On Wed, 15 Nov 2017 15:21:44 -0800, mike wrote:

On 11/15/2017 10:00 AM, Jeff Liebermann wrote:
On Wed, 15 Nov 2017 09:52:00 -0500, Bob Engelhardt
wrote:

On 11/15/2017 12:09 AM, Jeff Liebermann wrote:
...
If you're having problems charging cells which exhibit low terminal
voltage (under about 2.5v), try this:
https://www.youtube.com/watch?v=tbEfhPbqTDE

I came away with this clever tip: use small magnets to hold test leads
to the ends of cells.

Yep. However, I suspect that may have been an accidental discovery.
When one salvages 18650 cells from a laptop battery pack, the
resulting cells do not have a projecting button top on the positive
electrode. Getting such a cell to make a proper connection in a
battery holder or flashlight is problematic. So, someone determined
that a tiny magnet will stick to the positive terminal and provide a
suitable button top contact (and test lead connection):
https://www.ebay.com/itm/New-15pcs-Battery-Magnet-Spacer-Convert-Flat-to-Button-Top-All-Lithium-ion-/282548150944

Or you can get the ones not specified as battery extenders
https://www.ebay.com/itm/N35Super-Strong-Round-Disc-10x1mm-10x2mm-Magnets-Rare-Earth-Neodymium-N35/361874792648
for 3-cents each in hundreds.

Nice price. However, 10mm wide is MUCH too large for a button top
replacement. The eBay offering did not mention the size, but
measuring the one's I'm using produced:
6mm diameter x 0.84 mm thick.

Measuring the normal size of the stock button top on a typical 18650
cell, I get:
6mm diameter at the base x 2.5 mm thick.

If you get some square magnets, you can wrap a piece of battery tab
material around them. Leave some sticking out so you can
solder a wire or put a clip on it. Works for charging all types
of batteries with magnet-attractive connection points. And the current
doesn't go through the magnet or depend on the surface plating.

I don't have much of a problem with current going through the magnet.
Perhaps if I were using the cells for a high discharge application,
but not for the typical small flashlight (about 1 amp) or E cigarette
application (about 2 amps max).

My ESR meter is in my office so I can't measure the resistance of the
battery right now. I'll try to remember to check tomorrow.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558