EnerChip Solar Energy Harvesting Demo Kit

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DS-72-08 Rev15

Page 5 of 11

Pulse Discharge Current for a Wireless End Device

Pulse discharge currents place special demands on batteries. Repeated delivery of pulse currents exceeding

the recommended load current of a given chemistry will diminish the useful life of the cell. The effects can be

severe, depending on the amplitude of the current and the particular cell chemistry and construction. Pulse

currents of tens of milliamperes are common in wireless sensor systems during transmit and receive modes.

Moreover, the internal impedance of the cell often results in an internal voltage drop that precludes the cell

from delivering the pulse current at the voltage necessary to operate the external circuit. One method of

mitigating such effects is to place a low Equivalent Series Resistance (ESR) capacitor across the battery. The

battery charges the capacitor between discharge pulses and the capacitor delivers the pulse current to the

load. Specifying the capacitance for a given battery in an application is a straightforward procedure, once a few

key parameters are known. The key parameters are:

Battery impedance (at temperature and state-of-charge)

•

Battery voltage (as a function of state-of-charge)

•

Operating temperature

•

Pulse current amplitude

•

Pulse current duration

•

Allowable voltage droop during pulse discharge

•

Two equations will be used to calculate two unknown parameters:

1) the output capacitance needed to deliver the specified pulse current of a known duration;

2) the latency time that must be imposed between pulses to allow the capacitor to be recharged by the

battery.

Both formulae will assume that the capacitor ESR is sufficiently low to result in negligible internal voltage drop

while delivering the specified pulse current; consequently, only the battery resistance will be considered in

the formula used to compute capacitor charging time and only the load resistance will be considered when

computing the capacitance needed to deliver the discharge current.

The first step in creating a battery-capacitor couple for pulse current applications is to size the capacitance

using the following formula:

Discharge formula: C = t / R * [-ln (Vmin / Vmax)]

where:

C = output capacitance, in parallel with battery;

t = pulse duration;

R = load resistance = Vout(average) / Ipulse

Vmin and Vmax are determined by the combination of the battery voltage at a given state-of-charge and the

operating voltage requirement of the external circuit.

Once the capacitance has been determined, the capacitor charging time can be calculated using the following

formula:

Charge formula: t = R * C * [- ln (1 - Vmin / Vmax)]

where:

t = capacitor charging time, from Vmin to Vmax

R = battery resistance

C = output capacitance, in parallel with battery