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Permalink Reply by Brad Hughey on March 19, 2012 at 7:09pm

I agree he's okay, but not by such a margin.  I'm assuming he meant a load current of 26A each on a quad, which means 104A total.  We hope his quad will probably hover at something substantially less than the rated (maximum?) load current, though.

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Permalink Reply by Steve Warren on March 21, 2012 at 8:18am

Thanks guys - I am a little unclear on how to decode the 20C designation. Do you multiply that by mAh for the total current capacity? That's what it looks like you are doing. Have I missed a FAQ around here that explains things like that? I hate asking questions that have already been answered in a sticky somewhere. :)

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Permalink Reply by Brad Hughey on March 21, 2012 at 9:15am

To elaborate a bit, not knowing myself if this is on a FAQ page somewhere...  :-)

A "C" rating is the battery manufactures' way of specifying the maximum sustainable rate of current discharge in amps, as a convenient multiple of the energy storage rating of the battery in amp/hours.  For example, a 3300 mAh 20C battery can sustain a current discharge rate of 66 amps, while a 2100 mAh 20C battery can only provide 42 amps.

To further complicate things (as marketing hacks love to do), many companies are now starting to add "burst" current ratings, which indicate short-term capabilities.  In the following example, a particular vendor is quoting a "30C" capacity with "60C" bursts:

Hobby King 30-60 Cells

The devil is in the details of how such bursts are actually calculated, but know that such a figure should not be used (in my opinion) to determine the suitability of a battery pack for a specific application.

Also, note that the other main specification of interest is the xS/xP, which is an indication of how the pack is actually constructed.  In the example case, 2S/2P means that two cells were connected in parallel and then placed in series with two more cells connected in parallel.  Parallel connections increase current capacity and series connections add voltage.  So, to make the pack in the above example, 4 2650 mAh cells were used. I personally prefer the idea of 1P packs as having fewer possible points of failure.

The voltage rating can be a bit tricky too, as a fully charged LiPoly cell has an unloaded voltage of 4.2, but this number starts dropping the instant you apply a load (draw current).  It is not uncommon for some cells to drop as low as 3.3 volts when full rated current is drawn.  The 3.7 v "nominal" specification is just used for comparison purposes, but you can multiply the "S" number by 4.2 or 3.3 to get an idea of the voltage range under load.

The final "C" rating of interest is related to charging.  The example 5300 mAh battery has a 5C charging rate, which means your charger should be set to deliver no more than 26.5 amps.  The manufacturer claims this pack can be charged 5 times faster than a 1C pack.

Keep in mind that the maximum "C" ratings are just that - maximums.  Your battery pack will be happier and last longer if you don't approach maximums on a regular basis.  Lithium ion technology in particular does not do well in heat, and the higher the current, the more heat the pack has to dissipate.  Personally, I never charge packs faster than 0.5 C, simply because I give them enough abuse during discharge.  :-)

Too much information yet?

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Permalink Reply by Steve Warren on March 21, 2012 at 9:25am

No - keep it up - I've followed some of your other explanations in the forums and really appreciate the education. I'm a EE so this stuff ought to come easy right? :)

But as always the devil is in the details. So more of the right kind of info is highly valued.

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Permalink Reply by Cliff Dearden on March 21, 2012 at 9:48am

Excellant explanation Brad.

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DeveloperPermalink Reply by R_Lefebvre on March 21, 2012 at 10:26am

Good stuff as always Brad.

Now, one thing that I have thought about, sort of a rule of thumb, would be to say that a flight duration should be *at least* 60/C minutes long.  So, a flight with a 20C pack should last at least 3 minutes.  If not, you're obviously overdoing it.  In fact, I would bet the rule of thumb should be 30/C to be a bit healthier.  If your 20C pack lasts 6 minutes, you're averaging only half of it's rated current.

My heli uses an 8S5000 pack, and I get at least 10 minutes out of them.  So I must be discharging at ~6C, which is plenty low for a 20C pack.

Now, this maybe isn't the best time or place, but since we're on the topic of LiPo's, I have always heard that LiPos should be stored at the storage voltage, and in a freezer for long-term.  If you store them full, it ruins them.  Well, I stored two packs with a full charge at room temp for.... 8 months I think?  I am using them again, and can detect no performance reduction.

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Permalink Reply by Vernon Barry on March 21, 2012 at 10:43am

The key is that never ever let a LiPo go below voltage and especially not in storage. I was always told to charge normally then store. LiPos have and extremely low self discharge rate not attached to anything which is good, but for storage it's better to have as much "reserve" to ensure self disharge doesn't get to the low end of the safe voltage. I've had cells puff (basically ruining them) because I either stored them discharged after use for too long or left one attached to a balancing charger that the charger got unplugged and self discharged the battery to the point the cell puffed.

Basically the 3 rules, never over charge voltage (NE 4 volts per cell is about when they burst into flames), never let it go under voltage (around 3V per cell or less will puff them for sure) and don't overcurrent charge or discharge either way.

I've never had one catch fire but have ruined them by storing or over discharging so now, I charge them all full, store them and then periodically recharge them to make sure self discharge never got too low. Writing dates on tape with the last time charged is a good idea.

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Permalink Reply by Steve Warren on March 21, 2012 at 12:22pm

All great stuff but it seems like from what I've read there is also a lot of variation in lifespan depending on the manufacturer. It sure seems true with lead-acid batteries and I'm about to find out with Lipos since I have some really cheap ones right now. I'd be interested to graph the price/lifespan ratio of different brands but that's pretty hard to do without a standard methodology and a lot of money for a hobbyist...

For example if the battery lasts half as long but costs 25% of the premium priced brand then it's still a good deal but if it only lasts 10% as long then it's a bad deal, etc. But you would have to treat the batteries exactly the same during the lifetime test in order to get valid results.

And I would want a load test that looks like a typical quad-copter run (high current with peaks and valleys) because maybe the lifespan curves look totally different for different styles of loads.

I can envision an actual quadcopter sitting in a static rig and running a pre-recorded set of control signals to the motors over and over, recharging, waiting for cooldown, and repeating until the battery charge capacity drops below some level. That would be nice data to have for a bunch of different battery brands.

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Permalink Reply by Brad Hughey on March 21, 2012 at 1:48pm

Thanks for your kind words, guys.  I'm not proud to say that I've destroyed quite a few cells of varying types over the years during my self-edification process.  One of my most spectacular disasters was the revelation that NiMH cells actually DROP impedance when fully charged, leading them to charge faster.  Doh!  A smoking, sizzling mass of expensive batteries smells really nasty.

I'm an EE too, but the dynamics of power storage ultimately is a chemical thing.  Chemistry simply befuddles the hell out of me; how in the world can two oxygen atoms together be essential for life, yet three such atoms constitute a poison?

Anyway, I do know from some rather expensive lessons that there are significant differences the the properties of electric cells based on chemistry.  Even within the same family of cells, there are notable exceptions (the lithium ion cells of A123 with the nano phosphate technology come to mind as an example).

I've had good luck storing LiPoly cells at the recommended 40% charge level (about 3.7 volts @ no load), and only charging them up to the maximum 4.2 when I'm within a day or two of using them. 

Here's a good general primer on the characteristics of lithium ion cells that also applies to the "polymer" class.  In short, high voltage and high heat = short life.

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_b...

I don't know about freezing them for storage, but I do know they don't work very well when frozen.

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