[rpatton3]

Unvented Optima batteries are NOT recommended by Optima for 2002-2005 Thunderbirds. Here is a cut and paste from Thunderbirdnest.com:
OPTIMA BATTERY

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I thought an Optima battery would be an ideal replacement for my wife's car when the OE battery heads south.
See the response from Optima below.


Unfortunately we are not able to recommend a certain model for your vehicle. Your vehicle
needs a battery that can be vented to the outside. Although Optimas are completely sealed batteries, if your alternator malfunctions or the battery is overcharged, valves can open and gas can be released.

Please feel free to contact customer service at the number below if you have any other questions. Thank you for your interest and support of Optima Batteries.

Minh
Optima Batteries
Customer Service
1.800.292.4359
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I have no idea if all sealed batteries are contra-indicated for use in the trunk. Oddesy batteries might be different.