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I was lazy, and almost lost a model

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The Wright Stuff10/07/2017 11:41:34
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1383 forum posts
226 photos

I had a near-miss last week with my E-Flite Hurricane. Using 3200 mAh 3S batteries, first flight was 8 minutes, which normally leaves about 20-25% in the LiPo: on this occasion, it was 16%. A little loss of capacity, it seems.

Without loss of enthusiasm, I swapped in the next battery, checked charge at 98%, and commenced my second flight. All was good until I heard the rattle of the ESC cutting power to the motor. At first I thought the noise was the combine harvester in the next field, but then the power loss was unmistakable. I stuck the nose down to get some airspeed, stuck in some flaps and luck managed to find the strip. As I rolled out, there was no damage to the model but my confidence had taken a hit. The timer read 7 minutes.

Upon investigating, the LiPo was reading 1% with a 0.6 V imbalance, so clearly a dead cell. Needless to say, the pack was condemned.

Now this is the first time I’ve used these batteries for almost 3 years, so they have spent a long time in storage. They were impeccably looked after in the sense of charging at 1C, balancing, not over discharging, and were stored at exactly 40%. A value, I noted, they still had after 3 years in this state.

Moral of the story:

  1. LiPos can age, even if kept stored in nominally ‘ideal’ conditions.
  2. Check the actual capacity of the cells after prolonged storage, even if they accept charge perfectly. I was lazy and nearly paid for it with the model.

Question:

Is there any evidence to suggest that loss of capacity due to degradation is worse in higher capacity cells? I have used (and similarly neglected) 3S packs of 1000 to 2200 mAh capacities and they have exhibited much longer lives. The battery in question was a Turnigy 3300mAh 3S 30C.

Tony Kenny10/07/2017 11:47:21
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279 forum posts
62 photos

Thanks for the story, it's a god reminder for me to check each cell before I fly with the device I use to alert to loss of power.

When you condemn the pack, will you retain the functioning cells to build up a new pack later?

Frank Skilbeck10/07/2017 12:25:05
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4241 forum posts
100 photos

Yes the batteries do lose performance over time, I have some 3s 3300 mah batteries and they are good for about 2200 mah now! It's where the mah telemetry sensor comes in to, I'll often check the batteries on a lipo monitor after landing and do a quick calc on the mah used and comparing against the telemetry sensor (which is usually within a few mah of what the charger puts back in)

My condemned packs are now find a new life as power supplies for my IC flight box and starter.

The Wright Stuff10/07/2017 12:31:06
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1383 forum posts
226 photos
Posted by Frank Skilbeck on 10/07/2017 12:25:05:

My condemned packs are now find a new life as power supplies for my IC flight box and starter.

Brilliant practical idea, and more than a little ironic!

Josip Vrandecic -Mes10/07/2017 12:40:25
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2932 forum posts
247 photos

Thanks TWS , unfortunately I had similar experiences but then I bought telemetry for RX voltage on the plane (strong alarm)  the nightmare was disappeared.
If you can get telemetry for RX voltage on the plane and enjoy flying wink 2

Cheers

Edited By Josip Vrandecic -Mes on 10/07/2017 13:07:27

The Wright Stuff10/07/2017 12:50:26
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1383 forum posts
226 photos
Posted by Tony Kenny on 10/07/2017 11:47:21:

When you condemn the pack, will you retain the functioning cells to build up a new pack later?

Nope. Even if I had confidence in the other cells being healthy, to be honest, life is too short!

Chris Walby10/07/2017 14:39:21
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765 forum posts
148 photos

Hi, just to add it does not need to be an old pack, over the winter I had forgot to bring the Lipos in from the car. Next day was bright but cold day (just around zero) so headed for the field.

Checked a 4S 5000 no issue with capacity or balance so in it went, all checks complete and gentle taxi out, Turned into light wind and proceeded to full power and just climber to about 20ft at the end of the strip, then the ESC wound the power to almost nothing. With a gentle turn and downwind landing I check things over.

Result one cell would test okay (even balance check), but would go more or less short circuit under high load. Post field check results in very little capacity of the offending cell, but a voltage that indicated it was fully charged

Moral of the story, don't use cold lipo's as it appears to kill the lipo, dent the wallet and increase the heart rate!

PS I can't bring myself dispose of the lipo, but then I would not use the other cells in anything either, best to give it to the boating guys as they don't mind a bit of ballast!

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