UPS battery protection against overcharging. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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As a result of the power surge, several electrical appliances connected to the network failed, including an uninterruptible power supply (UPS) VK500 from ARS, which at the time of the accident was also connected to the network, although the switch on its front panel was in the "Off" position. An "opening" showed that a 0,125 A fuse-link burned out in the UPS, protecting the primary winding of the power transformer. As it turned out, this transformer is constantly connected to the network, regardless of the position of the mains switch.

Replacing the fusible link restored the source to work. However (apparently due to interturn short circuits formed as a result of the accident), the no-load current of the primary winding of the transformer increased to 200 ... 300 mA. Since the transformer got very hot, up to the appearance of a characteristic smell, it had to be replaced. It was not possible to find one of the same type, so a more powerful transformer was installed with a secondary winding voltage without load of 16,5 V (for the replaced one - 17 V) and an open-circuit current of the primary winding not exceeding 40 mA.

The repaired UPS remained connected to the network for several days. Measurements have shown that the charging current of a battery whose voltage has long exceeded 14 V remains approximately 200 mA. This far exceeds the value needed to compensate for the self-discharge of the battery, which is dangerous by overcharging and shortening its life.

The assumption is that the UPS node is faulty. controlling the degree of battery charge, which should turn off the charging circuit when a certain voltage value is reached, has not been confirmed. Moreover, the study of the UPS circuit (its fragment is shown in Fig. 1) showed that such a node simply does not exist.

Most likely, before the repair, the charging current of the GB1 battery was limited by the increased internal resistance of the transformer. However, an emergency increase in the mains voltage led to a significant increase in the charging current, which put the transformer out of action.

To avoid future troubles, a battery voltage monitoring unit has been added to the UPS, which is assembled according to the diagram shown in fig. 2. Here the diodes VD1-VD5, the fusible link FU2 and the battery GB1 are elements already present in the UPS (see Fig. 1). The circuit connecting the positive terminal of the VD1-VD4 diode bridge to the anode of the VD5 diode is broken. The transistor VT1 is included in the resulting gap.

While the voltage at the input (pin 1) of the voltage drop detector DA1 is less than the threshold, its output transistor is open, the collector of which is connected to pin 3, and the emitter to pin 2. As a result, transistors VT1 and VT2 are also open. The GB1 battery is charging. A slight decrease in the charging current caused by the presence of a transistor in the charging circuit is compensated by a smaller decrease in the voltage of the secondary winding of a more powerful power transformer under load.

As soon as the voltage from the output of the resistive divider R2-R4, proportional to the voltage on the battery, exceeds the threshold level, the output transistor of the detector DA1 will close, which will lead to the closing of the transistors VT1, VT2 and opening the charging circuit. Charging will be enabled again when the battery voltage decreases due to self-discharge or the UPS running in battery mode.

The exact value of the device response threshold (14,2 ... 14,4 V) is set by a trimming resistor R2. If you use another voltage drop detector (the response threshold shown in the KR1171SP73 diagram is 7,3 V, for other microcircuits of the same series - the value indicated by the last two digits of the name), you should also change the values ​​\u2b\u4bof the resistors RXNUMX-RXNUMX. Their approximate values ​​in kiloohms can be calculated using the formulas

where Udp - the threshold of the detector DA1, V; UП - voltage of the battery GB1, at which its charging should be terminated, V; Id - current of the resistive divider, mA. Its value is chosen within 0,2 ... 1 mA. Please note that this current continuously discharges the battery. With its capacity of 7 Ah and systematic charging, this factor can be neglected. However, if the UPS is to be stored unplugged for several months, it is best to disconnect the battery. To do this, just remove the tip of the red wire coming from the battery from the contact plate on the UPS board.

Author: A.Sergeev, Moscow

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