[Cialug] bios fubar?

[Nathan Stien]

On Jan 11, 2008 2:48 PM, Josh More <morej at alliancetechnologies.net> wrote:
> I have seen laptops that wouldn't even turn on if they had a bad
> battery.  My understanding is that the following logic applies to
> certain older laptops (no clue about new ones).
>
> If the battery exists, route all power through it and on to the
> mainboard.
> If the battery does not exist, send the power directly to the mainboard.
>
> I think that the purpose is to keep the battery charged and possibly to
> condition the power.  The flaw is that if the battery is in bad enough
> shape to not hold charge, but in good enough shape to advertise it's
> presence to the analog board, power will route into it... but not out.
>
> I suspect that this is what you are experiencing.

I have some experience designing devices that charge their own
batteries.  I have no idea if laptops do it like we did it, but I have
designed phones and other portable gadgets that all basically did the
same thing.  The logic was more like:
The device uses power from the battery terminals.  The charger
provides a voltage just a little higher than the maximum battery
voltage, and applies the voltage across the battery terminals.  If the
charger is around, current will flow into the battery if it needs
some, and some of it will come straight into the device.  If there is
no battery, you still have the charger providing power at the
terminals as if you have one.

Most of the device does not have to know or care how it gets the
power.  It actually takes *more* work, in terms of hardware design, to
find out where the power is coming from.

If you had a battery that acted like an open circuit, it wouldn't
matter as long as you had the power plugged in.  But if your battery
had failed such that it looks like a *short* circuit, then you hope
the engineer put in some overcurrent and/or thermal protection.
Li-Ions do often fail this way.  This could explain some of those
non-starter laptops.

Lithium ion batteries can, under some deep discharge conditions, decay
in such a way that they build a short circuit of conductive material
between the terminals which bypasses the rest of the battery.  (I wish
I could recall the specifics, but I haven't had to think about this in
a couple years.)  The same reaction that produces the adverse
conductor bridge also produces a gas.  And heat.  Absent a working
safety mechanism, you can end up with a lot of heat and high pressure
gases in a little package.  Then you end up with a smoking crater in
your laptop and possibly also your lap.

There are regulations that require you to put safety monitors inside
li-ion battery packs, and sometimes these make things worse.  They use
a little power from the battery all the time in order to make sure
it's safe, and if you leave a discharged one laying around for a long
time, sometimes the safety mechanism itself will discharge the battery
down into the danger zone, causing the exact problem it was supposed
to prevent.  If the safety discharges the battery to dangerous levels,
but fails to detect that [all devices have nonzero failure rates],
things can get 'splody when you go to use it.  Yay.

- Nathan