The Rune Stone Battery option that you can acquire for your rune cannon allows you to have a battery that the rune cannon uses for its attacks so that the rune cannon's Soul cost does not affect you. But, once the Soul runs out, you are left with an empty ylem stone attached to your rune cannon. This ability allows you to overcharge the stone, using the same rules that apply to making rune stones. If the ylem stone can normally take 20 Soul, this allows you to put in 40.
With this ability you can double the capacity of your rune stone battery. The stone now is holding double the amount of Soul until it is empty again. During this time the stone cannot be recharged - it must first be fully used up.
Each overcharging causes visible cracks in the stone that leak runic energy. After the third overcharging, the stone may explode. The explosion isn't instant and only happens when receiving stress, such as being hit, thrown, or used by a rune cannon. Each time a stress causing event happens - including each time the stone is used by a rune cannon - a Hard Luck check is rolled. Failing means the stone explodes. If anyone is near the stone (one space) they receive the stored Soul amount of magic damage.
The stone cannot be used again after depleting its third overcharge, even if the stone does not explode. Some have found that making the stone explode serves as a novel magical weapon, when used correctly.
Runic Amplification