Battery capacity / Accumulator Charge Meter

Posted on Dec 3, 2012

The instrument used to measure battery capacity. The battery may be charged to any current in the range 0-3 A. The load current may be constant, may change over time or even break off completely. Unloading stops when the battery voltage drops below the set limit. Measuring the battery at various discharge currents, we consider the purpose for which it can still be used. Meter is based on the current-frequency converter. The output pulses of the converter are counted in counter, whose contents are displayed on the screen. The appropriate choice of transfer constants are displayed directly passed through the hub size. For example, a current of 1 A for 1 hour with loads of pulses 1000. However, if diminishing current to 100 mA, the output frequency is 10 times lower transmitter and pulses will be only one hundredth To read in Pulse 1000, the same as in the previous case, the converter would have to pass through stream 10 times longer. For practical reasons, however, given the current transmitter frequency several orders of magnitude higher and the required number of pulses per unit time decreases divider. We do not have to monitor battery voltage, the meter is completed circuit, which disconnects the load is reduced when the voltage is below the set limit. Involvement of the current-frequency converter is in Figure 1 The converter circuit using MAX471, which is designed to monitor the battery charge in portable devices. Monitored current passes between the pin RS + and RS-circuit. The current creates a voltage drop across the internal resistor with resistance of about 35 milliohms. The voltage drop is in the direction of current flow amplified by the amplifier A1 or A2 and is employed by one of two sources of power transistors Q1 and Q2. Their outputs are combined and connected to a common outlet OUT. From this flows the outlet stream, which is in absolute value 2000x smaller than the current passing between the terminals + and MS-MS. Voltage outputs of amplifiers A1 and A2 control comparator. The output (pin SIGN) is a logic level indicating the direction of current flow - sign. Indication of the direction of flow meter is not used, only the expected battery. Stream from the output OUT is charged capacitor C is connected to a known timer 555th Reaches the voltage on the capacitor 2 / 3 power, turning it upside down to flip the timer circuit and the capacitor is quickly discharged transistor internal timer for 1 / 3 voltage. Then, the internal flip-flop timer tilted back and the capacitor can charge back the current flowing from pin OUT circuit MAX471. Since the rate of charging a capacitor is directly proportional to charging current, the output frequency proportional to the measured current.

Battery capacity / Accumulator Charge Meter
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Total connection counter is in Figure 2 Let's go back to the transmitter. According to catalog data conversion accuracy of current in the circuit deteriorates rapidly at low currents. This is probably caused by the output OUT is still a small stream flows. In my case, this current around 0.6 mA, which would correspond to the measured current 1.2 mA. When measuring current 12 mA residual current that would cause an error of 10%. The measured error, however, was considerably smaller, rather it seems to diminish the output current is below a certain threshold. I decided to compensate the current circuit with a transistor T3, diode D4 and trimmer P3. I of course setting the compensation current can significantly reduce the measurement error at low currents. For completeness, according to the catalog data may be OUT pin leakage current to 2.5 mA (10% error at 50 mA). Another problem is setting the correct conversion ratio. The conversion ratio of current in the circuit MAX471 can not change. Roughly the frequency is set condenser capacity. Since the capacitors are normally sold only in E6 and a tolerance of 10%, it is possible to obtain the required capacity of the parallel connection of capacitors C4 and C5. Thus it is appropriate to adjust the frequency to an accuracy of 5%. Then gently set the trimmer P2. When measuring current of 100 mA is needed to bring the counter to 100 pulses per hour. The output of the converter, in...

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