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Old 10th February 2012, 12:23 AM   #1
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Default Lithium battery charging mythology?

I have seen in a number of places dire warnings about the charging regime for lithium chemistry batteries.

These suggest that once a lithium battery has reached full charge the charging voltage must be removed, else the battery risks bursting into flames or exploding. It has been suggested that the cause of this is solid lithium plating out onto the electrodes.

Reading the National LM3420 lithium charge controller datasheet, I find only the following 'Once the charger is in the constant voltage mode, the charger maintains a regulated 8.4V [*for 2 cells] across the battery and the charging current is dependent on the state of charge of the battery. As the cells approach a fully charged condition, the charge current falls to a very low value.'

*My parenthesis.

...As the cells approach a fully charged condition, the charge current falls to a very low value.

This is what one would expect.

Nowhere in the datasheet can I find any suggestion that the charge controller removes the charging voltage once the battery is charged. In fact the LM3420 is merely a precise voltage reference to be employed in a standard constant voltage/constant current circuit as per the LM317 datasheet, it is intended to permit the construction of such a charger without the use of a trimpot for final adjustment of the output voltage.

Can anybody direct me to a manufacturers datasheet or other reliable source which suggests that using such a circuit presents a safety hazard, or can I safely assume that the dire warnings seen elsewhere are merely further evidence of the internet myth-generator at work?
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Old 10th February 2012, 01:33 AM   #2
agdr is offline agdr  United States
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Quote:
Originally Posted by counter culture View Post
I have seen in a number of places dire warnings about the charging regime for lithium chemistry batteries.
The dire warnings are quite true. I do a lot with high power LED flashlights that use lithium 18650 and 26650 cells. Here is what happens when a lithium ion cell is overcharged:

https://www.youtube.com/watch?v=SMy2_qNO2Y0

There is a photo over on candlepowerforums.com of a shorted lithium ion cell in series with others in a flashlight, causing polarity reversal in the shorted cell, which resulted in the aluminum flashlight tube exploding and taking out an entire sliding glass door. Luckily the owner heard a "pop" and dived behind a sofa in time.

This writeup is pretty accurate with current thinking on charging lithium ions

Charging Lithium-Ion Batteries ? Battery University

Note the second paragraph and table 2.

Some older charging chips for lithium cells did use to try to "maintain" the cell at 4.2 volts at end of charge. That didn't work out as the article explains. Current thinking in the flashlight community is best to cut off entirely at 4.2 volts, which is what the newer chips do. I see the datasheet on that national chip is dated 2000.

A good example is the venerable Ultrafire WF-139 charger that a lot of flashlight folks use for 18650 cells

Ultrafire wf-139 Lithium Ion battery charger 14500, 17500, 17670, 18500, 18650, 18670

The original version of that charger had a chip that "maintained" and went up to 4.3v at end-of-charge. It was famous for killing 18650 cells in quick order. The latest version out for the last couple of years has a chip that cuts off entirely at 4.20V. Works great. With lithium-ion that extra 0.1V really, really matters. My Sony laptop here (using 18650s in the battery pack as many laptops do) has a "battery preservation" setting that I use, which charges the cells to just 80%. From that article you can see in table 2 that is 4.1V, just 0.1V less.

I hope this helps!

Last edited by agdr; 10th February 2012 at 01:45 AM.
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Old 10th February 2012, 01:38 AM   #3
dtproff is offline dtproff  United States
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I agree... Flat charging at the top to maintain the cell at a constant voltage is not a good odea. Let them self discharge dow a ways and then top them off again. THis is easily done in a micro. I do the same sith SLA and NiMH batteries... Same algoritm of CC, CV for top off and then let them set until it's time to charge them up again.

Tony
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Old 10th February 2012, 01:40 AM   #4
GloBug is offline GloBug  Canada
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It's a relatively new technology.

Most of the chargers that I am aware of used in the R/C hobby stop charging when the battery is full.
Given charging is more aggressive to get faster charging times.

These batteries are more sensitive over and undercharging. Things get more complex if individuals start to drift over charging cycles, making balanced chargers ideal.

It appears your chip is not a balanced charger.

If you have a chip and follow the guidelines then I suppose you answered your own question.

For an application like this, I myself would prefer something that charges the cells individualy and shuts off when charged.
These batteries have excellent shelf life for a rechargeable, I can't see a real advantage to continue charging vs the risks of over charging.
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Old 10th February 2012, 03:16 AM   #5
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Hmm. 'Battery University'. I'd seen that site already. Here's a quote:

'Charging and discharging batteries is a chemical reaction, but Li-ion is claimed as an exception. Here, battery scientists talk about energies flowing in and out as part of ion movement between anode and cathode. This claim has merits, but if the scientists were totally right then the battery would live forever, and this is wishful thinking. The experts blame capacity fade on ions getting trapped. For simplicity, we consider aging a corrosion that affects all battery systems.'

This is not a standard of discourse calculated to inspire confidence in me. It's about the level of hogwash I expect from somebody selling hi-end cables. The site is just an advert for a book, and on the basis of what I gleaned, not one I would buy.

A reversed-charged cell in a flashlight blew up. What's that got to do with the price of cheese? The YouTube explosion was deliberately provoked by overcharging.

If I was of a nervous disposition I might be affected by these 2 pieces of 'evidence', but they're irrelevant, and worse, PREJUDICIAL.

I have many LED flashlights, Fenix, Ultrafire, Aurora with Cree, Seoul Semi and Luxeon LEDs. I stopped visiting candlepowerforums because I couldn't take those guys gloating over blowing more money on a pocket light than lots of 3rd. world dwellers have to live on for a year.

Anyway, enough of that...

Does anybody know of an integrated charge control chip that employs a more complex regime than the one I quoted from National (now TI)? Or a circuit in commercial production?

Yes, it's true that it's possible to use a uproc to switch charging on and off and it's possible to employ balanced charging, I fly model heli's so I'm familiar with such implementations, but it's all expense and board real estate. In the case of the heli's the cells are so abused, charge and discharge >> C, that balanced charging offers tangible benefits, but in a portable electronics device the current is << C. In many smaller heli's unbalanced chargers are employed. I have 2 sitting on the windowledge.

I have other features I am trying to cram into limited space in a product, and a budget to boot, so I'm going to go with something authoritative, like a datasheet, where somebody's commercial a$$ is on the line, not a bunch of received folklore from a gaggle of flashlight freaks with more money than good taste.

dtproff, you recommend the same for SLA and NiMH, but nobody else feels it necessary, so I think you are simply overcautious.

Has anybody got anything else?
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Old 10th February 2012, 03:59 AM   #6
dtproff is offline dtproff  United States
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Here is a good explanation of what to do on float charging. Float charging lithium ion cells - 2/1/2006 - Electronics Weekly

To my point... the final paragrapgh is telling:
"Float charging Li-ion cells is possible, but the voltage-temperature-lifetime-capacity trade-off is complex. And Broussely warns: “Certainly discuss this with your battery manufacturer. It is very application dependent.”"

The applications I did they were all wide temperature range (-20C to +50C). I used a thermistor to change the cell voltage for the top-off range. These were intended for maximum life, maximum holdup time applications.

I will follow up with a final comment... there are a lot of ways to do the same thing, some have advantage over other ways of doing things. I usually assume that the person chose a topology for a reason and rarely do I disagree with the approach unless there is a safety issue involved. So, when asked about my opinion that is what I give...

Good luck and please don't take offense to the comments above.

Tony
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Old 10th February 2012, 04:02 AM   #7
dtproff is offline dtproff  United States
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Sorry... One more thing... Read the Li charger data sheet from TI. It disconnects the battery also...

http://www.ti.com/lit/ds/slus606o/slus606o.pdf

There are others but I thought that one would do for a start.

Tony
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Old 10th February 2012, 04:47 AM   #8
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Designing a good charger for Lion batteries would need to understand batteries first. Or even if you want to go for some charging circuit on net you can select good design if you know Lion batteries.

Here are two good links.
Charging Lithium-Ion Batteries ? Battery University

How to Prolong Lithium-based Batteries - Battery University
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Old 10th February 2012, 10:14 AM   #9
agdr is offline agdr  United States
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Lol! I stepped into one there.
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Old 10th February 2012, 01:58 PM   #10
gmarsh is offline gmarsh  Canada
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Don't trickle charge Li-Ions. They don't recombine like Pb/NiCd/NiMh and can't "burn off" excess charge.

I'd go with a chip that has both a C/x cutoff and a charge timer. You want the latter for safety reasons - I've personally seen a failed Li-Ion cell keep accepting current and get hot despite the termination voltage not being reached, and you don't want your charger to keep dumping energy into a bad cell in that circumstance. Similarly if you're charging multiple cells in series and one of them fails, a timer will limit the overcharge to the other cells.

For smaller chargers, I've used the Linear LT1731 in a number of different designs and I'm happy with it. It has both aforementioned features, reasonably priced, and only needs a few R's/C's and an external pass transistor to run.
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