Take two...
Electrically Charged (Ex): Walkers are powered by electricity. A walker normally carries enough charges to function for up to one month (30 charges). After this time, the walker becomes inert until recharged. Walkers generally return to their cysts before this time elapses.
Each time a walker uses its electrical discharge ability, it expends one charge.
Within a cyst, a walker may fully recharge after one hour of inactivity. Alternatively, a walker can recharge "in the field" if exposed to an attack that deals electricity damage. An attack that deals electricity damage restores 1 charge for every 5 points of damage the attack would otherwise deal. For example, a walker hit by a lightning bolt gains 3 charges if the attack would have dealt 15 points of damage. A walker gets no saving throw against attacks that deal electricity damage. Regardless of the amount of electricity damage directed at a walker, it cannot exceed its normal maximum of 30 charges.
And change the electrical charge ability like so?
Electrical Discharge (Su): Three times per day, a walker can release stored energy as a powerful electrical shock. Its next successful slam attack deals an additional 2d4 points of electricity damage. Alternatively, if an attacker strikes the walker with a metallic weapon while the walker contains an excess charge in its carapace, the foe takes 2d4 points of electricity damage. This attack expends one of the walker's electrical charges, regardless of the damage dealt.