The “C” rating is a rating which tells you how fast the battery is capable of discharging its power.  Think of it as the size of a water hose, the bigger the hose (or the higher the “C” rating”), the faster the current can come out of the battery.  Most Lipos have a “Continuous” and a “Burst” rating.  The “Continuous” rating is the “C” rating that the Lipo can put out on a constant basis, and the “Burst” rating is how much it can put out for brief periods of high draw.

A battery will only discharge as much power as is being pulled from it by your electronics.  So if you have a basic system which is not very power hungry you do not need to spend money on a higher “C” rated battery as there is no need for it.

That being said, the higher the “C” rating of a Lipo the cooler it is going to run and the longer it will last as you are not “working” it as hard.  People who do aerobatic or 3D flying and those who have high power systems in their cars or trucks can really benefit from Lipos with higher “C” ratings.

A 30C rating is typically enough for the majority of applications you will run across, with the higher “C” ratings being used for the high performance applications mentioned above.  When in doubt though, always go with a higher “C” rating, you can never have too much in this department.

A good “seat of the pants” way of checking your batteries to see if they are being over worked is to feel them immediately after using them.  As a general rule, if a pack is too hot to hold comfortably in your hand then you should switch to a higher “C” rating.

Capacity, or “mAh”

The capacity of a Lipo battery, or the “mAh” rating is the same unit of measurement as used for other types of batteries.  The mAh rating is the capacity of the battery (measured in ampere-hours), like the size of a gas tank in a car.

Once you know the “C” rating and the “mAh” rating of a Lipo, you can determine the available current from that battery.  All you have to do is multiply the “mAh” rating by the “C” rating and it will tell you how many amps of power that battery can provide.  For example, if you have a 2200mah, 30C battery then you can safely provide 2200mAh x 30 = 66,000 mAh, or 66 amps with that battery.  If you know the current requirements of your electrical system you can use this formula to determine if you have the right battery.

Safety

A lot of the fear and mystery about Lipos revolves around stories of Lipos bursting into flames while using them or while charging them.  While these certainly are possible occurrences with lipo batteries, if you understand some basics and take the proper precautions then you will greatly reduce the possibility of a mishap.

Over-charging, Over-discharging

The majority of accidents with Lipos happen when they are either charged well beyond their rated voltage, or when they are discharged below their minimum voltage.  Each cell should never be charged to more than 4.2 volts (which is a 100% charged cell) and as a general rule should not be discharged below 3.2 volts per cell.  Some people may argue that you can go down to 3.0 volts per cell, and while that may be true it is getting dangerously low and may shorten the lifespan of your battery.

It is extremely important that you use a charger designed specifically for Lipo batteries and that you have it set correctly (this will be discussed more in the charging section).

A proper charger will ensure that your battery is not over-charged, and most ESC’s nowadays are designed with Lipo settings built in. Most ESC’s have an “Auto-Lipo” mode where it detects the number of cells in the pack and therefore will sense when the overall pack voltage is at or below the minimum voltage programmed into it, which is generally 3.2 volts per cell.  Most ESC’s are also programmable and allow you to select the cutoff voltage manually as well.  It is critical that you ensure that your ESC is set for Lipo mode or else damage to the batteries, electrical system or vehicl ecan result.  If you are using an older ESC or one that simply does not have Lipo mode, it is important that you install an external LVC (Low Voltage Cutoff) which can cut power once the voltage gets too low.

General Rules

-Use only a charger designed for Lipo batteries.

-Make sure the correct cell count is displayed on your charger.  Keep an eye on the charger for the first couple minutes of charging to make sure that the setting remains the same.

-Balance charge your packs periodically.  As a rule, you should balance charge your pack every 10thcharge, but it is easy enough to do more often than that.  This is explained in the charging section.

-Never leave a charging battery unattended.

-Charge batteries on a safe surface and away from anything that may catch fire should something go wrong.  Fireplaces, concrete surfaces away from flammable items, flower pots and pyrex dishes with sand in them are ideal spots.

-Use a Lipo charging bag.  There are special fire retardant bags made for charging Lipos that will contain any flames should a pack catch fire while charging.  These are available in our “Battery Hardware and Accessories” section.

-NEVER puncture a cell.  If a pack balloons or puffs up, set it aside in a fire safe area for at least two hours and then discharge it slowly before discarding.  Slow discharging can be accomplished with a charger which has a discharge feature, or you can wire a flashlight bulb of appropriate voltage to the pack (higher voltage is OK, lower voltage is not).  Once the light is completely out you can discard the battery.

-Check your pack after a crash.  A Lipo pack involved in a crash may look OK from the outside but may be shorted out internally.  It’s a good idea to remove and set aside a pack involved in a crash for at least 20 minutes to make sure that it does not short out.

-Charge batteries in a ventilated area.  Should something go wrong, hazardous fumes may spew from the battery.

-Keep a fire extinguisher or bucket of sand nearby in case of fire.

Remember, as long as you follow all of the precautions it is very unlikely that you will have an incident with your Lipos, but it is always best to be prepared!

Charging

As cool as Lipo batteries are, they aren’t much good to us if we can’t charge them back up after using up the power!

The first thing to know is that you MUST use a charger designed specifically for Lipo batteries.  Lipo chargers range anywhere from under $10.00 for a charger that is little more than a plug in wall transformer with some voltage detecting circuitry, to high current models that cost several hundred dollars.  The different models and types of chargers are a whole other subject but the important part is that they are specifically designated as Lipo capable.

A Lipo charger has to be able to know how many cells a particular pack has in order to not over charge the pack.  Most chargers can do this automatically or you simply select the number of cells that you have when you charge your pack.  Once the pack starts charging, it’s a good idea to keep an eye on the charger for the first couple of minutes to ensure that it remains in the correct setting.

Charging Rates – The rate at which a Lipo battery may be charged is also expressed in a number of “C”, just like the discharge rating.  This number is lower than the discharge rating though and is usually around 1C to 5C.  If you do not know the charging rate of a particular Lipo pack, assume that it is a 1C rated pack.  What the “C” rating means is at how many times the capacity of the battery you should charge it at.  For example, at “1C” a 2000 mAh pack should be charged at no more than 2 amps (2000 x 1 = 2000mAh, or 2.0 amps).  If you were charging this same pack at a rate of 2C, you could charge it at 4 amps (2000 x 2 = 4000mah or 4.0 amps).  While 1C charging rates used to be the norm, advances in Lipo technology have allowed most batteries to charge in the 2-5C range.  It is important not to exceed the recommended charging rate or you may shorten the battery life or risk a fire.

Balance Charging –  Earlier, in the safety section we mentioned balance charging.  It is important to periodically balance charge any Lipo pack with more than one cell in order to keep all of the cells at or near the same voltage.  The reason that this is so important is that because once in use, the ESC only sees the overall voltage of the pack and activates the Low Voltage Cutoff accordingly to prevent battery damage, or worse yet, a fire.  If you have one weak cell in say a 3 cell pack, the ESC may not know this because the other 2 cells are charged up enough to keep the overall pack voltage up and the one cell may become discharged below 3.0 volts.  Only one cell has to become over-discharged to cause a fire or damage the pack.

Because of this, nearly all modern Lipo batteries have balancing taps on them.  This is the smaller set of wires coming out of the pack with a small connector on them.  Through this connector, a charger can measure the individual voltage of each cell and stop charging if there is a problem with one or more cells. Since it knows the voltage of each cell, a balancing charger is able to charge individual cells more or less in order to “balance” the pack.

Through regular use the individual cell voltages can begin to creep away from each other which is why it is important to balance charge your packs at least one in every 10 charging cycles.

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