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Understanding Your PERC Battery

To keep up system maintenance on your Dell PowerEdge machine, it is a good idea to view your PERC log to see if there are any outstanding errors. The PERC log can be pulled through OMSA or a DSET report.

One thing that you will want to look out for is your RAID controller battery. If it goes to failed state and no longer holds a charge your RAID controller will not use the Write Back feature assigned to your Virtual Disks.

This can cause some  serious performance issue because your RAID controller will now be set to Write Through and write all the data to the disks before if can access more data.

Here is the line you will be looking for in your PERC Log:

Relative State of Charge : 98 %
Absolute State of Charge : 84 %
Remaining Capacity : 1428 mAh
Full Charge Capacity : 1456 mAh
Time to Empty : 65535 Min
Average Time to Empty : 65535 Min
Average Time to Full : 65535 Min
Cycle Count : 23
Max Error : 0 %

I highlighted the important information in red.

Absolute State of Charge

-The current maximum charge capacity of the battery as compared to its maximum specification capacity
-The battery at 100% Absolute State of Charge should maintain cache for up to 72 hours.
-You dont want the Absolute State of Charge to go below 33%. If it goes below 20% the battery will show in a predicative failure state.

Relative State of Charge

-The current maximum charge capacity of the battery as compared to the current Absolute State of Charge.

Math Formula

Here is the math to figure our how long the cache will remain when the turns off expectantly:

98% x 72 = 70.56 Hours
50% x 70.56 = 35.28 Hours

This battery will maintain it’s cache for 35.28 hours.


Here is another example problem:

A Battery showing 75% Absolute State of Charge and 50% Relative State of Charge will maintain the cache for approximately how long?

Answer: 27 Hours

Here is the math to work that out:

75% x 72 = 54 Hours
50% x 54 = 27 Hours




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