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CBC-EVAL-08 Datasheet(PDF) 5 Page - List of Unclassifed Manufacturers |
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CBC-EVAL-08 Datasheet(HTML) 5 Page - List of Unclassifed Manufacturers |
5 / 11 page EnerChip Solar Energy Harvesting Demo Kit ©2009 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com 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 |
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