The folks at EnerSys/Odyssey Batteries had some additional info on the
PC680 batteries:

1.) If you plan a firewall mounting beware of the temperature limitation
for their plastic version PC680:
"Metal jackets are required for operating temperatures above 113F which
is the max operating temperature of the PC680 without it. It is a little
conservative but these batteries were not originally produced for the
powersports industry and the higher temperatures that are often
associated with those types of applications. The issue being that the
plastic of the bottom case can become soft and can distort and bulge at
higher temps and the metal jacket prevents that."  Odyssey does produce
the PC680 with a metal housing.  The size is larger by about 3 to 4mm
for both the length and width. Overall dimensions in the printed
literature are 184.7mm by 79.0mm with the metal jacket.

2.) Odyssey reccomends against using a Battery Tender brand and similar
units to recharge and to top their batteries. The cycling is less than
optimal and would result in a shorter battery life.  The sugegested I
contact Bob Besch at Interacter, Inc. who produces 12V, 24V, 36V, and
48V chargers that can be programmed with an ODYSSEY chip. For a 24v buss
they suggest a 7A, 24V charger.  Part number is their Professional
Series PS2408-Odyssey.  Bob's number is (203) 630-0199.  List price is
$180 but for Lancairians the pre-Oshkosh sale price is $145.  Limited
time only, etc. etc. etc.

Details from their Web site (http://www.interacter.com):

Our PS charger utililizes our patented "3 Stage Plus" technology to
interact with the battery and provide exactly the right amount of
charge. Interacter "3 Stage Plus" charging technology eliminates
undercharging, overcharging, or damage to the battery, and is completely
safe. The Interacter PS can be used to recharge virtually any type
lead-acid battery including conventional liquid electrolyte,
maintenance-free absorbed electrolyte, deep-cycle, and gelled type batteries.  Interacter's microprocessor circuitry with patented program in EPROM,
controls four separate charging stages:
FIRST: A constant current bulk charge is applied to achieve the fastest
possible return of energy to the battery.
SECOND: This constant voltage stage is proportionally timed and
temperature compensated, minimizing electrolyte breakdown and gas
emission.
THIRD: The final standby float charge stage will replace standing losses
and keep the battery in a fully charged state & "Ready".
PLUS: As soon as the battery is connected, a de-sulfation circuit
encourages the breakdown of large sulfate crystals which can develop
during prolonged periods of deep discharge.

Robert M. Simon
ES-P N301ES