From: Joel M. Sciamma (joel_at_inventors-emporium.co.uk)
Date: Wed Oct 20 2004 - 16:29:06 PDT
I would like to suggest something that I think might be a safer solution to
powering the MP2K models as they get older.
I have now seen three machines which have been terminally damaged due to a
failed capacitor in the charging circuit/power supply while charging the
original NiMH battery pack with the Apple mains adaptors. When this
component fails, it tends to do so with some energy and leaves a bit of mess
on the PCB.
As the MP2K components get older, this kind of failure mode is to be
expected when high currents have to be switched in an environment of
elevated temperatures, both in the parts themselves and the battery pack at
end of charge.
As a consequence, I have stopped using the Apple NiMH battery packs (which
were getting to the end of their lives anyway) and instead I'm using NiMH AA
cells in the battery tray, charging them externally as alternating sets of
four cells. This way, one set is always ready to pop into the Newt when
necessary.
To ensure that the Newt's battery indicator gives the correct reading and
the Newt issues the appropriate low voltage warnings, I use Nick's
SoftCharge to select the NiMH cell type.
<http://www.unna.org/unna/utilites/system_enhancements/SoftCharge/readme.htm
>
When changing batteries, I either plug the sleeping Newt into its own power
adaptor (NEVER a third-party unit) or the external Newton Battery Booster
Pack which has some Alkaline cells in it. This makes the changeover seamless
without a restart.
<http://tools.unna.org/wikiwikinewt/index.php/BatteryBooster>
For the record, I am using Uniross 2,300mAH NiMH cells and their new 1 hour
'Sprint' charger which are widely available but any decent capacity NiMH
cell / charger combo would do.
<http://www.uniross.com/UK/info/pages/chargeur_sprint.htm>
Alternatively you could just use Alkalines (which gets expensive and is a
bit wasteful), rechargeable Alkalines like the Rayovac, or NiCd batteries
but I find the self discharge and memory effects of the latter a pain.
In summary, I feel the benefits are as follows:
Eliminates stress on the MessagePad power circuits and so possible failures
due to high currents and elevated temperatures.
Extended Newton uptime with high capacity NiMH cells.
Eliminates the usual 1 hour charging time so the Newt is ready to go again
in seconds if you have a charged set at hand.
Allows higher capacity cells to be easily adopted as they become available.
No modifications to the Newt or its accessories.
The only downside is that you need a battery tray and these can be expensive
but if you plan to keep your MP2K for years to come then it might be worth
it and it saves you having to recell the original battery packs.
Joel.
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