Choosing a LiPo Battery

by FliteTest | May 19, 2012 | (34) Posted in Tips

By: Chad Kapper

It seems like every month there are new types of batteries. The sizes, shapes and power are changing rapidly. For this article I’d like to talk about the most common powerplant for our hobby... the Lithium Polymer battery pack, or LiPo for short.

Choosing the right battery
There are a number of factors to consider when it comes to choosing a battery pack. Before making a purchase, think about these points:

  1. Voltage
  2. Capacity
  3. Weight
  4. Physical Size
  5. Connector
  6. Discharge
  7. Charging Rate

Voltage (S)
All lithium Polymer cells have a nominal voltage of 3.7v per cell. When fully charged a LiPo cell should be 4.2v and when discharged it should never be below 3v.
You will notice that LiPo packs are made up of layers of multiple cells. A 1S = 1 cell (3.7v). If the LiPo pack is listed as being 3S this means it is 3 cells x 3.7v (per cell) which is 11.1v. It has 3 layers of 3.7v each. The rating for each cell is always referenced by its nominal voltage (3.7v), NOT the maximum voltage (4.2v).

Capacity (mAh).
This is usually the biggest or most obvious rating shown on the LiPo pack and is measured in mAh (Milliamp/hour) or Ah (Amp/hour). The capacity is an important factor to consider. Think of capacity (mAh) as the amount of fuel in your car's fuel tank. A higher capacity tank will run your car for longer. A 4,000mAh battery will run for twice as long as a 2,000mAh battery. A 2,000mah battery will (in theory) run for 1 hour if drained at a constant 2,000 Milliamps.
You can see an example of a capacity test HERE.

It's tempting to choose the biggest and most powerful battery your model can handle. Always double check the model’s CG (Center of Gravity) to help you decide on the right battery weight. In most cases higher capacity battery will be physically larger and heavier.

Physical Size
For a battery to be right for your model it must fit within the model’s battery compartment. Not all batteries are the same size. While most manufacturers try to stay consistent in sizing, the dimensions can still be quite a bit different. Sometimes a 2mm difference in size might keep it from fitting in your model.

This is the main ‘plug’ on your battery. There are SO MANY different connectors out there. The chances of the right battery having the right connector are slim. Be prepared to have to re-solder your own connector on a new battery. We did a video showing how we do it HERE.
You will also notice a secondary plug with multiple wires. This is your balance plug. It feeds into each cell separately. This will plug into your charger to make sure the cells are balanced properly. I could fill another whole article on balance plugs, balance charging and connectors! That’ll have to wait until I get more time.

Discharge (C)
Discharge is the amount of power the battery can supply your power system. The ‘C’ rating is an multiplication of the capacity. Example: A 20C battery can discharge at 20 x 2,000mAh which is 40,000mAh or 40Amps. This is an important number to consider. It’s always good to use a wattmeter to measure the demand of your system.
In addition to this, batteries have a 'Burst' rate, which is the amount of power the battery can discharge for a short period, usually 10-20 seconds. A typical battery label may show 20-30C, this would mean a 1,000mAh battery can discharge 20,000mAh constantly or give a sudden and short 10-20 second 30,000mAh (30A) burst of power.
Tip: A higher 'C' rated battery will last longer if run at a lower 'C' rate. Example: a 30C battery run at 20C maximum will have a longer cycle life than a 20C run at 20C each flight.
You can see an example of a discharge test HERE.

Charging Rate
Some batteries will display a ‘C’ rating for charging. MOST batteries are rated at a 1C charge rate. The formula is this:
XXXX mAh / 1000 x (Charging C rating) = XX amps
For example: If you had a 2200mAh LiPo pack with a charge rating of 2C you would not want to charge the battery at rate more than 4.4 amps.
It’s the lower amperage you charge your batteries, the longer they will take to charge. However, a slower charge is always better for the health of your battery. If you have the time... take it slow. If you can’t find charge rating on your battery, then play it safe and charge at 1C.

Charging Safety

  • Always use a lithium Polymer battery charger and never charge the battery above 4.2v per cell. (example: 2S, never above 8.4v)
  • Never leave a charging battery unattended.
  • Never allow the battery's voltage to fall below 3v per cell. (example: 3S, never below 9v)
  • Never charge batteries inside a vehicle.
  • You should always charge and store LiPo batteries in a fire-safe container.
  • Do not charge batteries near anything flammable.

Battery Storage: If you don’t plan on using your lipo pack for more than a week, it’s good practice to store it at 60% of the pack’s rated capacity. To maintain good healthy battery packs you should also cycle your packs at least once a month. Storing them without use for long periods of time will decrease the overall life of your batteries.

Here is a nice thread with additional info:

Understanding Batteries 101 -



747Pilot on May 19, 2012
This was a good article. I am a newbie (Glow fuel for 25 years) and I'm just about to venture into electric flight since my backyard is considerably closer than the local flying field. Thanks Chad.
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stonekap on May 19, 2012
No problem! Glad I could help.
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colorex on May 20, 2012
Great article, I will send this to a friend!
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JerMalk on June 14, 2012
I highly agree with L33n0x that you need to do one on help with choosing a motor. I am still confused about kv ratings not really meaning anything??? And how do you tell what size a motor is, because I heard that is what really matters, but I could be wrong. Most motors don't even tell you the size when you go to buy one!!?? So what really matters when choosing a motors strength?? Like how come a motor that says its 300kv costs soo much more than let's say a 1700 kg motor?? PLEASE do either an episode, d# or even just an article on this subject!

Thank You.
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Geinstein on May 20, 2012
As a physicsteacher I have to make a comment.
mAh is not milliamp/h(milliamp per hour) but mA*h (milliamp times hour).

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cwozny on July 6, 2012
Exactly, although doesn't it mean that you'd be able to draw, for example, 2200 milliamps from a battery for an hour if it's rated at 2200 mAH? The notation of the mAH unit is still a little fuzzy to me.
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Geinstein on July 7, 2012
A 2200 mAh lipo has the following caracteristics:
you could draw 2200 mA (=2,2A) for 1 hour
or 1100 mA for 2 hours.
So: mA x hours = capacity.

Please note: don't empty a lipo all the way. It won't survive that.
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L33n0x on May 20, 2012
Good article ! It will be really nice to have a same one "how to choose a motor". :)
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Steve Bowman on May 21, 2012
Thanks, I always had a few question and you got them answer.
Great web --site

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House Of Noob on June 7, 2012
Love it - it would be nice to have a reference library where this type of fundamental information is accessible as a set. People getting into the hobby such as myself end up trolling around a lot gleaning information out of hundreds of pages in a thread somewhere. Makes it really hard to find the second time too which is frustrating beyond belief.

Good job
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Chrisryalls on January 17, 2013
Very informative
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The Big Ox on May 30, 2012
Great article answered a LOT of my questions!
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Foskje on May 19, 2012
And 60% store capacity is not 4,2V x 0,6 = 2,52V,
but (3,0V - 4,2V) * 0,6 + 3,0V = 3,72V
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colorex on June 27, 2012
True - yet so easily overlooked!
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GPLot on May 20, 2012
You have got the cat by the tail regarding the first section. 1S means that there is one cell connected in Series. 1P means there is 1 cell connected in Parallel. Typically a 11.1V 3 cell 2200mAh battery pack should read 3S1P, meaning there are 3 cells connected in Series and only 1 cell per Parallel connection. A battery pack labelled 3S2P 2200mAh will typically be a battery pack made up of 6 1100mAh cells, connected in 3 Series sets of 2 Parallel cells ((2x1100mAh)x3).
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Pablete on June 4, 2012
really helpfull, tnx
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Juandre on May 21, 2012
I've been looking for a article like this for some time now but not one of them sums up the important facts like this one. Thanx Chad
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stephen on June 14, 2012
That's was just the info I needed. Last time I was into flying was 30 years ago, OS Max control line flying wing combat. Stumbled across flighttest reviews of electric planes now I'm hooked. Can't get enough info quick enough. Great read simple to the point.

Thanks Chad
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jalkin on February 16, 2016
You seem to have your units a bit mixed up. You say "A 20C battery can discharge at 20 x 2,000mAh which is 40,000mAh or 40Amps"
"mAh" is a measure of the capacity of the battery, not a measure of discharge current.
40A is certainly a measure of discharge current, but you can't say "40,000mAh or 40Amps", as you can't say that capacity = current. To elaborate on your analogy of capacity being like the amount that your car's fuel tank can hold, discharge current would be the speed of out-flow along the fuel line to your engine. Getting the units mixed up gives the impression that you are writing about something you don't fully understand, which undermines confidence in what you say. You continue "battery can discharge 20,000mAh constantly", which makes no sense to an ectrician or electronics engineer. If you read "fuel tank outflow is 1 gallon", you would be asking yourself "1 gallon per minute? per hour? What?".
I came here looking for the meaning of the C rating (what does 30C mean) because I read on another web site that a 2000mAh 15C battery can have a maximum discharge
of 2000 x 15 = 30000mA (or 30A), while a 4000mAh 15C battery can have a max discharge rate of 4000 x 15 = 60000mA (or 60A). Well, maybe that's true, but it makes no intuitive sense to me that it should be. Why should a battery be capable of delivering a higher current because it has a higher capacity? I can see that a battery of twice the capacity will deliver the same current for twice as long, or double the current for the same time (supposing that the current is within the capability of the battery), but I'm still confused over the meaning of the "C" specification.
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Choosing a LiPo Battery