The Storwize V7000 is a modular storage system. This means you buy it in enclosures that are two rack units (2U) in size. The first enclosure you buy is a control enclosure which contains two node canisters and two power supplies (with integrated batteries).
The purpose of the batteries (that are only found in the control enclosure) is to protect the node canisters from brown-outs (so they can ride through sudden drops in input voltage) and to give the node canisters time to gracefully shutdown should power be lost. Any data in the cache of the Storwize V7000 will be persistently written to the internal solid state drives. So the batteries are only needed if power is lost and only used while the nodes are shutting down. The Storwize V7000 Information Center contains a great write-up on how this works here.
In the image below you can see a control enclosure battery unit on the right, with an arrow to show where it fits into a control enclosure power supply.
On August 9, 2011 IBM announced that the feature code (8001) for the Storwize V7000 battery was going away without replacement. This left some people a little bit concerned as to what was happening with the batteries. We don't want to have a Storwize V7000 without batteries! The answer is that IBM had separate feature codes for the control enclosure power supplies (Feature Code 9801) and the batteries (Feature Code 8001). Since you cannot have one without the other, IBM have decided to remove one of the feature codes (the battery feature code). Thus this change is purely an administration change and the machine will continue to ship with two batteries. Phew!