185. Master battery pack + internal modem
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The early type of Lithium 'incendiary device' was replaced by a rather bulky
pack of 3 'AA' size 1.5v Duracell batteries, which occupied the space
intended for the internal modem. Despite the warning on the pack, I see no
reason why the owner should not replace the individual cells when necessary.
The positive lead has an 'idiot diode' in series, to prevent damage to the
computer if the batteries are inserted wrongly, and to prevent the computer
from attempting to charge 'flat' batteries. There is also a 100 Ohm resistor
in series with the negative lead, which prevents damage to the batteries if
the output is accidentally short-circuited. When plugging the pack back into
the main PCB, note that the black lead goes to the North pin, (nearest the
rear), the red lead goes to the central pin, and the South pin is
unconnected. Hold down the "R" key whilst turning the power back on, so as
to reset the RAM ready for you to enter the *CONFIGURE options. (The latter
are best held on disc as a *SPOOLed file, ready to *EXEC back in when
required.) Finally, you should reset the internal real-time clock/calendar.

The current type of battery pack still uses 3 Duracells, but they are in a
shrink-wrap plastic covering, which makes it much more compact. It fits
neatly down the small slot immediately to the left of the keyboard PCB, and
immediately in front of the PSU. This frees the internal modem space for
whatever you want to put there, including a modem of course. Replacing the
individual cells oneself is not as easy as with the older pack, but it is by
no means impossible. The pack costs 3-68 from Beebug, (3-50 to Beebug
members), plus 1 pp, and the part number is "0807B - Master Battery Pack".
Beebug also do an internal modem, which frees the RS423 port for other uses,
including software on ROM, for 119, (113-05 to Beebug members), plus 3-75
pp. The modem is multi-standard, with auto-dial, auto-answer, and various
other bells and whistles. The part number is "0759E - Master Internal Modem".