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failures, like an AC power loss, to dump the data cache contents safely from RAM to a non-volatile

memory location before the controller powers down completely. Customer data shows that these

catastrophic failures for enterprise-level servers are rare, and the advantage of supercap’s high

charge/discharge cycles is minimal or immaterial in this application.

Energy density and physical size

Energy density in a Li Ion cell is significantly greater than a comparable, capable supercap. Per unit

weight, a Li Ion cell stores 30 to 50 times more energy than a supercap:



Standard supercap: 3 – 5 Wh/kg



Standard Li Ion cell: 100 – 250 Wh/kg

Because of the significant difference in energy density, coupled with a low maximum output voltage, a

Supercap-based power pack design requires multiple Supercaps connected in series and/or in parallel.

This increases the physical size of the pack as well as the design complexity. Dell EMC server designs

are extremely dense and the system designers did not want to sacrifice valuable space to accommodate

multiple mounting points for tethered supercap solutions or to waste power to increase airflow

unnecessarily.

Degradation and life estimates

Both Supercaps and Li Ion batteries experience degradation when subjected to higher temperatures in

the

range of 60°C to 65°C. In general, for every 10°C rise in the ambient temperature, a Supercap’s

lifetime is

reduced by a factor of 2 as compared to a Li Ion battery. When designing a power backup

solution for

Enterprise-class servers, careful consideration must be given to life expectancy and

ambient conditions.

The design should have sufficient margin to accommodate degradation over the

expected life of the

product.

Conclusion

While there are Pros and Cons for batteries versus Supercaps Dell EMC’s choice of a Li Ion battery in the

PERC RAID controllers took these criteria and more into

consideration, and delivered a solution that is

maintenance free with well understood technology. In addition, the capability to withstand higher

temperatures enables your business to save operational expenses through reduced cooling

requirements

in your data center. Going chiller-less altogether for your next data center will have a

huge impact on

capital expenditures, while delivering reduced risk—and helping your organization

be greener by reducing

its carbon footprint. Dell EMC continues to methodically assess the best back up power source for every

generation of PERC.

Warranty

The Li Ion battery pack used in the new generation RAID controllers is designed to support

a 3-year warranty in the worst-case conditions of 55°C to 60°C ambient temperature.

PERC batteries

Use industry standard prismatic Li Ion cells with approximately 1.6Wh capacity with

Transparent Learn Cycles (TLC) and are integrated directly onto the controller.

Point of Failure

Supercap based power source would require multiple

parts connected in series and/or in

parallel in order to support the energy requirements of a RAID

controller backup solution

Reference: A Comparison between LI Ion Battery and SuperCap technologies