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11-30-2013, 03:31 PM | #1 (permalink) |
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Magical Equation?
What is it? Where is it?
I'm trying to find the amount of time it would take to charge four 6s3000mh batteries, parallel charged, using a 12v 500w charger. Please explain, or point me to, the equation you used in your determination. I have tried to find the answer, but am having no success. |
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11-30-2013, 04:52 PM | #2 (permalink) |
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The simplified guideline is a 1C charge will take 1 hour (by definition), for a fully-drained battery.
So all of this also depends on how far you are discharging your packs. If you drain 80%, they will take longer to charge than if you drain 50%, naturally. It also makes generalizations that don't include how long it takes your charger to balance the packs. Let's assume you drain 75%. So a 1C charge will take 45 minutes (1 hour * 75% drained). We'll also assume you're using a 1C charge, to start. You have a 12V, 500W charger. Really, we need more info, including the amps the charger can put out. What model charger is it? The output power from a 12V source also comes into play. Even a 1000W PL8 will have significantly less output power if running at 12V. Every charger has an input-current limit. On the PL8, that's 60A (significantly higher than most chargers). Based on that, 12V * 60A = 720W input power. It's around 90% efficient, so can output around 720W * 0.9 = 648W. Anyways, we'll generalize further, and assume your charger can output a full 500W from 12V, and however many amps you need. (4) 6S 3000mAh packs are seen as a single 6S 12,000mAh pack. Which requires 12.0A for a 1C charge. A fully charged 6S is 6 cells * 4.2V/cell = 25.2V. 12.0A * 25.2V = 302W for a 1C charge (this is worst-case, as it nears fully-charged. Again, we're making assumptions) So, if the charger can do 12A output, for 1C, and you drained 75%, you could charge them in about 45 minutes. Let's assume you can make use of the full 500W. How many amps can your charger give you, for charging 6S? 500W / 25.2V (worst-case for 6S packs) = 19.8A for 6S For you, a 1C charge is 12A, and you can do 19.8A. So 19.8A/12A = 1.65C, which would be your max charge rate. 1 / 1.65C= 0.61 hours for a 100% charge. Since we're assuming only 75% drained, 0.61*0.75=0.46 hours. Or just under 30 minutes. When you speed things up significantly (3C, 4C, etc), you get to a point of diminishing returns. Maybe 4C takes 60% the time of a 2C charge, not the 50% you'd expect. But that's not too important for our purposes. Again, this has assumptions, etc etc. With your charger model, we could at least incorporate what it can really do. But maybe this will get you started. Sorry if this isn't all presented as cleanly as it could be. Let me know what I explained poorly (or out-of-order) and I'll try to clarify.
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11-30-2013, 05:45 PM | #3 (permalink) |
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Magical Equation?
Red, congrats!
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11-30-2013, 06:54 PM | #4 (permalink) |
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RedOctobyr, you've answered my question exceptionally well. My purpose was to get info that I could clearly understand. Thank you!
My situation is this, I have (2) 120vac HP power supplies rated output showing 12vdc of 910w and 74a. I don't have a charger, but I used an I306B for my question. I have a TREX 500 that I plan on building over the winter and used that battery info for this question as well. I can now see how the numbers work. Thanks again, Scott |
11-30-2013, 07:06 PM | #5 (permalink) |
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Red is right for diminishing returns.
A huge portion of charge time is the final stage, to top off the pack as well as the get them all nicely balanced. The 306b is good, but balancing power isn't that fast. I've found that poorly balanced packs can take a long time to balance. For high capacity packs combined like you have, the topping off and balancing is usually referred to as CV or constant voltage phase, and is the slowest part. Expect 10-20 mins additional time spent here. As red says, even if you charged at light speed, to get to this phase, once this phase kicks off its an adding 10-20 mins, pretty fixed, hence diminishing returns
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11-30-2013, 07:42 PM | #6 (permalink) |
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Is the constant voltage phase (CV) known by any other name?
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11-30-2013, 07:53 PM | #7 (permalink) |
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Not really, so far as I know. LiPo charges have two phases, Constant Current, then they go to Constant Voltage (which is also typically when the balancing happens).
Since the #'s line up so well, I'm guessing this isn't news to you, but page 11 of the 306B manual shows you can get 500W output at 12V in. You'd be able to tap into the full 30A/750W output once you reach 22V input You don't need (2) 12V supplies that can each supply 74A (148A total). The charger has a 50A input limit. However, reading up on how to modify them to run the two in series, to get 24V, 74A, would unlock the charger's full power. It would also help keep things cooler, as the input current would be less. For any charger over ~300W, you really want 24V to get the full power (or even almost-full-power) of the charger. Some, like the PL6, don't hit their full output until 30V.
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11-30-2013, 08:06 PM | #8 (permalink) |
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I now understand how CC drops off ,and CV finishes the job. I also spent 4 days reading this entire thread.
http://www.rcgroups.com/forums/showt...hp?t=1005309is |
11-30-2013, 08:56 PM | #9 (permalink) |
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Great explanation RO. In addition, here some other resources.
Online power calculators http://www.soko-heli-tools.com/en/ch...alculator.html http://www.progressiverc.com/calcs http://www.revolectrix.com/Calculators_main.htm Excellent Reference info on LiPo charging. http://www.tjinguytech.com/charging-how-tos
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11-30-2013, 09:21 PM | #10 (permalink) |
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Greg, It was a graph you posted from a different thread from TJinTECH's site, that showed me how CV differed from CC and allowed me to understand how it works.
I understand a whole lot more now that I understand the "Magic Equation". I have been at each link you posted numerous times without success of trying to understand what I didn't understand. Thankfully RedOctobyr came in for the save. I can only hope it gets easier from here. Scott |
11-30-2013, 09:36 PM | #11 (permalink) |
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The online calculators are good quick references. But I still prefer to run the numbers by hand as RO described.
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12-01-2013, 04:33 AM | #12 (permalink) |
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I had the same question when I first got me trex 500pro dfc. I had an 80w charger and it took forever. I upgraded to a 308duo with s1200 powersupply. Now I can crank out the amps for 3c charging.
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