UPS External Batteries


   How to calculate and configure external battery count for UPS units.

Battery Packs:

   UPS units can run for days with a sufficient number of external batteries -not so for some APC units.  The UPS uses battery count for runtime calculations.  For example, some UPS models cap the number of external batteries.  In other words, runtime won't benefit from additional batteries that exceed this cap.

   UPS units use external battery count for runtime calculation.  This process provides maximum runtime and prevents premature shutdowns.  Configure external batteries in the NMC GUI.

   At first glance this process seems simple enough.  However, determining the correct number of external batteries gets confusing.  For example, does it want a count of all physical batteries?  Or does it want a count of battery packs?

   In most situations, battery count equals the number of physical batteries each battery pack.  N.B., this rule only applies so long as the internal and external batteries are identical.  For example:
  •  750XL UPS uses two 12V18AH batteries.  
  • SUA24XLBP battery pack uses four 12V18AH batteries. 
  • The network management card (NMC) external battery count equals four.
   However, there are situation when internal and external batteries are different.  Consider the following:
  • 750XL UPS uses two 12V18AH batteries.
     ~18AH per battery; or a total of ~36AH.  
  • UXBP24 battery pack uses four 12V75A.
     ~75AH per battery; or ~300AH total.  
   In this situation, the external battery count is not four.  Rather, it equals 16.  Huh?

   In both examples, the external battery count is the total sum of Amp Hours (AH) of all external batteries; divided by the AH of ONE internal battery.

     AH for total external batteries:  300AH
     AH for single internal battery:    18AH

               Figure 2.  Calculating External Battery Count 
  The battery amperage is usually listed on the battery.  Alternately, look up the UPS or battery pack model at  

Console Control:

   Alternately, use the console (i.e., not the NMC) to set the battery count:
  1.  Shut down UPS.
  2.  Remove NMC.
  3.  Use an APC serial cable to console to the UPS:
         2400 baud,
         uncheck FIFO.
  4.  Enter Smart mode:
       Press Shift+Y (i.e., CAP "Y").
       Prompt changes to SM#.
  5.  Enter Program mode: Y, 1, 1
      N.B., each comma represents a pause. 
  6. Use the "+" or "-" to change the battery count.  
  7. Type "R" to save and exit
That's It!


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