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 Post subject: Power question
PostPosted: Mon Aug 26, 2013 7:57 pm 
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Okay, I think I'm starting to understand this whole how fast I can charge thing, but I want to ask.

First, I understand that watts = volts * amps.

So, looking at my charger, if I put in 120 volts AC and my charger is limited to a 90 watt maximum output, then the best I can hope for is 0.75 amps of current out. That means that the fastest I can charge a 2600 mah battery is about 1.75 C. Correct? However, if I put in 12 volts DC for which my charger is limited to 150 watt max, then I can expect up to 12.5 amps out. That means I should be able to charge a 2600 mah battery at 4 C (and beyond if I want to cook it) with no problem. Correct?

Thanks!

Jonathan

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 Post subject: Re: Power question
PostPosted: Mon Aug 26, 2013 8:49 pm 
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I think you're still a bit confused. ;)

Ignoring the input voltage for now, if your charger can output 90 watts, that is the important number. If you are charging a 3S pack (12.6V fully charged) then the maximum charge rate would be 90 / 12.6 = 7.14 amps. If you're charging a 6S pack (25.2v fully charged) then you'd only be able to charge it at 90 / 25.2 = 3.57 amps.

Make sense?

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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 6:13 am 
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I'll continue with Chris' explanation:

The input voltage (120vac/12vdc) isn't used in the calculation that you are interested in other than that the output wattage capacity of the charger increases. The voltage of the battery at full charge is the voltage that you are going to be using. Also, to make the math more "readable" I'm going to call your battery a 2.6Ah battery (2600mah / 1000 = 2.6 Amp hour).

A few figures to get out of the way:

A fully charged LiPo battery cell is 4.20vdc and a 80% discharged LiPo cell is 3.7vdc. I'm going to assume that your battery is 80% discharged when you recharge it so you are going to be replacing 2.08Ah (2.6ah x 0.8) with each charging cycle.

Full voltage of different cell count packs (number of cells x 4.20vdc)

1S = 4.2vdc
2S = 8.4vdc
3S = 12.6vdc
4S = 16.8vdc
5S = 21vdc
6S = 25.2vdc

The equation that we are going to be using is watts / volts = amps.

(The output of your charger in watts) / (number of cells x 4.2vdc) = charging amps.

So applying the equation:

1S battery is: 90watts / 4.2vdc = 21.43A charging output
2S battery is: 90watts / 8.4vdc = 10.71A charging output
3S battery is: 90watts / 12.6vdc = 7.14A charging output
4S battery is: 90watts / 16.8vdc = 5.36A charging output
5S battery is: 90watts / 21vdc = 4.29A charging output
6S battery is: 90watts / 25.2vdc = 3.57A charging output

The charger will also have an amperage output limit, let's assume that it is 10A. This means that when charging a 1S or 2S LiPo battery the output amperage is going to be 10A not the 21.43A or 10.71A as above.

With the charger amperage output when the battery is fully charged we can estimate the charging rate, aka the "C" rating. The equation is:

(Charging amperage when the battery is full) / (battery Ah rating x 0.8 aka 80% discharged) = charging C rating

So applying the equation:

1S battery is: 10A / (2.08Ah) = 4.81C charging rate (using the 10A charging limit)
2S battery is: 10A / (2.08Ah) = 4.81C charging rate (using the 10A charging limit)
3S battery is: 7.14A / (2.08Ah) = 3.43C charging rate
4S battery is: 5.34A / (2.08Ah) = 2.57C charging rate
5S battery is: 4.29A / (2.08Ah) = 2.06C charging rate
6S battery is: 3.57A / (2.08Ah) = 1.72C charging rate

You can plug in the numbers and get your charging rates when you are using a DC power source and being able to output 150watts, but remember to apply the charger's maximum amperage output limit.

If you want to try parallel charging, just add the Ah ratings of the batteries into one battery for the equation. The Ah ratings of the batteries can be different, but the cell count of the batteries must be the same.

For example parallel charging two 3S 2.6Ah batteries:

7.14A / (2 x 2.08Ah) = 1.72C charging rate


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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 6:54 am 
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Very nice tgiencke! :thumbup:

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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 7:37 am 
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Yeah, very nice. Can we steal that as show content? We'd give you full credit. :)

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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 9:55 am 
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JustPlaneChris wrote:
Yeah, very nice. Can we steal that as show content? We'd give you full credit. :)


Sure, no problem. But I don't need to be given credit, but if you do my last name is pronounced GING' key. The t is Todd.

If you wanted expound this into a show topic may I suggest the subject of The Economy of High Current Charging. This idea came from listening to James talking about having three batteries and two chargers, and cycling them to be able to almost continuously fly; and batteries are solid fuel. I took this idea to the next step, two batteries and one high wattage/amperage charger for almost continuous flying.

(If you are interested I'm willing to come on the show and talk about this stuff)

I could go into it, but to cut to the chase:

Charging system: $140 Powerlab 6 or $190 Powerlab 8 V2, two 100+Ah deep cycle batteries.

Flight battery: Pair of 6S 5000mAh batteries.

Charging time:

Powerlab 6: 19.5 minutes (14.5 minute charging + 5 minute balancing timeout)

Powerlab 8: 17 minutes (12 minute charging + 5 minute balancing timeout)

Supporting info:

1. A lot of inexpensive batteries are rated for 5C charging. So let's charge at 5C, while monitoring battery temp accurately If the battery is not increasing in temperature while charging , we are not doing damage to the battery. After testing with expensive Pulse and inexpensive Turnigy batteries, they both don't mind being charged at 5C

2. $190 Refurbished Cellpro Powerlab 8 V2 40A/1344W charger or $140 Refurbished Cellpro Powerlab 6 40A (~33A @ 24vdc)/1000W charger (need to add a $10 balance lead adapter). Add to this "inexpensive" refurbished charger a pair of ~$90ea 100+ Ah deep cycle batteries.

So here is my charging math for parallel charging a pair of 6S 5000mAh batteries at 5C on my Powerlab 8:

1344watts / 25.2vdc = 53.33amps (I'm limited to 40 amps at 25.2vdc)

40A / (2 batteries x (5Ah x 0.8)) = 5C maximum charge rate.


With the less expensive $140 Powerlab 6 doing the same charging:

1000watts /25.2vdc = 39.68amps (limited to 33A by the 24vdc power source)

33A / (2 batteries x (5Ah x 0.8)) = 4.13C maximum charge rate.


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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 10:27 am 
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Todd,

Good stuff buddy. These kind of things are tricky to talk about on the show because it becomes difficult for people to follow numbers while listening. However I bet we could create a page on the website, then talk about it on the show just toning it down a bit. Just a thought. :thinking

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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 11:16 am 
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Yes, that's what I had in mind Bobby! A "tech" section of the main website, with content such as this presented (maybe with some nice charts or something if applicable). Then we could have a companion thread here in the forum, with a handy link from the page.

I'm just trying to think of ways to make this crazy RCTS endeavor more helpful to our loyal listeners! :thumbup:

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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 12:00 pm 
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Do what you will with whatever I have done. I think you guys have done well in bringing down the stigma of inexpensive batteries. I would like to support the idea of investing in charging systems and not batteries. Kind of like investing in radio/servos and power train equipment and not airframes.

Another topic in the same thought process is calculating appropriate discharge C ratings for different power trains. ie as the Ah increase the need for discharge C ratings decrease.


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 Post subject: Re: Power question
PostPosted: Tue Aug 27, 2013 4:11 pm 
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Location: McKinney, TX
Does anybody know of a good formula for estimating the required power supply wattage for a given charger wattage output, assuming power factor correction is used? I would guess it would vary widely between charger models, but it'd be great if there was some sort of "worst-case" computation. I was mainly wondering if 1500W of power supply wattage would be enough to drive 1100W of charging power.

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