Damages Isaac in exchange for fully recharging Isaac's current activated item.
Sharp Plug costs two hearts of health to use.
Sharp Plug's heart cost is one half-heart per charge slot filled. The health cost is not increased in later stages where half-heart damage is upgraded to full heart.
If the item has no charges, the Sharp Plug will not activate.
Notes
If Isaac has less than two Black or Soul Hearts, it will remove them but will not remove any Red Hearts.
Sharp Plug attempts to draw from Red Hearts first.
Sharp Plug cannot deal fatal damage. If used at half a heart, it will not work.
Synergies
Damage-triggered items: Each half heart used up by activating Sharp Plug triggers damage effects separately, allowing multiple payouts.
Habit: Each damage taken triggers the extra charge, which can be seen when Isaac has The Battery, or when using Sharp Plug without having enough health to fully charge the active.
Glowing Hourglass: As long as Isaac has more than 2 full hearts, time can be rewinded indefinitely since the health used to recharge the item is refunded upon use.
Jacob's Ladder / 120 Volt: Electricity deals 75% of Isaac's tear damage if he owns 1 of these items, to 100% if Isaac owns both.
Keeper: The Keeper takes only one heart of damage.
Expends health for charges at the rate of 2 charges per 1 Coin Heart. If an item is 1-charge, removes 1 Coin Heart.
Schoolbag: The active item in the main slot is fully charged as normal and the secondary slot gains as many charges as the main one did.
: Only the front item is charged.
Tainted Magdalene: Tainted Magdalene's Yum Heart can be used infinitely, allowing for infinite invincibility frames from taking damage. Paired with Tainted Magdalene's melee attack, this can easily win a run.
The Wafer: Reduces the health cost to half a heart (except for Keeper).
Interactions
9 Volt: Renders the Sharp Plug unusable. Sharp Plug can only be used if the item is at 0 charges, but 9 Volt instantly gives 1 charge to the item upon activation.
As Sharp Plug can now be used with charges and the cost is based on charges filled, 9 Volt now synergizes with Sharp Plug, lowering the cost of using it by half a heart on 2+ charge items.
Alabaster Box: Expends up to 6 full hearts if possible with no charges given.
Bethany / Tainted Bethany: Soul/blood charges are used before Sharp Plug when available.
Eraser: Attempting to recharge the Eraser with the Sharp Plug fails and only hurts Isaac for half a heart.
Holy Mantle: Holy Mantle blocks the damage from Sharp Plug if the shield is active, but also keeps Isaac's active item from charging.
Bypasses the Holy Mantle and gives charges as usual.
The Lost: No benefit from this item unless he has a respawn item because his active item's charge will not refill if the Holy Mantle effect activates. Sharp Plug only activates upon receiving actual damage.
No effect, as The Lost doesn't have health.
Tainted Jacob or White Fire Place Lost form: Activating Sharp Plug will take the health cost from the original character's health. Holy Mantle will not block the health cost.
Lost form + Heartbreak: Every point of health taken to activate Sharp Plug is considered fatal damage, leaving Isaac with as little as half a soul heart. It is possible to get 12 Broken Hearts in addition to this final half heart, past the normal limit.
Mega Blast: Sharp Plug will not work with Mega Blast.
Works as intended, giving 1 charge per half a heart.
Sharp Plug costs two hearts of health to use.
Sharp Plug's heart cost is one half-heart per charge slot filled. The health cost is not increased in later stages where half-heart damage is upgraded to full heart.
Habit: Each damage taken triggers the extra charge, which can be seen when Isaac has 
The Battery, or when using Sharp Plug without having enough health to fully charge the active.
Glowing Hourglass: As long as Isaac has more than 2 full hearts, time can be rewinded indefinitely since the health used to recharge the item is refunded upon use.
Jacob's Ladder / 
120 Volt: Electricity deals 75% of Isaac's tear damage if he owns 1 of these items, to 100% if Isaac owns both.
Keeper: The Keeper takes only one heart of damage.
Schoolbag: The active item in the main slot is fully charged as normal and the secondary slot gains as many charges as the main one did.
Tainted Magdalene: Tainted Magdalene's 
Yum Heart can be used infinitely, allowing for infinite invincibility frames from taking damage. Paired with Tainted Magdalene's melee attack, this can easily win a run.
The Wafer: Reduces the health cost to half a heart (except for Keeper).
9 Volt: Renders the Sharp Plug unusable. Sharp Plug can only be used if the item is at 0 charges, but 9 Volt instantly gives 1 charge to the item upon activation.
Alabaster Box: Expends up to 6 full hearts if possible with no charges given.
Bethany / 
Tainted Bethany: Soul/blood charges are used before Sharp Plug when available.
Eraser: Attempting to recharge the Eraser with the Sharp Plug fails and only hurts Isaac for half a heart.
Holy Mantle: Holy Mantle blocks the damage from Sharp Plug if the shield is active, but also keeps Isaac's active item from charging.
Isaac's Heart: Renders the Sharp Plug unusable.
The Lost: No benefit from this item unless he has a respawn item because his active item's charge will not refill if the 
Tainted Jacob or 
White Fire Place Lost form: Activating Sharp Plug will take the health cost from the original character's health. 
Heartbreak: Every point of health taken to activate Sharp Plug is considered fatal damage, leaving Isaac with as little as half a soul heart. It is possible to get 12 Broken Hearts in addition to this final half heart, past the normal limit.
Mega Blast: Sharp Plug will not work with Mega Blast.
Active Item Modifiers 
The Binding of Isaac: Rebirth
