There are certain modules that are—for one reason or another—rarely used in EVE. Some were made irrelevant by other modules or shifts in design, some are shunned because they are seen as sub-optimal, and some never seemed to have a niche in the first place. While L33t PVP “experts”, neckbeards, and bittervets alike will likely decry most of what I have listed here, each of these modules does have a niche, even if it is exceedingly narrow.
Whenever Capacitor Batteries show up in a fit, several pages of “OMG U R TERRIBAD!” sperg are sure to follow. Most of these posts will be from PVP players, whose preference is a quick, high-powered burst of energy and not a slow rebuild. To be fair, you do want quick bursts in PVP builds, because it doesn’t matter what your ship is carrying if you lose it. This is the same concept behind overheating modules: A rack of guns on the verge of burning out is preferable to a scrapped ship. The setting is a bit different for PVE applications, though. In PVE, you’re usually looking for staying power and you don't want to be burning through a hold full of capacitor charges to achieve that. The Capacitor Battery gives faster recharge times by applying the constant capacitor recharge rate to a larger pool. It's the same principle as buffer-tanking a Drake. If the pool promises to refill in 100 seconds, regardless of how big it is, why not just make it hold more?
Some argue that a Capacitor Recharger module is more efficient at raising that recharge rate. However, if you’re using multiple capacitor-related modules, you usually want a mixture of improved recharge time and expanded pool to maximize the total recharge. Blame synergistic mathematics for that.
Besides boosting your recharge rate, there is another situation where the Capacitor Battery is the clear victor: when you just plain want a bigger capacitor pool. For example, the Capacitor Battery allows you to warp across longer distances in small ships with ease. When warp is initiated, an amount of charge is taken off the top to begin, and then the remaining cost is applied versus your remaining capacitor charge. Caldari ships in particular are not well-known for huge capacitor reserves; smaller ships will be forced to make multiple warps in some areas. I’ve been on nullsec roams where battleships pulled ahead of my frigate because I had to make three separate warp jumps to get across the system. With the Capacitor Battery, the Capacitor holds more charge, ergo you can make longer warps with it.
Hyperspatial Velocity Optimizer
The Hyperspatial Velocity Optimizer is a rig you don’t see much of, mainly because it won’t help you kill your enemies faster. It might help you get close to begin killing your enemies, but it doesn’t help in the actual killing. Have you lost interest yet? Most people skip right over this rig when outfitting their ships, and even transport ships don’t use them much, favoring cargohold optimization rigs instead. However, if there’s a time and place for everything, this rig's time and place is on bastardized Tech 3 cargo runners.
Every time someone fits an Interdiction Nullifier and Covert Reconfiguration to a Strategic Cruiser, gate campers die a little inside. If you’re doing that, chances are you’re carrying very compact, very expensive things in your hold and would rather not be inconvenienced by roadblocks. Strategic Cruisers are fast, but there are faster ships out there and in pipe systems it’s fairly obvious where you’re heading. If you’ve equipped a Hyperspatial Velocity Optimizer or two and—god forbid—a Capacitor Battery on top of that, you stand a very good chance of getting to and through the next gate before even Interceptors and Covert Ops Frigates can get in front of you. On top of that, you'll have an increased chance to lose anyone chasing you over multiple systems. The effect is more pronounced in larger systems where you will be at maximum warp speed longer, but showing up at the destination gate several seconds before you’re expected will still greatly enhance your survivability.