The issue with computer batteries in flight has to do with the large amount of energy stored in modern lithium-ion laptop batteries (about 1/6th that of a hand grenade), and the nasty fire (and poisonous fumes) you would get if one were punctured or shorted intentionally.
The little coin-sized alkaline watch battery that keeps your system clock and BIOS running while the computer is unplugged is far too weak to be weaponized in any way. Not that a clueless security goon is obligated to know the difference, but it's not something anyone should be afraid of. Tell the goons that it's just a "watch battery", and they might relent. You might even show them that the computer can't be turned on unless it's plugged-in. Because it doesn't have the kind of battery that they're afraid of. Heck, put it through the x-ray machine.