Lithium Ion battery packs are not for the faint of heart. Sony and Apple lost
facilities to fires caused by Li-ion failures, and the company I used to work
for had several small fires, contained by elaborate engineering controls.

The protection circuit usually consists of a voltage comparator chip and one
or two FETs used to interrupt the battery current during overcharge and
underdischarge conditions. The voltage comparator senses each cell in the
battery stack separately. You have to disconnect the cells and
charge/discharge the cell that is out of balance before you can reconnect the
circuit. Permanently disconnecting the circuit is asking for trouble.

Barry Gershenfeld wrote:

> Li-ion has this really neat runaway failure mode which can make
> them catch fire or explode.  Thus they are very careful about
> their treatment and no one will sell packs without that circuit.
>
> The circuit will disconnect from the battery if it is overcharged. It
> will also disconnect if overly discharged; this also protects
> the battery.  Despite all this if you charge a discharged pack or
> put a load on the overcharged pack the battery should reconnect.
> I played with one of these myself on a lab power supply.
>
> Of course yours might really be broken in which case I don't
> know.  But check the circuitry for a fuse.
>
> Barry
>
> >Does anyone know why lithium ion battery packs fail so easily? I've had
> >two die on me in under a month, two separate types of devices too. I've
> >been told that it's not the cells, but the little circuit in the pack
> >that dies, which renders the pack useless. Anyone have any idea why this
> >happens, what causes it, and why they aren't a bit more robust? Also a
> >general question, should I be keeping the battery packs charged when not
> >in use (i.e. does allowing them to self discharge over a period of
> >months adversely effect their life)?
> >
> >Thanks for any help,
> >
> >Josh
> >joshy@mb.sympatico.ca
>
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