More Battery questions...

rca@rle-vlsi.mit.edu
Tue, 14 Apr 92 13:55:49 edt

>Reply-To: ultralite-list@grot.csd.mot.com
>From: brian@grot.ca41.csd.mot.com (Brian Smithson)
>Date: Mon, 13 Apr 92 22:31:20 PDT
>X-System: grot R3V7 920115 M68030
>X-Mailer: ELM [version 2.3 PL11]

>>
>> From: Wayne Fiori <fiori@chemistry.ucsc.edu>
>> Here's an even more perplexing battery problem. I thought that my molicel was
>> dead, so I pulled it out and hooked it up to a voltmeter to see for sure. To
>> my surprise there was a constant 10 V across the leads. The low battery light
>> won't go on when the battery is connected, but the computer won't boot on the
>> battery alone. The CHG light on the adaptor won't go out even after days of
>> being plugged in. Admittedly I didn't put a load on the battery (i.e. a
>> resistor) when I checked it, but the leads should provide enough of a load.
>> The charger is putting out the correct voltages and current. Any ideas???

>I think that the open-circuit voltage on those batteries should be at least
>12V. Also, a good voltmeter won't put any significant load on a battery and
>so you should really stick a resistor on there (50-100 ohms should be good)
>to give the battery something to do while you measure it. I'm not sure
>why the LOW BAT indicator doesn't come on, unless it is triggered by a
>threshold crossing instead of a level.
>--
>-Brian Smithson
> Motorola Inc., Computer Group, Commercial Systems Division
> 10700 N. De Anza Boulevard, Cupertino, CA 95014 USA, (408)366-4104
> brian@csd.mot.com, {apple | pyramid}!motcsd!brian

One of the cells in the battery is probably shorted (maybe two). I believe
there are seven 1.8V(nominal) cells in the Molicel module (7x1.8 = 12.6
= voltage stamped on label). The charger only cuts off when the battery
voltage reaches 15V. I just measured a fully charged battery with no
load and got 14V (the voltage quickly drops under 13V under load.)
If the battery read 10V after a few days of charging, you have probably got
two shorted or otherwise dead cells.

REPLACE THE BATTERY NOW!

You are undoubtedly overcharging the remaining good cells and this is the
situation where a cell may explode! This may be more or less risky depending
on the design of the cells (conventional nicads are designed to withstand
continuous overcharging at the recommended charging rate.) Best not to
take a chance with the molicel. If the new battery has the same problem,
then the problem may be with the charger rather than the battery.
In that case, get the charger fixed immediately.

I suggest that anyone who notices that the battery is taking much too long to
charge should measure the battery voltage to see if there seems to be a
shorted cell. 16 hours is a typical full charge from complete discharge
-- I have never had to charge longer than about 10 hours in normal use.
If the charge light doesn't go out after 24 or 36 hours you should get
suspicious. If the voltage after this time is still below the nominal 12.6,
it's time for a new battery. (If the new battery has the same problem, then
it may be time for a new charger.)

----------------------------------------------------------------
Bob Armstrong
Research Laboratory of Electronics
MIT, Cambridge, MA 02139
Email: rca@rle-vlsi.mit.edu