Informational - LiPo Batteries

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patelldp
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Informational - LiPo Batteries

Postby patelldp » Tue Dec 22, 2015 9:47 am

I was asked some information about LiPo's this morning, figured I'd post what I sent for posterity!

LiPo's are interesting, I know very little about them. I do know that the "S" number is the number of cells wired in series within the battery therefore adding the voltages of the individual cells. LiPo's also report a higher voltage than their rating when fully charged, so a 1S will report ~4.2V vs. a rating of 3.7V, a 2S ~8.4V vs. a rating of 7.4V, etc.

The next things to consider are the current (mAh) rating, or the capacity, and the C rating (A), or discharge rating. The higher the mAh number and lower the C rating, the longer the battery will perform under load. A high C rating and high mAh can damage the altimeter's FET's if you have a prolonged short after the ejection charge or igniter fires. Consider that a 9V battery is ~500 mAh. Here's a link explaining what the C means: https://sites.google.com/site/tjinguyte ... variable-c

Let's take a look at a battery that I know works on the RRC2+ and RRC3 as they are what I Beta tested those units with.: http://www.hobbyking.com/hobbyking/stor ... FPack.html

It's 2S (7.4V), has a 180 mAh capacity, and a 25C continuous discharge rating, 40C burst. Here's the math:

C = 180 mAh = 0.180 Ah = 0.180 A

25*C = (25)*(0.180) = 4.5A
40*C = (40)*(0.180) = 7.2A

As you can see, this battery will operate primarily at 4.5A, while the Perfectflite SLCF and MW RRC3 have capability up to 5A. You could do damage at the 40C burst, but you're above my pay grade when considering what would cause that to happen.
Dan Patell
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MarsaSystems
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Re: Informational - LiPo Batteries

Postby MarsaSystems » Tue Dec 22, 2015 3:11 pm

Dan, excellent post! I would like to add some points.

The C rating of the LiPo is the discharge rating of the battery where it can safely operate for somewhat continuous current. (Think RC planes and helicopters). This is generally based on the safe heating level of the battery. The higher the C rating the lower the internal resistance of the battery' Less power is dissipated in the battery itself.

However the C rating does indicate the maximum current the LiPo will deliver. For example I use HK 300ma 35C batteries with my altimeters. It is incorrect to assume that the maximum current you will get out of these batteries is 35x0.3 or 10.5 amps. During a test of these batteries they supplied over 25A into a dead short.

Bottom line is you cannot use the C rating by itself for current limiting estimation. Most any decent sized LiPo (300mah or greater) can easily very large amounts of current into low resistance. The only way to know the maximum current is to (safely) measure it.

Edit: Also the 1S 400ma battery from Sparkfun is not a choice because it has a battery protection circuit in it. Do not use batteries with protection circuits. I want the battery to protect the rocket, I care less about protecting the battery.

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patelldp
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Re: Informational - LiPo Batteries

Postby patelldp » Tue Dec 22, 2015 3:41 pm

MarsaSystems wrote:Dan, excellent post! I would like to add some points.

The C rating of the LiPo is the discharge rating of the battery where it can safely operate for somewhat continuous current. (Think RC planes and helicopters). This is generally based on the safe heating level of the battery. The higher the C rating the lower the internal resistance of the battery' Less power is dissipated in the battery itself.

However the C rating does indicate the maximum current the LiPo will deliver. For example I use HK 300ma 35C batteries with my altimeters. It is incorrect to assume that the maximum current you will get out of these batteries is 35x0.3 or 10.5 amps. During a test of these batteries they supplied over 25A into a dead short.

Bottom line is you cannot use the C rating by itself for current limiting estimation. Most any decent sized LiPo (300mah or greater) can easily very large amounts of current into low resistance. The only way to know the maximum current is to (safely) measure it.


Thanks for this additional information. I knew that these are primarily used in RC vehicles and people often look right at the capacity figure and little else. We're more likely to run into these maximum current dump scenarios with our application vs. a RC helicopter!

MarsaSystems wrote:Edit: Also the 1S 400ma battery from Sparkfun is not a choice because it has a battery protection circuit in it. Do not use batteries with protection circuits. I want the battery to protect the rocket, I care less about protecting the battery.


I had heard that before, and understand that is why Altus Metrum uses unprotected batteries. I agree with you.
Dan Patell
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MarsaSystems
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Re: Informational - LiPo Batteries

Postby MarsaSystems » Tue Dec 22, 2015 3:56 pm

Another thing and this is my own personal opinion....

The whole advantage of LiPo is their current output capability (low internal resistance) and their storage capacity. So to me selecting these wimpy low C high resistance low mah Lipo's for rocketry use just doesn't make alot of sense....

You want high capacity so I have have long pad time and not worry about the charge level.
You want unfettered high current output so if there is a short or transient high load you don't reset the altimeter.

So we throw that away because we are worrying about frying our FETs? That's like putting a low HP 4-cylinder engine in your Vette so you don't spin your tires.

So here is my recommendation...
1 Pick a lipo with high capacity and high current output.
2.Select an altimeter which has output current limitation (protection) - like the Marsa54L (shameless plug)
3.If you can't do 2, put a 1ohm carbon composition resistor in series with your ematches, That will limit your current to the battery voltage in the event of a short.

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patelldp
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Re: Informational - LiPo Batteries

Postby patelldp » Tue Dec 22, 2015 4:02 pm

MarsaSystems wrote:Another thing and this is my own personal opinion....

The whole advantage of LiPo is their current output capability (low internal resistance) and their storage capacity. So to me selecting these wimpy low C high resistance low mah Lipo's for rocketry use just doesn't make alot of sense....

You want high capacity so I have have long pad time and not worry about the charge level.
You want unfettered high current output so if there is a short or transient high load you don't reset the altimeter.

So we throw that away because we are worrying about frying our FETs? That's like putting a low HP 4-cylinder engine in your Vette so you don't spin your tires.

So here is my recommendation...
1 Pick a lipo with high capacity and high current output.
2.Select an altimeter which has output current limitation (protection) - like the Marsa54L (shameless plug)
3.If you can't do 2, put a 1ohm carbon composition resistor in series with your ematches, That will limit your current to the battery voltage in the event of a short.


More great points, and I edited my original post to reflect that. Thanks for sharing!
Dan Patell
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DBMcCann
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Re: Informational - LiPo Batteries

Postby DBMcCann » Sun Dec 27, 2015 11:17 pm

I'll be honest. I like lipo's because they're smaller.... I'm moving over to lipos on my RRC3's and RRC2's, mostly because they recommend a specific battery.

As long as someone smarter than me picks one, I'll run with it.... I'm just way too lazy/focused on other aspects to get into the entire science of batteries.

DBMcCann
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Re: Informational - LiPo Batteries

Postby DBMcCann » Sun Dec 27, 2015 11:23 pm

LOL. I just put 2 + 2 together here, excuse me for that.

But what it breaks down to.... if it works.... I'll run with it.


I like my MARSA..... Probably going to be switching to LiPo on it this winter so I can stuff it into my 54mm MD Tomach :)

FMarvinS
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Re: Informational - LiPo Batteries

Postby FMarvinS » Mon Dec 28, 2015 9:24 pm

Dan,

Thanks for the informative thread. Which Lipo charger are you using for the Lipo's described?

Thanks,
Fred


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