Lithium Polymer Batteries and Chargers mini-primer

Lithium Polymer batteries are an attractive power source for electric flight due to their high energy density. The combination of lithium polymer batteries and brushless motors now rivals glow power in terms of power and duration.

The main difficulty when using lithium polymer batteries is choosing what batteries and charger to use. Batteries are specified in terms of voltage, capacity, and discharge rating.

Voltage is generally listed as 1S - 6S. The S number refers to the number of cells connected in series that the pack uses. You can calculate the nominal (mostly discharged) voltage by multiplying the S number by 3.7 volts, so a 3S pack has a nominal voltage rating of 11.1 volts. Fully charged, each lipoly cell will measure 4.2 volts, so a fully charged 3S pack will have measure 12.6 volts.

Capacity is measured in Milliamp Hours (mAh). So for example a 2000 mAh 1S pack can be expected to provide a voltage of 3.7 volts at a current of 2000 mA (2 amps) for 1 hour. In a nutshell, more mAh is better (and heavier).

One of the more confusing specifications is the C rating. The C rating refers to the maximum sustained discharge rate that the battery can tolerate. The formula is 1/C hours. In the example above where we discharge our 2000mAh 1S pack using a 2amp load for one hour, we are discharging it at 1C. If that pack is rated for 20C, we could discharge it in 3 minutes (1 hour / 20C = 60 min / 20 = 3 minutes) with a load of 40amps (20C * 2000mAh = 20 * 2A = 40A). As I understand it, lipo packs should be charged at a maximum of 1C; so 2 amps maximum charge current for the pack in the example above.

The main drawbacks to lipoly batteries are cost and sensitivity to abuse. Now your pack may not burst into tears if you insult its paternal lineage, but it may explosively combust if charged improperly or if it becomes damaged. See here, and here for examples of lipo combustion.

I personally use, and recommend using a balance charger, and battery packs that have a balance connector. I would not use any batteries that did not have a balance connector. I use the IMAX B5 charger, which can balance charge up to a 5S pack, and will refuse to charge unless it determines that the battery is connected properly and is of the correct type.

I also use 20C rated packs with JST style connectors(see photo below).

I have found that has excellent prices and quick shipping(from Hong Kong).

Also PLEASE READ the sticky post about Lithium Polymer batteries on rcgroups for much more in depth (and accurate?) information about the care and feeding of Lipo batteries.

Happy flying!

Views: 1155

Comment by hypergolic on July 23, 2008 at 11:39am
Thanks, that was helpful!
Comment by Cory on July 23, 2008 at 11:56am
You are very welcome.

3D Robotics
Comment by Chris Anderson on July 23, 2008 at 12:16pm
Excellent post!

Which AC adapter do you use for that charger, and does it come with the most common balancing connectors?
Comment by Cory on July 23, 2008 at 12:26pm
If you use the link for the charger, there are two power supplies listed on that page. I have both. Get the IMAX one because the other power supply connector will not fit the charger. I had to hack it to use it. The balance connectors on the charger are JST style. I am not sure if that is the most common one.
Comment by dincer hepguler on July 23, 2008 at 12:44pm
this site is also a very good source for brushless motors and esc alternatives. very cheap and too many motor types and escs. i bought several motors and esc from them and i am very happy about the prices and the shipping...
Comment by Lorentz on July 24, 2008 at 1:15pm
Great post, thank you. I managed to sample an 4S LiFePO4 (Lithium Iron phosphate) battery and plan to test it. Every cell is rated 3.2 Volts, capacity is 3000 mA/h.
Compared with LiPo-s, LiFePO4 batteries have pros and cons:
"C" is 10 - 15, while LiPo-s reach 25, heavier than LiPo-s
less sensitive to overcharge and less likely to explode.
Comment by Cory on July 29, 2008 at 6:19am
One more thing: see this page for information on how to make your own packs, and an explanation of how to wire balance connectors


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