I'm not in a position to test but here's the info from misc_equip.ini on the shield batteries:
[ShieldBattery
nickname=ge_s_battery_01
ids_name=263748
ids_info=264748
volume=0
mass=10
hit_pts=300
loot_appearance=ge_s_battery_01_loot
units_per_container=15
lootable=true
The hit_pts controls how many points a single battery will restore to your shields. You might try adding a power_usage line with a number around 1000. After that, try using a shield battery and see if the power drops tremendously. If so, then we at least know that we can link power_usage to a shield battery. Next step would be to try using a negative number on power_usage to see if it will give you power.
The only problem you'll run into at the get go is that freelancer is hardcoded to use up as many batteries as it takes to replinesh the shields to full power.
-V
The Nets Edge
Freelancer .INI File Parser
Freelancer IDSName Matcher