Choosing a LiPo Battery

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.


Weight
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.

Connector
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 - http://forum.flitetest.com/showthread.php?437-Understanding-Batteries-101