A micro-charge were a type of explosive charge used by Starfleet and possessed by Ferengi merchant Ulis, during the mid-22nd century. (ENT: "Fight or Flight", "Acquisition")
In 2151, Lieutenant Malcolm Reed of Enterprise NX-01 suggested using two micro-charges to blow open the airlock of an Axanar cargo ship. (ENT: "Fight or Flight")
A green blinking micro-charge
In June 2151, Reed set a group of remotely triggered explosives in the monastery at P'Jem. He then detonated them himself, blewing off a wall in the atrium. (ENT: "The Andorian Incident")
The final draft script of "The Andorian Incident" referred to these devices, which are unnamed on screen, as micro-charges. Also, though their quantity is unclear from the episode, the script clarified that Reed used four of these charges. The call sheet of the episode is also listing "4 micro charges" in the props section. However, only two were seen in the episode and Reed's remote control featured three green-blinking lights he activated previously.
When Ferengi pirates who raided Enterprise in the same year believed they had discovered the ship's vault, which was actually sickbay's imaging scanner, it was suggested that they use micro-charges to blow it open, as a means of circumventing what they believed to be a magnetic seal. (ENT: "Acquisition")
As part of escape plans to break out of Tandaran Detention Complex 26, Ensign Travis Mayweather told Lieutenant Malcolm Reed and a Suliban inmate named Sajen that charges were set. These micro-charges blinked when they were ready to be detonated. Six of them were located on a wall of the internment camp. Using a remote detonator, Reed set off these explosives during the actual prison break, which incidentally killed Tandaran Major Klev and two other Tandaran guards as well as destroying the wall. (ENT: "Detained")
Similar to the unnamed explosives in "The Andorian Incident", the charges in "Detained" were referred to as "micro-charges" in the episode's final draft script, despite the fact they are merely termed "charges" in the episode itself. Also in the script, the explosives were set up by Reed, observed by Sajen and another Suliban prisoner, Danik. Sajen criticized the charges, while they were being prepared, as looking "awfully small," though Reed assured him the devices would "do their job." It was Danik, not Reed, who later detonated the charges in the script.
On a Xindi-Reptilian warship in February 2154, a micro-charge was used by MACO Major J. Hayes to blow open the door of a cell imprisoning Ensign Hoshi Sato. (ENT: "Countdown")
The final draft script of "Countdown" referred to this device, which is unnamed on screen, as a micro-charge.
Micro-charges were at one point intended to appear in "Dead Stop", though they are not actually shown in that episode. In the episode's final draft script, an explosion aboard an automated repair station, set off by Lieutenant Reed, was described as being due to three micro-charges. However, in the actual installment, a large metallic warp plasma canister, with two blinking lights atop the device, is instead shown to be used.