Did your battery pack puff up? Should you throw it away?
I got a really interesting email recently. A pilot flew an RC model airplane with a six cell (6S) battery pack on a warm day. After the airplane landed, he noticed that the battery pack had puffed up. Much stranger, after he cycled the battery pack the puffyness went away. He wanted to know if it was safe to fly with this battery pack.
In an ideal world, I would say just throw it out. But for all I know he has $100 invested in that big battery pack. I know that I would not want to just throw it out. Can that big bad boy be trusted to fly your airplane again?
From my experience, puffed and dented (crash damaged) battery packs suffer a permanent loss in capacity. I have also seen packs like these go on and deliver reliable performance for many discharge cycles after the damage was done. What tests can you do to check it out?
The best test would be a high current discharge test. In other words, test it just like you fly it. Few chargers on the market can do this, unfortunately. As a second best alternative, do a low current discharge test. In fact, do a couple. Is the remaining capacity good enough to safely fly your model airplane?
Is one cell weak, reaching the cut-off voltage much sooner than the others? That is a big warning sign. Very few speed controls monitor the individual cell voltages. If you have a very weak cell, get rid of the pack.
I would be wary of flying a battery pack that had a 25% or greater sudden loss in capacity. Also keep in mind that a lower capacity pack will be getting discharged at a higher effective C rate.
I would also measure the internal resistance of each cell in the pack. This is not a replacement for the discharge test, but might yield useful information.
In a large and specially an expensive model airplane, you should be using a separate receiver battery pack anyway. If you have been following my posts, you know that being able to do a dead stick landing is a required skill with any model airplane.
What about the puffyness that went away? That is really odd. I would look for a puncture hole or some other break in the battery’s seal. If you find one, throw it out.
Heat alone can make a fully charged battery pack puff up. It has happened to me. The pilot may not have done anything wrong. But it is up to you to diagnose the issue and decide what is the best action to take.