EPTOverview
The Shield Battery is a rarely used defensive building which can repair the shields of an injured Protoss unit. It generates energy which can be quickly converted into shield hit points on nearby units, either a single unit or a group. When constructed it starts with 100 energy, each energy point can heal two shield points. Unlike the medic's heal, the unit receiving the shield recovery cannot move or attack while it happens. To the majority of Starcraft players, the Shield Battery is a mysterious and misunderstood building.
Useages
Because of their low cost they can be effective in supporting carriers and archons, both of which can take a lot of shield damage. They are also sometimes used in PvP early game when a defender needs to hold his base against a greater number of zealots. Because of their lack of mobility they are most useful when placed in direct support of a defensive position, such as at the top of a ramp the player is trying to defend. In very rare cases a protoss player proxying a pylon near an opponent's base will proxy a Shield Battery to make the attack more powerful.
On maps where starting locations are close to each other, like on hunters, you can build a Shield Battery to help you contain an enemy.
Shield Battery useages are situational, meaning that you can't often predict using them beyond the spur of the moment, an example being when you expect a tough fight. It is not viable to put Shield Batteries well in advance and to plan on using it. Because going both Photon Cannon and Shield Battery is overkill, you have to opt for one or the other. Opting for a Photon Cannon with range will also enable you to use the required Forge for a +1 upgrade to two hit kill Zerglings. Photon Cannons may be better defense overall than Shield Batteries, however you may not have adequate time to make them. If this happends to be the case, you should opt for the Shield Battery when you don't have time to make the Pylon, Forge, and Cannons. A Forge takes 50 seconds to morph in, and then you have to build Photon Cannons that take another 50 seconds each. A Shield Battery only takes 30 seconds to morph in, which is about a minute faster than making a single Photon Cannon.
As defense, Shield Batteries aren't effective to use on units being attacked by heavy damage dealers. In the amount of time the unit spends not attacking and charging, he may lose more than the regained Shields. This is a reason why they aren't extremely effective in the late game. In the late game it may not be worth it to use on units when you have too many and cannot take them out of combat for enough time to recharge. Also, it may be ineffective to use a Shield Battery since you will only be able to recharge a limited amount of units in your maxed out army.
When pairing a Shield Battery with Archons or any other unit, you have the disadvantage of having to bring the unit away from the fight to the Shield Battery. You will have to wait for a stop in the fighting to take your units back to the Shield Battery.
On certain maps, Shield Batteries become even more viable. On maps without a geyser at the first expansion, like Nostalgia, you will not be able to make the large amount of gas intensive units that are so crucial. You may want to preserve Archons, Reavers, High Templars, and Dragoons; And in doing so, it may be as effective as having another of those units. In island games, where gas is also limited due to difficult to obtain expansions and where air units and dropping is more common, Shield Batteries become more viable as well.
A Dragoon or two with a Shield Battery can be used in combination with Micro to great effect against many Marines. You may need several Shield Batteries to recharge a large amount of your own Carriers. You may benefit from using a Shield Battery in defensive PvZ situations where you use it with a Reaver to hold off an all-in enemy. Corsairs, as well, can benefit from the Shield Battery recharge. In PvT and PvZ and PvP, a Protoss can benefit from having a Shield Battery in aiding his Shuttle/Reaver harass. Low shields on Shuttles or Reavers can be recharged. After a fast expansion, you may benefit from a Shield Battery while being behind on numbers of units.
2v2 Useages
You may opt for getting a Shield Battery in 2v2 when you have a Zerg ally since you won't need the detection Photon Cannons offer. In fact, Shield Batteries are more viable in 2v2 games. You may need to hold your ramp as long as possible while your ally's painfully slow Marines or Zealots come to your aid. In conclusion, in 2v2 Shield Batteries are more viable than in 1v1 because there are more "spur of the moments" in games when you may need them, and you may need to hold off the enemy until your ally arrives to help you.
A Shield Battery can be used in conjunction with just about any other unit. Although it requires more clicking around, they can help you fortify a defending location. However just like the previous paragraph mentioned, they aren't terribly viable in 1v1 but in a 2v2 they can really help you defend both you and a teammate, especially with a couple archons with them.
Archons and a Shield Battery are particularly effective against masses of Zerglings. A couple Archons being used in conjunction with a Shield Battery at your ramp will rack up dozens of kills against two Zerg opponents massing Zerglings. If you Fast Expand, you may want to put another Shield Battery at the Expansion so you don't have to retreat back to the original. Also, a Shield Battery can be placed near your mineral line, for your Archons, if you anticipate Mutalisks.
Methods to use the Shield Battery
There are three methods to using the Shield Battery. 1) You can use the Shield Battery on an empty area near your own units to recharge multiple units at a time. 2) You can use the Shield Battery on individual units. 3) Finally, You can select the desired units and then click on the Shield Battery.
If you use the Shield Battery on any units out of range, however little the amount, they will have to stop attacking and walk back to the Shield Battery. Whenever you use the Shield Battery to recharge units, all units targeted will have to stop attacking while being recharged. This may have a bad outcome for you when you are being attacked on your ramp. An alternative is only targeting the weakest units. Perhaps the most effective method is pulling the wounded units out of the line to the rear to recharge and regroup later. One should note using the Shield Battery on a unit results in it losing its hold position. Zealots can be lured away by Zerglings.
