TL;DR - Is the immediate failure of a replacement battery, for a failed battery, on a battery backed accelerator for a Smart Array P400 controller a common occurrence? Or are we likely to have an storage controller with an impending and critical fault?

We have a slightly confusing situation with a Smart Array P400 storage controller with the 512mb battery backed accelerator addon on an HP DL380 server.

The storage controller is (afaik) running the latest firmware and driver:

Model:  Smart Array P400 
Controller Status:  OK 
Firmware Version:  7.24 
Serial Number:  *snip* 
Rebuild Priority:  Medium 
Expand Priority:  Medium 
Number Of Ports:  2 

The storage diagnostic (both on the both boot-up screen for the controller and within the 'Management Homepage' and the 'HP Array Diagnostic Utility') recently starting showing the following status a fault for the battery for the accelerator:

Accelerator
Status:  Temporarily Disabled 
Error Code:  Cache Disabled Low Batteries 
Serial Number:  *snip* 
Total Memory:  524288 KB 
Read Cache:  25% 
Write Cache:  75% 
Battery Status:  Failed 
Read Errors:  0 
Write Errors:  0

We replaced the battery with a new unit (a visual inspection of the P400 card showing nothing unusual) and saw the same fault - but expected this to disappear over the course of a few hours/days as it charged. This didn't happy, and the fault status remains the same as above.

Given the battery is a genuine part from HP, I wouldn't have expected a replacement battery to fail straight away, or to be dead-on-arrival (is that naivety on my part?). Is the immediate failure of a replacement battery, for a failed battery, on a battery backed accelerator a common occurrence? Or are we likely to have an storage controller with an impending and critical fault? Is there any diagnostic that could tell me more about the failed battery, without cracking the server open again?

Many thanks!