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Monitor battery discharge with power supply

27 Jan 2016  | Samuel Nork

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Predicting remaining capacity with many primary cell batteries is often a difficult task, but is especially challenging with lithium thionyl chloride. As shown in the discharge curves in figure 4, the open circuit voltage for a typical Li-SOCL2 battery remains fixed at a nearly constant voltage until virtually no capacity remains in the battery. At this point, the battery voltage drops abruptly. As a result, battery voltage monitoring provides little useful information until battery capacity is very close to zero.


Figure 4: Li-SOCL2Vage vs. Output Current (source: Tadiran).


Furthermore, both the open circuit voltage and the battery impedance have a strong temperature dependency, so even if measuring such parameters provides sufficient warning to avoid unscheduled downtime, discerning the difference between the knee of the discharge curve and a change in temperature or load is impossible without additional monitoring – all of which consumes unwanted power.


"Zero" Quiescent Current Coulomb Counter
The most simple and direct way to monitor battery usage is to count the coulombs discharged from the battery. Traditional methods involve continuous monitoring and integration of the battery current, which consumes considerable amounts of power even under no load conditions. However, the LTC3335's power conversion architecture accurately self-monitors the amount of charge passing from the battery to the load each time the DC/DC converter needs to boost the output into regulation. The key difference is that during the DC/DC SLEEP periods, the coulomb counter consumes zero current.

Whenever the DC/DC converter is enabled, current flows from the battery only when switches A and C are on. Current in switches A and C will flow until IPEAK is reached, and then switches B and D turn on and the inductor current discharges into the output capacitor as it ramps down to zero. Once the zero current point is detected, the cycle repeats until VOUT is in regulation. With an appropriately chosen inductor, the current from the battery will ramp linearly from zero until the programmed peak current value is reached each time switches A and C are ON, as shown in figure 5.


Figure 5: Battery Discharge Measured During Switch AC ON Time.


The time required to reach IPEAK in a given AC ON cycle is primarily a function of the battery voltage, inductor value and IPEAK setting. By measuring the amount of time required to reach IPEAK, the number of coulombs transferred during each AC ON cycle may be determined for a given IPEAK setting using the formula below:




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