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BATTERIES VERSUS SUPERCAPACITORS
Why Dell EMC PERC Controllers Use Batteries
Tech Note by:
Gene Chesser
This tech note provides an overview
of the critical differences between
Lithium Ion (Li Ion) battery and
Supercapacitor (Supercap)
technologies, and the key
considerations that went into
selecting Li Ion technology as the
preferred choice for the newest Dell
EMC PowerEdge™ RAID controller
(PERC) designs.
Pros and Cons:
 Growing non-volatile memory (NV)
densities require a dense solution.
Battery makes more sense because
of the size of a supercap (SC)
o Very large packaging due to
number of caps required for power
dump
o No place to mount a huge SC
 Major cooling challenges with SC vs.
battery
 Have to be positioned in a place for
maximum cooling.
o SCs run very hot and if not cooled
properly will fail more quickly
Dell EMC servers are optimized for
“Fresh Air” environments. The
smaller size of a battery (to get the
same energy density) allows for a
larger heatsink on the RAID
processor, which allows you to keep
the card in a hotter environment with
less airflow and still stay within temp
specification.
Protecting RAID controller cache has been debated in the industry
for years, comparing batteries to supercapacitors (supercaps). Dell
EMC engineering developed its own Flash Backed Write Cache
architecture under the PowerEdge RAID Controller (PERC) brand.
The engineering team spent seven months comparing the pros and
cons of batteries and supercaps with regard to energy densities and
capabilities around size and heat.
There is a big ‘myth” that supercaps are infallible and ultimately
superior. They are NOT. They fail just like batteries do and they
have their own set of challenges. Dell EMC uses batteries due to
their higher energy density, reduced physical size, and ability to
handle elevated thermal environments in PowerEdge servers
Several customer needs were evaluated during the design phase of
the PERC RAID controllers for developing a Li Ion battery pack or a
supercap to handle the task of moving cache data into non-volatile
flash memory. These design factors included:
 Thermal profile to support Fresh Air data centers
 Maintenance free without impacts to performance as seen in
previous controllers
 Discharge capacity over time to support a 3-year warranty in
worst-case extreme conditions
 Physical size and location in the server that would allow
server development teams to maximize memory and IO
density
In addition to customer criteria, several critical design parameters
were analyzed and consequently supported the decision to choose
a Li Ion battery pack over a Supercap.
Cycles
Supercaps support a high number of charge/discharge cycles when
compared to Li Ion battery packs. In the RAID subsystem,
supercaps or Li Ion packs are always used as a backup power
source. This power source is used only in the case of catastrophic