Best way to prevent over charging of 18650 cells?

Even when using protected 18605's can I use a voltmeter as they are charging to read the voltage and unplug when they reach the max to make sure they don't over charge? Will the voltmeter read right or do they need to be under load to see if they're fully charged?

The voltmeter will correctly read resting charge, which is supposed to be 4.2. So yes, it will work.

joshconsulting said:

The voltmeter will correctly read resting charge, which is supposed to be 4.2. So yes, it will work.

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Do I just put on the probes when they are in the charger charging or do i have to take them out to test?

It's much simpler to take them out. LiIon aren't harmed by stopping and starting charging, there's no memory effect.

Another one from you in the wrong forum - please note where the Batteries section is.

Hello Jimbo231,

The best way to prevent over charging of Li-Ion cells is to use a quality charger that follows the proper charging algorithm for charging this chemistry...

A quality charger will limit the charging voltage to 4.2 volts (plus or minus 0.05 volts) at any time during the charge. If you check the voltage of the cell while it is being charged, you should never see the voltage rise above 4.2 volts. If you do, get a different charger.

Tom

It's important to understand, that when charging a Li-Ion cell with a charger that uses a proper algorithm (from B.U.), the cell will be @ 4.20 Volts for a good portion of the charge. Removing the cell as soon as the charging voltage reaches 4.20 Volts, will result in a cell that is only 70-80% charged.


When using most "cheapo" chargers, the algorithm used, skips the second stage (CV) of the proper three stage process. Some of these chargers simply indicate that the charge is complete (green LED, or whatever) once the charging voltage reaches 4.20 Volts, resulting in a 70-80% charged cell.

Other, less than desirable charger algorithms, continue to let the charging voltage rise above 4.20 Volts, allowing a more complete charge, at the expense of possible cell damage, as well as promoting a potential safety risk.

In either case, most (but not all) of these inexpensive chargers, include a "trickle charge", which is also very undesirable in a Li-Ion charger. Trickle charging Li-Ion cells promotes cell damage and can be hazardous.

Dave

Addendum: I see SilverFox has already intecepted.

Aside from the specialized hobby type chargers, is the Pila charger considered the "best" Li-Ion charger available for general use?

Peter

Pretty much, LitFuse. In that price range the Pila is about it. I was fortunate to get a YOHO-122 (now apparently discontinued) years ago. It's algorithm isn't quite as good as the Pila's, and neither are perfect, but both are close enough to call good chargers and are quite acceptable.

Dave

I don't know what a Pila costs, but an Accucel (or equivalent) 6-cell, 5A hobby charger is about $35.

I just remembered i have a rc car battery charger that you can set the amount of cells , current, and charge rate. And it does lithium ion cells. I just need to figure out how to connect the battery. It just has alligator clips.

jimbo231 said:

I just remembered i have a rc car battery charger that you can set the amount of cells , current, and charge rate. And it does lithium ion cells. I just need to figure out how to connect the battery. It just has alligator clips.

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Some time ago I bought some small magnets that each had a wire spot welded to it, for just that purpose. Got them at the Marketplace, but unfortunately I don't remember the seller's name. They might be available again from time to time. Work great. You might be able to work a variation of that method by pinching copper tape between a magnet and the battery terminal.

c_c

Magnets are cool and simple to use. But i did notice they have quite high resistance, so it would be nice if you could bypass them with something like copper... I have yet to find a solution to my final magnet balancing/charging thingie.

I did strip down a cheap chinese li-ion charger i bought of ebay at first for a few bucks. Gutted the internal charging circuitry (now theres an overstatement) and the gazilion tiny wires that made no sense, and rewired it so i can simply connect the bay to my hobby charger. It can take 2x18650 or 2x 16340 (and variations of that It suffices for my humble collection of cells :] Now i only need to get some quality AA holders, and i should be good.

Anyways look around for some of the options people use around here. Basicly you have the magnet holders, the classic spring holders (C size cut in half, extended and glued to a surface for example), and clamp holders. And some custom made ones for sale even.

At one time you could induce a properly welded magnet lead out of CPF member LuxLuthor with the right incentive.

A trick to working with magnets would be to place a piece of ferrous metal between the magnet and the battery. Then the alligator clip can grab the metal and not the magnet. Or the magnet can be "inside" of the clip's jaws; there are lots of ways.

Why not use one of these http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=393250&k=18650

Employing a hobby charger to CC/CV charge a single 18650 I've taken a graph of the progress. You can clearly see the Constant Current part in the beginning and, when 4.2V is reached, the subsequent Constant Voltage part.

Voltage is shown blue and current is shown red.
The small tables show the various values reached at different times.



Charging is finished when the cutoff current of (in this case) ~ 60mA is reached. So any overcharging is prevented.

Wulf

Nice job there, Meterman. :thumbsup:

You need to re-size your image though, as it is bigger than allowed.

Dave

Now what charger performs as you show graphically?

aehaas

Sorry for the former size of the picture :sigh: , it simply was of much better quality.


@AEHaas

The charger used is an Orbit Pocketlader.

Manufacturer

Manufacturers' representative in USA

Wulf

rmteo said:

Why not use one of these http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=393250&k=18650

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Do those little tabs on the back connect to the positive and negative so I can use alligator clips to charge?