On Feb 14, 2005, at 2:41 AM, Philip Pemberton wrote:
>
> I've left the safety protection circuitry in place - hopefully if my 
> PSU
> loses regulation, the protector should stop the cells at 4.2V.
>
I don't know if you can count on that.  "protection circuits" for
li-ion normally do UP TO three things:

1) under-voltage protection : prevent the cells from discharging
    below that 3V/cell magic number.

2) over-current protection : prevent the cells from discharging at
    a rate that would be dangerous.  Usually this includes fuse-type
    structures, and sometimes electronics as well.

3) charge protection.  what you're hoping for.  I think this is
    actually pretty uncommon, and left to the chargers, but that
    may depend on the complexity of the packs.

While NiCd and NiMH batteries can overheat and "explode", there
are a couple of things that make li-ion cells especially dangerous:

1) There's apparently a thermal runaway reaction that occurs at a
    moderately low temp (185F?)  Internal components of the battery
    start to react with one another in a thermite-like way, leading
    to that "vent with flame" condition.

2) Both the electrolyte and electrode materials in li-ion cells
    are flammable.  An Ni-xx cell can overheat and violently spew
    hot electrolyte and metal pieces everywhere, or short out and
    heat wires to incandescence, but when it's all over you have
    basically metal bits and aqueous electrolyte.  No "burning."
    The corresponding components of a Li-ion cell burn just fine.

(there's some other things to note.  For instance, if you're
rescuing li-ion cells from the dumpster and they're 'free', you
don't have to worry as much about getting the maximum capacity and
maximum number of charges from them.  You can charge to only 4V and
still have a very useful battery, and you can discharge below 3V and
not care too much that that might destroy your cell pretty fast!)

BillW
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