[The Rocketry Show] Episode #4.53: Season 4 Premiere! Straight Fins and Lithium Polymer

*** NOTE from CG: Do not use the LiPo salt water disposal method mentioned in the episode!  While it has been “common knowledge” within  hobby circles to use this method of disposal, I have found very good reasons not to do this, and I’ll talk about it in Episode 54!!!   Just take bad LiPo batteries to a recycling center***

We’re back for Episode 1 of Season 4 of the Rocketry Show!

This is a show packed with lots of information for you!   Daniel, The Rocket n00b starts with a discussion on a technique he mastered for making curled balsa and basswood rocket fins straight again!

CG chimes in during the second half to give a basic beginner’s primer on Lithium polymer (LiPo) batteries for powering your electronic systems on board your next rocket project!

References for you to follow up on what CG was talking about:

Since the show was running a bit long, and there were details CG didn’t get to cover, he has provided some quick, but important notes here:

What does the Battery “C Rating” mean?

The C rating of a Lithium Polymer (LiPo) battery (or cell) is used as a guide to show what kind of continuous current draw, in amps,  the cell in question will support.

By multiplying the C rating times the cell capacity in milliamperehours (mAh), the continuous current in milliamperes (mA) of a cell can be calculated.

In my case, the batteries I use for Nesaru are rated as 180 mAh, 25-40C.  I’ll use the lower “C” number, as I’m pretty sure the 40C is “peak” capacity – more on that in a moment.

The Turnigy LiPo battery CG uses for his rocket, Nesaru.

For this battery, the safe continuous current draw value is found by multiplying 180 (mAh) x 25 (C), which gives you 4500 mA, or 4.5 Amps.

Now, for the “Peak value”.

This is the amount of current the battery can safely deliver for very short periods of time.   In this case, the battery says its peak value is 40C.  Using the same math above, we get a peak current rating of 7200 Ma, or 7.2 Amps.

Note on Charger setting:

The battery in my example is rated for 8C max charge rate.   Doing the math (180 x 8), the safe charge rate for my battery is about 1.44 Amps.

As for the charger settings for this battery, I should stay below the 1.44 Amp charge value to stay safe.   Not pushing the battery to its limits in the charger means the cells will stand a much lesser chance of becoming unbalanced (in multi-cell batteries).  So, I typically charge this battery at a 0.5 Amps on the charger.

Why?  Because unbalanced battery cells = a really bad day in the field – if you’re lucky!

Links I found for further reading on the topic, and packed with many of the tips I gave, plus much more:

The Lithium Polymer Safety Guide

The Drone Girl blog

That swollen cell phone LiPo battery CG talked about during the show!

liPo cell phone battery gone bad!

The swelling is apparent, and even though the battery has a big “Li-Ion” stamp on the lower right corner, the fine print on top says what CG figured out when the swelling pushed his old cell phone apart…

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