Electronics > Projects, Designs, and Technical Stuff

DIY cheap powerbank (technical suggestions R welcome !)

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ym58:

--- Quote from: Kilrah on August 06, 2016, 06:35:24 pm ---Nothing requires you to charge a Li battery at 1C, it's the maximum you can do on normal cells. The cell count in the powerbank is also irrelevant, it's the energy capacity that matters.
--- End quote ---
You're bloody damn right, but on the other hand nothing requires us to be happy with long-lasting situations ... otherwise there would be no chance for any progress !
Personnaly, I always charge my batt packs at 1C, be it with my lab P/S or my iMax B6 !

Kilrah:

--- Quote from: ym58 on August 06, 2016, 07:08:07 pm ---You're bloody damn right, but on the other hand nothing requires us to be happy with long-lasting situations ...
--- End quote ---
Of course not, but given the current situation if you wanted to do better $5 Chinese boards is not really the right place to look. Of course there are solutions to do better, but then you need to forget the condition I quoted in my first post becasue you're going to be making what amounts to a "premium product".

Siwastaja:

--- Quote from: Ranger14 on August 06, 2016, 02:20:19 am ---Most power banks run 18650 cells in parallel which most people recommend not doing..
--- End quote ---

... Which is a prime example why you shouldn't base a design on a group vote by random people who have no freaking clue what they are talking about.

18650 cells are most definitely used in parallel, which is their intended way of use, and has always been.

Siwastaja:

--- Quote from: ym58 on August 06, 2016, 07:08:07 pm ---Personnaly, I always charge my batt packs at 1C, be it with my lab P/S or my iMax B6 !

--- End quote ---

Charging any random energy-optimized (as opposed to power-optimized) li-ion packs at 1C is very likely to cause severe lifetime issues on some, if not most cells.

Most energy cells are rated at 1C absolute maximum charge rate, compromising the lifetime already, even with non-aged cells which still work optimally.

On random, aged, salvaged cells I strongly suggest limiting yourself to about 0.5C maximum occasional charge rate, preferably less than 0.25C as your everyday charge rate, to maintain good cycle life.

RC toy cells are optimized for high power discharge, and as a consequence, can tolerate higher charge rates too, so 1C is quite a normal everyday charge rate there, occasional peaks being somewhere around 3C. But this is not the case with consumer electronic cells which are optimized for higher energy density and lower price, not high power. There will be serious lithium plating, causing the cycle life to plummet. It can happen suddenly, a certain cell can work for 500 cycles just fine at, say, 0.6C, and be destroyed in less than 50 cycles at 0.8C. You can't know without destructive testing, so keep that charging slow enough.

ym58:

--- Quote from: Siwastaja on August 07, 2016, 08:24:40 pm ---18650 cells are most definitely used in parallel, which is their intended way of use, and has always been.

--- End quote ---
I agree, and I suppose this remark should also work for LiPos.
The best example being laptop battery packs that contain Li-Ions or LiPos which are ALWAYS paired in parallel.


--- Quote from: Siwastaja on August 07, 2016, 08:40:22 pm ---On random, aged, salvaged cells I strongly suggest limiting yourself to about 0.5C maximum occasional charge rate, preferably less than 0.25C as your everyday charge rate, to maintain good cycle life.
RC toy cells are optimized for high power discharge, and as a consequence, can tolerate higher charge rates too, so 1C is quite a normal everyday charge rate there, occasional peaks being somewhere around 3C. But this is not the case with consumer electronic cells which are optimized for higher energy density and lower price, not high power. There will be serious lithium plating, causing the cycle life to plummet. It can happen suddenly, a certain cell can work for 500 cycles just fine at, say, 0.6C, and be destroyed in less than 50 cycles at 0.8C. You can't know without destructive testing, so keep that charging slow enough.
--- End quote ---
Thanks for the advice, that's good to know and take into consideration when it comes to pricey devices.
---
Concerning the prototype described in this thread don't forget that LiPos have been salvaged (from a Sony Vaio laptop battery pack) and that I don't really care about their durabilty ...
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 !

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