EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: BloodySword on March 06, 2014, 07:00:44 pm
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Hello People,
things in my live changed a bit and there is no need for that super bright flash light I wanted to make. But I have antother task for my four 18650 cells I bought very expencive...
I know there are plenty of cheap 18650 USB Emergency Chargers out there, BUT:
I have four unprotected 18650 cells by Panasonic. Quite expencive and decent cells with high capacity.
My Problem is: I want a USB emergency Charger built on my own, because I want to monitor each cell alone.
What is smarter:
1. Use one cell (all in parallel) and a boost buck converter for 2.4 - 4.2 Volts to constant 5.1 Volts, 2 Amps? (I want to be able to charge Phablets or Tablets with it, therefore 2 Amps. The cells can be discharged safely 1 times C, ~3 Amps).
2. Or use two in series, giving us 7.4 Volts that are buck converted to 5.1 Volts, 2 Amps?
And there is another problem: I want to charge these cells individually, so every cell keeps balanced and monitored seperately. This is a MUST because it are unprotected cells. To avoid deep discharge or damage, it is really important to monitor these cells individually and charge them individually.
It also would be nice if there is a possibility to show a percentage of charge of this whole assembly with a 2,5 digit LED-Display. (188 segments) while I hold down a button.
Does anyone know a ready to assemble solution for this?
Thank you!
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If they where all in parallel like your first though then they are balanced by design (one may take more current than another due to a capacity mismatch, but the voltage is the same) i would imagine this would also make the fuel gauge measuring simpler, but would make the efficiency of the switcher slightly lower,
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Hi, thanks for the very quick answer.
I found the test of these cells again: http://lygte-info.dk/review/batteries2012/Panasonic%20NCR18650B%203400mAh%20%28Green%29%20UK.html (http://lygte-info.dk/review/batteries2012/Panasonic%20NCR18650B%203400mAh%20%28Green%29%20UK.html)
You can find some more specs of them there, too. Hard to belive, bit this are the BEST cells on the market you can get. With 3000 mA discharge current, you still get 3100 mAh out of them.
(http://i57.tinypic.com/nlsxf9.jpg)
But how do I charge them? All in parallel?
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That will work, and they will all reach balance as well at cut off, providing they are all at around the same charge level when connected. That will be the simplest and safest method other than charging individually.
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Charging in parallel is ok (if your charger can supply the current to get it done in a reasonable time :)), but make sure they are each individually fully charged before you initially connect them up, don't have one flat one and slap it in parallel with 3 charged ones, might not be pretty.
As long as they are fully charged (and therefore close to the same voltage), they will equalise themselves once paralleled.
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"close" in this case should be ideally within a few mV; the internal resistance according to that link posted above is only 110mohm, and that's the only thing that will limit the current if two cells not at the same voltage are connected together. And from the protection test, these cells can easily supply 15A :o
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before joining them for there final mounting, hook some 100 ohm resistors across the positive terminals with the negatives tied together, this will result in a more graceful cell balancing before you shunt them with a few milli ohm,
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Wouldn't it be easier before putting them together: Charge them individually and wrack them into the parallel wired sockets right after they are full?
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Another question: Any suggestions for a compact 10 Amp DC connector? Thanks.