Re: [NTLK] Using AAA batteries in a MP2100

From: Frank Gruendel (
Date: Fri Mar 25 2005 - 08:15:14 PST

> i have four adapter which translates four AAA
> 850Ma NiMh batteries, into the AA batteries
> dimensions.
> So i put them in a MP2100 batery tray.
> Then soldered a 10Ohm resistor directly to the
> motherboard to be able to recharge them. Being
> succesful on recharging, i left the pda without
> AC adapter and turned-on for testing the
> functionality of the AAA batteries. It's=20
> still turned-on. 36hours passed off just
> turning on backlight from when to when but with
> no particular activities and everything seems fine.
> I found my MP a lighter now, and fully operational.
> The question is: will the AAA batteries burn my pda?

Oh my God...

There's nothing inherently wrong with using AAA size
cells instead of AA size cells. If done conscientiously,
the Newton will charge them just fine.

Your setup, though, is the very opposite of
"conscientiously", and it is not without danger.

As you can see when having a look at my battery rebuild
page here

there are two components in a 2x00 battery pack. One is
a so-called polyswitch whose job it is to prevent the pack
from going up in flames if you accidently short it out or
if a defect in the Newton's charge circuitry does this for
you. The pack will work fine if you replace this component
with a piece of wire, but you don't want to be near your
Newton if ever the battery IS shorted out.

The second component is a resistor that changes its value
with the temperature. Absence of this resistor tells the
Newton that there's no rechargeable battery pack in it, and
it won't charge. If the resistor is present, which you made
the Newton think by soldering 100 Ohm directly on the
mainboard, it will charge the batteries.

The Newton check's this resistor's value during the charge
cycle. If it decreases, it means that the batteries get too
hot, which they will when the end of the charge cycle is
reached. You deprive your Newton of this information.

In your case, a value of 100 Ohm would (as you have noticed)
make the Newton think that the batteries' temperature is
almost 70 degrees celsius. It normally stops a discharge
cycle after the cells have a temperature of about 45 degrees
celsius after reaching a capacity of 100%.

If the charge circuitry in the Newton worked as it should,
it shouldn't even start charging your cells as long as what
it thinks the cell temperature is doesn't go down to a more
healty temperature. If you say the Newton charged the pack
nonetheless, I consider this proof that the Newton's charge
circuitry and -software doesn't know what's really going
on and what it should do.

If you want to make your setup halfway safe, remove the
resistor from the mainboard, get an old battery pack, open
it as shown on the page mentioned above and replace the
AA cells with your AAA cells, making sure the little white
resistor is placed close to their surface. If you want to
make it really safe, make sure you carefully remove the
polyswitch from the old pack and solder it to the AAA
cells. If you want to keep using your battery holder, you
will have to fit it with the two contacts an original pack


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