Power-Sonic PS-1251FP Battery, Sealed Lead-Acid, 12V, 5.4 Ah
This is the perfect battery for a "portable" system including a Pocket PC and GPS flight recorder.   I use one to power my Cambridge 302A which then powers my iPAQ.  I have another one that I use to power  GPS-NAV and Compaq Aero 1550.
Connectors:

• Positive terminal: "FASTON" quick disconnect tab (0.250" x 0.032"), mate with AMP. INC. FASTON "250", See the AMP-FASTON-250.
• Negative terminal: "FASTON" quick disconnect tabs (0.187" x 0.020"), mate with AMP. INC. FASTON "187", See the AMP-FASTON-187.

Dimensions: 
Length: 5.50 inches (139.7 mm) 
Width: 1.89 inches (48 mm) 
Height" 4.02 inches (102 mm) 
Height over Terminals: 4.06 inches (103 mm) 
Approximate Weight: 4.20 pounds (1.9 kg)

Charging
Cycle Applications: Limit initial current to 1.6A. Charge until battery voltage (under charge) reaches 14.40 to 14.70 volts at 68º F (20º C). Hold at 14.40 to 14.70 volts until current drops to approximately 54mA. Battery is fully charged under these conditions, and charger should either be disconnected or switched to "float" voltage.
"Float" or "Stand-by Service": Hold battery across constant voltage source of 13.50 to 13.80 volts continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.

NOTE: Due to self-discharge characteristics of this type of battery, it is imperative that they be charged after 6-9 months of storage, otherwise permanent loss of capacity might occur as a result of sulfation.

Data Sheet: ps-1251fp.pdf