This Speed Force battery is a battery that is said to be able to charge a speedster's speed, but Caitlin's statement disproves this, it can be used as a strong power source, though.
While Eobard was talking with Eddie, Cisco reveals to the team that Eobard was using this device to radically increase his speed and is why he was so much faster than Barry. It is assumed to be future technology such as the Reverse-Flash ring, the particle accelerator power source Eobard created, the DNA harvester used to transform Eobard's appearance into that of Harrison Wells, and Gideon, as Cisco claimed the battery could power all of Central City, as no device that small can do something like that in the present time. Later, Cisco uses it to neutralize the meta-humans' powers while team Flash were transporting them to a plane heading to drop off at Lian Yu, but either Captain Cold or his sister, Lisa, rigged it so the power drained, allowing the metas to escape.
After Ronnie Raymond died, Professor Martin Stein's body started to destabilize due to not bonding with Ronnie for a prolonged period of time. Cisco used the battery to stabilize Stein, but said that most of the power was used up during the transport of the meta-human criminals. The battery was then put into a power cane so that Stein could walk while Team Flash found a new partner for the Firestorm Matrix.