The problem is that frontal charges are useless, even when performed by celtic elites. I doubled the charge value of Arkoi and I barely noticed any difference, things improved only when I doubled melee hit rate in battle_config.
Not related to the infantry charge issue, but the same thing happen with regular javelins (those carried by most celtic melee units and thureophoroi for example) against line infantry, they become relevant only after raising the accuracy of ranged weapons in battle_config. Otherwise the enemy infantry can just tank them as they do very little, if any, damage, which seems odd to me.
Jurand' suggestion about raising charging time could be good too, but I don't know if it's possible.