Electronics > Projects, Designs, and Technical Stuff

DIY cheap powerbank (technical suggestions R welcome !)

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qnh:

--- Quote from: ym58 on August 08, 2016, 01:53:58 am ---My main concern is to reduce significantly the charging time that stretches over 12-14Hrs at 0.6A (that is 0.3A per battery) !
I would prefer to have them charged in 3-4hrs (as most smartphones/laptops do) even if this means I will have to dispose off them after 50 cycles !
--- End quote ---

I agree with you because Lithium batteries are getting cheaper and cheaper, and even free (as those you salvaged !).
For me, it makes no sense to 'maintain cycle life' (quote from @Siwastaja) at the expense of charging time ... it simply does not worth it : 12-14hr versus 3-4hr ... this range is just too wide !
In Germany, we have recycling bins in nearly all shops, with treasures inside : Li-Ion, Li-Po etc ...  recycling and using those batteries after testing and selecting them is definitely a sustainable attitude ...

Siwastaja:
When charging at 0.25C, the charging time to almost full is four hours. At 0.5C, it's two hours. Talking about 12 hours is nonsense. We are talking about topping the final <1% in the CV phase, which is irrelevant. CV phase is never much more than one hour max in any practical case, and you don't need it at all if you don't want to do it.

At low C rates, the cells are already very close to full when the CV phase begins. For example, at 0.5C, a typical laptop cell charged starting from 0% will reach approximately 90% charge after 0.9*2hours = 108 minutes, and topping off the last 10% would take maybe another hour. However, there is absolutely no reason to go to 100%, if you are happy with, say, 95%, which is quickly reached.

So, when you seldom start from real 0% (but something like 10% instead), and don't need to get to 100% (95-97% being enough), C/2 charge is really very close to 2 hours.

Note that the CV phase gets shorter when the charging current is lower, so it compensates for the longer CC phase!

About maintaining cycle life: what I mean is that your salvaged cells abused at 1C charge may be dead after just 5 such cycles! It's not just worth doing, building the pack takes your time and physical resources. You are exceeding the design margins. Just go a bit lower. Even a tiny bit helps, there may be a huge difference between 1C and 0.9C. I recommend below 0.5C to have some margin. Only charge at the rate you really need. If it must be fast, then sure. But note that you can't fully fast charge anyway, the CV phase will take longer accordingly when you increase the CC current.

Note that "Battery University" is quite bullshittish. This graph, for example, shows li-ion cell being charged starting from 1V, which is utter bullcrap. As usual, they have no clue what they are teaching about.

Many sources, including cell datasheets, also list CV phases that are way longer than actually needed in real life, giving the idea of very slow charging. The time required blows up exponentially when you are trying to squeeze the last 0.5%, but I never understood why you would want to do it. It's logical for the manufacturers to specify the capacity this way, however.

Kilrah:

--- Quote from: Siwastaja on August 08, 2016, 07:53:19 pm ---When charging at 0.25C, the charging time to almost full is four hours. At 0.5C, it's two hours. Talking about 12 hours is nonsense. We are talking about topping the final <1% in the CV phase, which is irrelevant.

--- End quote ---

Not at all... we're talking about commercial powerbanks that house 4 or more 18650s i.e. 12Ah@3.3V or so, yet still charge them with only 600mA@5V out of a USB port... which is means charging with less than 1/10C and is what results in the whole thing taking those 12h or so.

ym58:

--- Quote from: Siwastaja on August 08, 2016, 07:53:19 pm ---When charging at 0.25C, the charging time to almost full is four hours. At 0.5C, it's two hours. Talking about 12 hours is nonsense. We are talking about topping the final <1% in the CV phase, which is irrelevant. CV phase is never much more than one hour max in any practical case, and you don't need it at all if you don't want to do it.
--- End quote ---

No one will argue on this ... you're right.
But as Kilrah just explained in the previous post, we are not talking about an "ideal" single 16850 charge process but about a "strong" pack of two (or more, but in my case it's two) 8.2Wh 3.7V LiPos wired in parallel !
1C, in such a configuration, would theoretically be 4.4A (for the two batt)... that means that charging them at 0.6A equals to charging them at 0.15C or so, hence the unacceptable charging time ... and hence my quest on this thread to get a better circuit than FM9833E with its low 0.6A max charging current !


--- Quote from: Siwastaja on August 08, 2016, 07:53:19 pm ---Note that "Battery University" is quite bullshittish. This graph, for example, shows li-ion cell being charged starting from 1V, which is utter bullcrap. As usual, they have no clue what they are teaching about.
--- End quote ---

"Battery University" is not that crappy in the sense that it sums up in a readable and structured form all pertaining knowledge of the battery world including the latest technologies.
Some credit should be given to them, at least on these grounds.
That being said, you surely have noticed that  there are numerous reader comments attached to nearly all the articles published in "Battery University" ... which means that some constructive criticism is posted there, allowing readers (like me) to put the published articles into perspective !
---
Do you know of any other knowledgeable site about the battery world that you could recommend us to read ?
---
BTW, I have corrected the  improper 1-volt extrapolation that you came across in their graph, that will make it more acceptable I guess ...  :P


 

Siwastaja:
OK, I understood you guys wrong, because of how the OP stressed the need to charge at 1C, at the same time talking about 12 hours. Indeed, charging at C/12 takes 12 hours by definition, and it's impractical because it's considerably longer than a night. To fix that, I recommend charging at C/2 if absolutely necessary, or preferably a bit less (C/3 or C/4).

The problem with Battery University is the amount of misinformation and urban myths originated from there. The way the information is presented clearly does not help; quite the opposite, it leads the reader to enjoy reading and to believe it's a credible source. However, it appears to me that the quality has gone up considerably. Just some 5-10 years ago, it was total bullcrap to such a high level that it was a huge cringe to try to read through if you knew how the things are in reality. Still, most readers took their word because of the nice style of presentation, and most internet forum urban myths regarding li-ion originate from there, even though most of that stuff has already been removed or rewritten.

Showing faked graphs with severe misinformation "just as examples to point something else out" is a huge problem, you should never ever do it, especially when trying to teach someone else! People will always refer to such graphs and interpret every piece of information they can get from those, because graphs are powerful format of conveying facts using data. This graph doesn't even state that the voltage information is nonsense, which would be the minimum requirement. Although, it would be easiest to simply use a graph with correct information - but that, of course, requires that you have the correct information, which is not true with "Battery University". I guess the author just simply thought that li-ion cells are discharged down to 1V. From the charging algorithm point of view, the starting voltage is an extremely important parameter - a cell starting at 1.0V doesn't go though that kind of charging graph at all.

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