EnerChip Solar Energy Harvesting Demo Kit
Designing for Pulse Discharge Currents in Wireless Devices
Pulse currents of tens of milliamperes are common in wireless sensor systems during transmit and receive
modes. Pulse discharge currents place special demands on energy storage devices. 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. 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 main energy storage device. The storage device charges the capacitor between discharge pulses and the
capacitor delivers the pulse current to the load. Specifying the capacitance for a given energy storage device in
an application is a straightforward procedure, once a few key parameters are known. The key parameters are:
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Storage cell impedance (at temperature and state-of-charge)
Storage cell 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
main energy storage device such as the EnerChip solid state storage cell.
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 EnerChip device 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
an EnerChip-capacitor couple for pulse current applications is to size the capacitance using the following
formula:
Discharge formula: C = t / [ R * ln (Vmax / Vmin) ]
where:
C = output capacitance, in parallel with EnerChip;
t = pulse duration;
R = load resistance = Vout(average) / Ipulse
Vmin and Vmax are determined by the combination of the EnerChip 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 [ (Vmax - Vchg) / (Vmin - Vchg) ]
where:
t = capacitor charging time, from Vmin to Vmax
R = EnerChip resistance
C = output capacitance, in parallel with EnerChip
Vmax = final voltage to which the capacitor must be charged prior to delivering the next current pulse
Vmin = initial voltage on the capacitor when charging begins
Vchg = applied charging voltage on the capacitor
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DS-72-08 Rev18
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