AN-1057: Capacitance to EnerChip Battery Charge Conversion Guide

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Doc AN-72-1057 Rev B

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Preliminary

Figure 1 profiles do not account for the effects of self-discharge, which are measurably worse for

supercapacitors than for the EnerChip solid state battery.

Another frequently asked question is: “How much storage capacity do I need for my application?”

The simplest calculation involves multiplying the RTC/MCU/SRAM operating current (at the backup power

source voltage) times the number of hours needed for backup. The product is the amount of charge capacity

needed, in ampere-hours. When calculating the device run time in backup mode, it is sometimes necessary

to include de-rating factors such as storage device characteristics vs. temperature and aging, as well as load

current consumption over temperature and voltage. This is particularly true if precise knowledge of minimum

available backup time over life of product is required.

Comparing - in absolute terms - EnerChip storage capacity to supercapacitors in a comparable energy storage

range, we turn to Figure 2. Recalling the state-of-charge dependence typical of supercapacitors, three lines are

drawn, representing the minimum operating voltage (Vmin) at which the load can operate. Note that as Vmin

rises, a diminishing amount of useful charge is available from the supercapacitor. (Again, the relatively high

self-discharge of supercapacitors has been ignored here for the sake of clarity.) For reference, EnerChips range

from a few microampere-hours to several tens of microampere-hours, corresponding to equivalent capacitance

of tens to hundreds of millifarads.

Figure 2: Charge Capacity of EnerChips vs. Supercapacitors at Several Discharge Cutoff Voltage Levels.

In addition to the battery (EnerChip) management functions, the EnerChip CC has additional functions that

are useful and often necessary in backup power applications - such as supply voltage threshold detection and

a power-good indicator. EnerChip CC devices also have temperature-dependent battery charge compensation

included to improve the service life of the product. For more technical information related to the EnerChip and

EnerChip CC products, refer to the Cymbet Corporation web site: www.cymbet.com.