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Old Oct 24, 2010, 01:48 PM
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My first lipo failure - and some questions

Hey all, I wasn't sure where I should put this thread, as the power systems thread title says nothing about lipos, so I thought I'd put it in Scratchbuilt cuz thats where I generally hangout.

Anyways, I have a Cox micro Warbird that I finally got round to finishing, had a brushed/lipo esc hanging around so I thought I'd just go with the stock motor on this little guy.

Here's what happened on maiden 2x in a row, hand-launched and plane got about 40 feet and motor died suddenly.

I figured it was a motor or esc issue, but esc is a 10 amp so should've been fine.

I measured the voltage on my 2S 450mah pack after charging and it read 8.43, seemed high but guessing that's OK from what little I've read?

Next I measured the amp draw with this pack, getting close to full-throttle it only got to around 1.8 amps then motor died again.

Measured with another larger 2S pack and it got to around 3 amps, and kept running just fine.

Conclusion: bad lipo pack.

Couple Q's:

1) is this 2S voltage reading normal or am I overcharging?

2)My 2S packs did not come with balancing lead, how important is balancing and how often should I do it?

* Just ordered an FMA Cellpro 4S charger after reading very positive reviews, will this balance the cells properly or do I need a seperate balancer?

3) I have pretty much been charging at 500mA regardless of the C rating of my packs, since this is slightly higher than 1C for the 450mah pack, do y'all think this was the problem?

4)As an aside, its interesting to me that the ESC cut off because the battery wasn't putting out good current, did this cause Voltage to go down too ?

5)I generally fly until I see a noticeable decrease in power to my planes,then land. Should I be more rigid on my flying times? I was under the assumption that the lipo cutoff of the ESC should be good enough to avoid damaging my packs.

I know, too many questions, but I'm just trying to learn from my mistakes and hopefully avoid them in the future.

PS: Never hook up a multimeter to a lipo pack without first making sure you are connected to the Voltage leads, and not the ammeter leads.....ask me how I know this.....can you say massive short, ballooning and quick run to the fire extinguisher just in case? Doh !!
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Old Oct 24, 2010, 02:09 PM
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The Batteries and Chargers forum is usually good for questions about batteries (and chargers) .

1. It's a bit high...max of 8.4 is normal
2. Not very important when you only have 2 cells
3. No it's fine.
4. Too much current drags the voltage down and it's the low voltage that causes the ESC to cut off....that's why the feature is called LVC (Low Voltage Cutoff) .
5. Timed is better. Many ESCs cutoff is too low to keep the battery safe.

Steve
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Old Oct 24, 2010, 02:18 PM
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Thanks for the fast reply. I'm not sure how to move this thread. Guessing I need to invest in a decent digital timer. Still not sure what gave my e-tech 2S cells such a low lifespan.

re #4) I had too little current out of these cells (1.8 amps max), I'm assuming thats what you meant?

Thx again, good info.
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Old Oct 24, 2010, 08:18 PM
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rotagen: you could close this thread and ask the question in the batteries thread. But you will still get answers here too, I'd think!

I wouldn't buy a pack without balance connectors, when I can get decent packs for a few bucks with them (Hobby king, Dealextreme and now Hobby partz for a few). I have found that some packs will consistently charge one cell higher, or discharge one cell more than the other(s). Without a balance charger, that differential will grow til you get enough imbalance to cause problems. I'd get a needle poked through the wire insulation, or some other way to check the voltage of each cell, for sure.

The little Esky charger cost me 8 bucks from deal extreme and it charges each cell to 4.17 or 4.18v every time. Only drawback is it only charges one cell at a time (in series) I also have a Hextronix charger that cost me $28 at Hobbyking. It charges all cells simultaneously, but frequently not to the max. I frequently charge packs up to 1500mah with my Hextronix charger then use the esky to top them off.

Most of the new cells will charge at 2c, so your rate is probably not an issue

I think the comment about current was if one runs the pack at higher than appropriate current draw (too big a prop for instance), it will suck down the voltage to LVC abnormally fast. Not your problem, as I understand it. But if you have a weak cell, or a way out of balance pack, the voltage will be pulled down fast by the weak cell. The only counter argument to that is that getting an out of balance pack to 8.43 with one cell below the other, means that the "good" cell will be way overcharged, and should have puffed or worse. You really need to know what each cell is doing.
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Old Oct 25, 2010, 12:47 PM
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Thanks springer, your avatar animation cracks me up! Looks familiar.

I ordered a charger that looks like it does everything except cook dinner. The nice thing is it has a "storage function" that should get the cells to the right voltage for long term storage (around 3.7V per cell I think). Also it balance charges, which is cool. I've never balanced my packs, hoping I would get away with it but now I'm changing my ways.

One of the things I realized from lots of research online, is that my lipo storage conditions are all wrong, and will seriously decrease the capacity of my packs. I've been sorta willy nilly about storing my packs, and was thinking that storing them fully charged is better.

Storing packs fully charged at room temperature is about the Worst thing you can do.

I'm Thinking of storing them in the freezer as some people do since I sometimes don't get to fly for months at a time.

Here's a useful chart (lower right):

http://uterc.org/files/LipoStorageTips.pdf
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