On lithium-ion battery cells

[Andre Kesteloot]

  From the LWCA reflctor, a mesg from Clint KA7OEI/CT

André N4ICK

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I don't claim to be an expert, but I have used them (rechargeable 
Lithium-Ion, anyway) in several projects and in so-doing, I've done a 
bit of research.

In a nutshell, here are some DOs and DONTs:

- DO always include some sort of overcurrent protection - a fuse, for 
example.
- DO NOT EVER allow them to run down below 1 volt.  At this voltage, 
some sort of irreversible chemical change occurs that makes recharging 
them potentially hazardous (e.g. they blow up!)
- DO NOT EVER overcharge them.  Most Li-Ion cells have a maximum charge 
voltage of 4.2 volts (specified with a +-0.05 volt tolerance.)  If you 
go above this, they blow up!
- DO NOT charge them at too high current.  Modern Li-Ion cells can be 
charged at 1C to 3C (depending on the cell...)  If, however, you have 
run them down too low (below about 2.5 volts - this "threshold" varies 
wildly depending on the particular cell chemistry - you'd need to check 
with the manufacturer) you need to "trickle charge" them until they get 
above that magic voltage - and then you can "quick charge" them.
- DO NOT float charge them.  All of the manufacturers specify that the 
charging should stop after a certain amount of time.  Often this 
specification is a bit vague, but they just want to keep you from 
floating them for days at a time.
- DO NOT charge or discharge them at too high a temperature.  This is 
true of any battery/cell, actually.

Pretty much all Li-Ion packs that I have seen come with a circuit board 
that does the following:

- Has a fuse to prevent against a short circuit.  Often, this is one of 
those "chemical fuses" (thermal type) that self-reset once the fault has 
cleared.
- Has a "soft fuse" to prevent against overcurrent.  This "fuse" 
(usually a MOSFET switch) will open if the current is too high and is 
reset either on the load being completely removed, or upon (even brief) 
application of a charge current.
- Has a "low voltage cutout" circuit.  The MOSFET switch will open if 
the voltage across the cell (or any cell - if this is a series battery 
pack) drops below the critical voltage (usually between 2 and 3 volts 
somewhere.)  This switch is reset upon application of charge current 
once the "reset threshold" voltage has been crossed.
- A "charge prevention" circuit if the cell (or any cell) has too low a 
voltage.  This is to prevent charging a cell that has had the 
irreversible chemistry change.
- An overcharge prevention circuit.  This same "charge prevention" 
circuit for too low a voltage can also kick in at too high a cell voltage.

While all batteries/packs that I have seen have the first three of these 
things (fuse, overcurrent, undervoltage) some do NOT have the latter two.

For charging my Li-Ion packs, I have a precisely set constant-voltage 
current-limited supply and simply charge the pack for 12 hours:  Nothing 
special.

Just for yuks, I decided to see just what would happen were I to 
purposely overcharge a Li-Ion cell.  To do this, I'd connected a 5.0 
volt regulated supply directly to a fully-charged cell (one that wasn't 
working too well, anyway) and stood back.  (This was done in a protected 
area...)  After an hour or two, it went "bang" as the pressure vent 
abruptly gave way and the cell spewed a caustic goo.

I've read about "counterfeit" Li-Ion batteries for cell phones that are 
reported to explode:  Apparently, these batteries don't have the 
protection circuits and the explosions are a result of cell rupture in a 
confined space, usually due to overcharging.  (Exactly what is happening 
seems to be made purposely vague in the articles, but that's what I 
gather is happening...)

I've used Li-Ion cells with my FT-817 for quite a while as shown here:  
http://www.ussc.com/~turner/ft817_roar.html

and I have retrofitted one of my old FT-530 battery packs such that I 
now have a 3+ amp/hour Li-Ion pack for it, too.

Clint
KA7OEI/CT