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9 Volt is a passive item.
- Fully charges the current activated item upon first pickup.
- If the current activated item has a recharge time of more than 1 bar, then it will grant 1 bar of charge whenever the item is activated.
- If the current activated item has a recharge time of 1 bar, then it will change the item to have a timed recharge of 15 seconds. The recharging of the item only triggers so long as there are enemies present in the room in any fashion. Empty rooms do not cause recharging to start.
- An enemy spawned in a cleared room will also cause recharging to start.
- If the current activated item has a real-world unit-based recharge time, the effect of the 9 Volt may vary.
- Sharp PlugUsing an activated item while it's not charged will charge it and Sharp Plug does 2 full heart damage to Isaac. - Renders the Sharp Plug unusable. Sharp Plug can only be used if the item is at 0 charge, but 9 Volt instantly gives +1 charge to the item upon activation.
- 2-bar items - Fills the entire gauge in a single room, effectively granting the opportunity for the item to be used every room.
- 3-bar items - Quadruple rooms and L-shaped rooms give 2 bars after clearing them, allowing one free use of the item in those rooms if the gauge is full upon entering them. It can also be used at the beginning of a Boss Challenge Room to get the full charge back after beating both bosses.
- 4-bar items - In a Challenge Room, if the item is used during the first wave, it will get all its charge back at the end.
- AAA BatteryAutomatically recharges the last energy slot of an activated item, effectively reducing recharge rates by one. - Even faster item charging (9 Volt gives 1 extra bar after any item use, while AAA Battery reduces the number of bars required for using the activated item).
- AAA Battery + 3-bar items - The item can be used every room as with 2-bar items.
- AAA Battery + 4-bar items - Same as the 3-bar item synergy without AAA Battery.
- The BatteryActivated Items can be "overcharged" to allow for an additional use. - Charges faster and stores an extra level of charge, although AAA Battery synergies do not work with the extra level of charge.
- The CandleThrows out a blue flame that can pass over gaps (but not through rocks). The flame damages anything in its path. - The Candle starts at half charge after use.
- Red CandleThrows a persistent flame that deals high damage and diminishes as it damages enemies or blocks shots. - Red Candle recharges quicker than normal.
- Brown NuggetSpawns a stationary fly familiar that fires tears at nearby enemies. - Brown Nugget starts at half charge after use.
- Enemies generated through a slot machine, shell game beggar, or breaking pots/skull will restart the charging process, even though a cleared room still counts as cleared if enemies are spawned in it.