Yeah, in the Ten no Koe and Populous you'll find a battery in there. In SFII' and the Arcade Card Pro you'll find nothing.
No battery in Populous. Apparently.
Really? It says, "ROMRAM HUCARD" on the spine like the Bank does, and Mednafen specifically states, "Support for Populous's backup RAM" as a feature.
I don't own this game, so I can't look into it. Maybe it uses some sort of micro capacitor or...flash? It does save without a booster/duo/etc, right?