Car Battery 
Effect
Synergies
- (Added in Afterbirth †)
Broken Shovel: Prevents the Mom's Foot from appearing for two rooms (empty or not), or for two waves of a Challenge Room or the Boss Rush, essentially removing the stomping attacks for the entire duration of the Boss Rush
- (Added in Afterbirth † / Removed in Repentance)
Crooked Penny: Splits the 50% chance to succeed into a 25% chance to quadruple all items and a 25% chance to duplicate all items while consuming the originals; the latter will make Devil deal and Shop items free.
- (Added in Repentance) The 25% chance to duplicate all items while consuming the originals now generates 2 pennies instead.
- (Added in Repentance)
Sulfur: Will count as giving double Brimstone, giving extra damage and a x1.2 damage multiplier for the current room.
- (Added in Afterbirth †)
Void: Doubles effects of all absorbed activated items. Absorbing passives will not give extra stat ups.
Wooden Nickel: 25% chance to spawn nothing, 50% chance to spawn one coin, and 25% chance to spawn two coins.
Interactions
- (Added in Repentance)
Berserk!: Will only increase rage duration if no enemies are killed in the first half, effectively only affecting bosses that can't spawn enemies.
- (Added in Repentance)
Book of Virtues: Doubles the spawned wisps on activation.
Book of Secrets: Activates twice, (Removed in Repentance) but has a chance to uselessly activate the same effect twice.
Book of Shadows/
The Gamekid/
My Little Unicorn/
Unicorn Stump: Doubles the length of invincibility, but no effect on damage where applicable.- (Added in Afterbirth †)
Clicker: If the Car Battery is removed by the first activation, a second activation will not happen. Two activations in a row is mostly detrimental, removing two items and possibly cycling through a character that lowers health.
- (Added in Repentance)
Spindown Dice: Lowers ID by 2, good for some items, bad for others.
- (Added in Repentance)
Dark Arts: Doubles the duration, giving more time to walk through more enemies, if you end the effect before the end of the first second, the item will reactivate itself; allowing to target the same enemies twice.
D4/
D100: Instantly re-rolls Isaac's items twice, which doubles the speed at which the item pools are exhausted and transformations are achieved.
- (Added in Repentance) As items are returned back into the item pool when re-rolled, the item pools aren't exhausted at all. In addition, you need to actively be holding a transformation's respective items to retain them now, so using the double re-roll to gain multiple transformations is no longer possible.
The D6/
D8/
D12/
D20: Re-rolls twice instantly without any benefits. (Removed in Repentance) The D6 depletes item pools at double the speed.
D7: Doubles the amount of enemies summoned, but does not give double rewards.
Dead Sea Scrolls: Activates 2 random effects.- (Added in Afterbirth † / Removed in Repentance)
D Infinity: Activates 2 dice. The second activation die cannot be predicted.
- (Added in Repentance) Activates 2 dice. The second activation die is determined by the seed and never changes.
- (Added in Afterbirth †)
Dull Razor: The invulnerability time is not increased, but damage-triggered items will activate twice if applicable.
- (Added in Repentance)
Eternal D6: Re-rolls twice like The D6 (see above), but the 30% chance of the item disappearing is increased to 51%.
- (Added in Repentance)
Everything Jar: Instead of doubling its effect, Everything Jar will spawn a poop alongside its normal drop.
Head of Krampus: Doubles the damage the brimstone beams deal (for a total of (Removed in Repentance)880 damage/ (Added in Repentance)380 damage per use). Doesn't double the brimstone beams' duration.
The Hourglass: Doubles the length of the effect.- (Added in Repentance)
Mega Mush: Makes the effect twice as long.
- (Removed in Afterbirth †)
Mine Crafter: Spawns a Movable TNT and instantly detonates it, damaging Isaac.
- (Added in Afterbirth †) This has been changed to spawn two Movable TNT barrels instead, which you can detonate at will.
- (Added in Afterbirth †)
Moving Box: Activation swaps the contents of the box with the contents of the room.
Spider Butt: Deals 10 damage to enemies twice but does not increase slow effect.- (Added in Repentance)
Stitches: Swaps places with Isaac twice in quick succession, rendering Stitches useless for collecting pickups & items beyond pits, rocks etc.
- However, it can still be used to telefrag enemies.
The Bean/
Mega Bean/
Kidney Bean: Deals double damage, but charms/poisons/petrifies for the same amount of time.
Tear Detonator: Multiplies detonated tears, resulting in 6x more tears than normal.- (Added in Repentance)
The Scooper: Will summon 2 peepers in a bifurcated direction
Ventricle Razor: Places both trapdoors in same location.
- (Added in Afterbirth †) Ventricle Razor works normally.
Notes
- Having more than one Car Battery has no effect.