This quality issue comes from the poor quality batteries that have flooded the market. There is a reason why Tesla uses the Panasonic NCR18650 batteries because they seem to have the best manufacturing reliability. I've seen USB batteries from China that have no markings and covered with masking tape. That's why I use the same batteries Tesla uses. Using any other brand seems to have very little documentation, false claims, or just bad quality. I've been using the NCR18650A and B batteries for years and they have not died or caught fire despite camping and biking with them exposed to heat, rain, vibration, and even undercharged.
If the wrong charge controller is matched with a battery that claims to be high performance and isn't, then a fire will occur.
Isaac
-------- Original Message --------
Subject: Re: [dorkbotpdx-blabber] Lithium-Ion USB Charger Workshop Idea
From: Doug Ausmus <[email protected]>
Date: Mon, July 14, 2014 11:09 am
To: "A discussion list for dorkbot-pdx (portland, or)"
<[email protected]>
FWIW,Just thought I would pass-on this very recent development, especially since hobby electronics design issues have been mentioned by several engineers in this arena as being an additional safety concern... not sure what direction this will eventually go.Just a note to add a bit of quite recent detail to Lithium charging..This symposium was attended by one of my engineers (who also presented), and he came back with lots of interesting and depressing issues WRT Lithium batteries... depressing since we are in the middle of a project involving this stuff and have had to tell the principles the bad news about the direction safety certifications are going with Lithiums. We are still trying to work out a safe path for getting our project done and out the door (Certified packs and chargers? Deep due diligence in design,? Buy more insurance?). Not quite sure what the legal ramifications are since we are now aware of these issues, but the issues have no engineering direction for solution yet, and meanwhile Lithium continues to march along without slowdown nor action...
A symposium for reliability engineers and product safety engineers earlier this year discussed some recent findings that trickle-charging lithium chemistries can cause cobalt (and other things depending on exact design) to migrate and form nodules on the surface and and then potentially mechanically puncturing their dielectric and after which some nice exothermic reactions can/may/might/will occur.
This failure mode has been found (and others), but they are still trying to figure out why some will exhibit this behavior easily and some (from the same batch) may do not do so as readily. Many other charge/discharge things which make fire were discussed for which there are (AFAIK) no "controller solutions" for... yet. Lots of variables in manufacturing, materials, designs and chemistry and a number of metallurgical variations.
DougOn Mon, Jul 14, 2014 at 10:10 AM, <[email protected]> wrote:
Yeah, I use a MAX8903G myself. It has all those features and also has "smart selector technology", which gives the charger intellegence on when to store and release energy. I would like to cover that in the workshop too.Isaac-------- Original Message --------
Subject: Re: [dorkbotpdx-blabber] Lithium-Ion USB Charger Workshop IdeaFrom: [email protected]
Date: Mon, July 14, 2014 9:34 am
To: [email protected]
I would strongly recommend you make sure you understand the relationship between voltage-mode and constant current mode when working with Li+ chargers. A common booboo happens when the current flow into a Li+ cell decreases as the charging cycle is nearing completion, and is not detected within some reasonable amount of time, which causes over-charging. That would be an (ahem) exothermic reaction phase. Think of the news stories about early Apple lapbooks catching fire, which was largely caused by a slow current sense switch in the charger. When you feel you switch has reached the inflection point of current load vs cell potential, then a current limiter needs to be switched-in to keep the cell in low-current trickle charge. If you want to roll your own controller for that would be fine, but it is important to realize that the charging curve would have to be recharacterized each time you change from one Li+ cell manufacturer to another. Or try to have some kind of self-servoing current sense amplifier somewhere on the load. Another thing that is fun to do (I mean that) is figuring out how to switch MOSFET's so that a load can remain connected to the cell even when the charging source (USB) is disconnected (or reconnected to charge up the cell). Make sure, in that case, that you have an output L-C filter on the load-side to squelch the switching transients from the charger. I had to design in all these things in my Blivit charger system, but did not have the real-estate to do it all in discrete elements. I opted for the simple way to do it with a Maxim part as the control element for the charger (MAX1874)._______________________________________________
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