On lithium-ion battery cells
andre.kesteloot at verizon.net
Sat Apr 2 19:47:42 CST 2005
From the LWCA reflctor, a mesg from Clint KA7OEI/CT
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
- 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
- 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
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:
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.
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