Mortar Battery is the final upgrade of the Mortar Tower in BTD4, right after the Bloon Buster. The Mortar Battery is the same as the Mortar Tower except for the fact that it attacks three times as fast. They cause serious bloon damage for anything except for zebras and Black Bloons, although not as effective as MOAB Maulers, making MOAB Maulers the better choice. It is unlocked at Rank 23 in BTD4.
When this upgrade is purchased, the monkey disappears.
This also applies to the Triple Shot upgrade for Spike-o-pult.
In Bloons TD 4 iOS, when this upgrade is unlocked, it is misspelled as "Tripple the pain", instead of "Triple the pain".
The monkey operating the mortar tower disappears, and the tower can still attack, meaning that this upgrade makes the tower mechanical like the Tack Shooter which attacks with no monkey controlling it.
The upgrade button icon shows 3 mortar cannons and no monkey controlling it so the monkey disappears when you purchase it.