If you're managing a storage array or RAID-equipped server, you've probably run into two different terms for the same basic job: protecting cached write data during a power loss.  One is the conventional Battery Backup Unit (BBU). The other is a supercapacitor module, sometimes branded as CacheVault, ZMCP, or FBWC (flash-backed write cache). 

They solve the same problem in different ways, and how you maintain it, budget for it, and replace it is contingent upon which way your controller uses.

The Problem They Both Solve

Writing to cache is significantly quicker than writing to a spinning disk or flash memory. Before writing the data to the disk, write-back caching RAID controllers store the most recently written data in the onboard cache. The issue is that cached data is transient. The data is lost and the array may be corrupted if power is lost before it is purged.

BBUs and supercapacitors are present to prevent this. The disparity is in the manner in which they safeguard the data.

How a Traditional BBU Works

A BBU is a compact lithium-ion or NiMH battery pack that is directly connected to the controller. In the event of a power outage, it maintains the cache memory's continuous operation for a period of 24–72 hours, or until the data is manually flushed or the power is restored.
Tradeoffs:
Batteries require periodic "relearn" cycles to maintain their accuracy as they degrade over time.

The typical service life of a battery is 2–3 years before it requires replacement. If the battery fails silently, the controller typically reverts to write-through caching, which is safe but slower, until the battery is replaced. Batteries are a frequent source of RMA and support tickets in older server fleets.

How a Supercapacitor (CacheVault/FBWC) Works

A supercapacitor-based system employs a different strategy than maintaining the cache powered indefinitely. It stores only the necessary energy to flush the cached data to onboard flash memory in the seconds following a power failure. Once the data is securely stored on flash, no ongoing power is required; restored.
Tradeoffs:
the flash memory retains it indefinitely until the system is reconnected and reads it. Tradeoffs: Supercapacitors are physically smaller and frequently mounted directly on the controller, eliminating the need for a remote battery pack or cable. Slightly higher upfront cost on the controller itself is offset by lower long-term maintenance.

So Which One Does Your System Use?

This isn't a choice you make after the fact — it's determined by the controller you already have. A few ways to check:

Look at the controller model. Older LSI/Avago and IBM ServeRAID controllers (e.g., M5015, M5014) typically use BBUs. Later-generation controllers (e.g., M5210, M5225, many 9-series MegaRAID cards) commonly shifted to CacheVault/supercapacitor designs.
Check the management software. MegaRAID Storage Manager and similar tools will identify the cache protection type under the controller's properties.
Look at the physical hardware. A BBU is a separate battery pack, often cabled to the controller. A supercapacitor module is usually a small flash-mounted unit sitting directly on or near the card.

Do You Still Need One at All?

A write-back unit (BBU) or supercapacitor—or some other type of cache protection—is required if your controller uses write-back caching. Most controllers will safely deactivate write-back caching and revert to write-through mode, thus running without one isn't always catastrophic. Unfortunately, this usually explains why an array that "used to be faster" has subtly become slower; it comes at a genuine cost to performance.

An commonly missed reason of write performance concerns is a degraded or missing BBU/supercap, therefore it's worth examining if you're not sure if your array currently has protection or if it's functioning.

Replacing or Upgrading

If you're using an earlier BBU-based controller and your battery dies, you're stuck with the same ecosystem because the controller doesn't allow supercapacitors. The little premium for supercapacitor-based cards is more than justified by the reduced maintenance required in the long run, making them an excellent choice for new infrastructure or controller replacements.

Whatever you do, be sure the component is a perfect fit for your controller model. 

It is important to verify compatibility before purchase to avoid a second procurement cycle, especially for BBUs that are frequently cross-referenced under multiple part numbers due to OEM and revision.

Are you having trouble finding the correct replacement item or determining if your controller uses a BBU or supercapacitor? Please provide us your controller model so that we can verify if it is the correct fit.