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Doc AN-72-1057 Rev B

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AN-1057

Application Note

Capacitance to EnerChip Battery Charge Conversion Guide

Introduction

EnerChips are solid state rechargeable batteries that have a high charge-discharge cycle life, very low self-

discharge, and are constructed with solid state materials, making them durable, non-toxic, and safe to use in

a range of environments. EnerChip CC products include an EnerChip battery management circuit co-packaged

in low profile solder reflowable, surface mount QFN and DFN-style packages. EnerChips are often used as a

real-time clock (RTC) and memory backup power source. Supercapacitors are also used in these applications,

although there are significant differences in the operating characteristics of these two categories of energy

storage devices. An important distinction to understand when comparing effective charge storage capacity is

that the EnerChip delivers charge at a relatively fixed voltage, while supercapacitors deliver charge at an ever-

declining voltage, the voltage-charge slope being a function of discharge current.

A frequently asked question is: “If I’m using a supercapacitor as a backup power source for an RTC and want

to replace it with a solid state battery, what battery capacity is equivalent to the supercapacitor rating?”

In answering the question, one must recognize the differences in discharge profiles between EnerChips and

supercapacitors. In relative terms, the percentage of supercapacitor storage capacity available to the load is

linearly dependent on the minimum voltage at which the load device (e.g., RTC) will continue to operate. In

contrast, the EnerChip CC output voltage is nearly constant over the entire discharge, as depicted in Figure

1. EnerChip CC products have a self-contained battery management circuit that controls battery charging and

delivers >95% of the stored energy at a relatively steady output voltage of 3.3V before going into cutoff.

Figure 1: Relative Charge Capacity Delivered to a Load - Green Line Represents the EnerChip CC; Blue Line

Represents a Supercapacitor.

Figure 1 Notes:

1) Capacitor charge voltage = 3.3V. 2) Constant current discharge. 3) Assumes negligible self-discharge of capacitor and EnerChip

4) EnerChip CC voltage derived from 3.8V terminal voltage, less 0.5V diode drop to Vout.

Preliminary