Also getting the bat/lap pack + armor issue. Jetpack off I take a hit, armor goes down a smidge. Jetpack on I take a hit, it drops to roughly 30% durability no matter the armor, tried leather all the way up to diamond, second hit, armor breaks, on normal difficulty. Have the latest versions of Modloader, ModloaderMP, Forge, PlayerAPI, NEI/Core, Audiomod, Optifine. Those are ones directly effecting my jar. I've tried with all my other mods, and just IC2 alone, same results. I don't seem to get any error messages when it happens from MCPatcher, armor just takes a beating. Oddly from fall damage it will lose durability, then make the breaking sound, but go back to full durability, so go figure on that one.
Update: Energy-based armors (Nano, ect) do not seem to be effected by this