Automatic battery shutdown. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Automatic battery shutdown. Encyclopedia of radio electronics and electrical engineering

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

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If you frequently use nickel-cadmium batteries, you probably know that in a series-connected circuit, cells that are discharged to a voltage below 1,1 V turn from a voltage source into a kind of additional load on the cells that have not yet been discharged, causing a sharp drop in voltage at the battery terminals. batteries. In addition to reducing the energy intensity of the battery, this can also lead to damage to its individual elements.

The proposed device, the scheme of which is shown in the figure, prevents too deep discharge of the cells in the battery. It is connected between the battery and the load. The principle of operation is based on the control of the voltage on the load. When it drops to the level of 1,1N V (where N is the number of cells in the battery), the load and the device itself are turned off by the relay contact group - the current through the battery cells stops (if there are no malfunctions in the battery itself).

Automatic battery shutdown

When the SB1 button is pressed, both the load and the control device itself are connected to the current source. The voltage at the inverting input of the DA1 microcircuit (pin 2) is determined by the zener diode VD1 and is 3,9 V, and at the non-inverting input (pin 3) - by a voltage divider across resistors R1 and R2, at normal current source voltage it is slightly higher than at the inverting input. In this state, the output of the microcircuit has a high voltage level - the relay K1 turns on and its contacts K1.1 leave the load and the control device on even when the power button is released.

When the voltage on the battery drops so much that its value at the non-inverting input becomes less than 3,9 V, the voltage at the output of the microcircuit will become low and the relay will de-energize, breaking the power circuit. The switching moment depends on the voltage on the battery and the resistance value of the resistor R1, which should be selected in accordance with the table.

Automatic battery shutdown

This device may give false alarms if too powerful a load is connected to the power source, at which the battery voltage instantly “sags down”. In this case, the disconnection of the load does not yet mean that the cell (cells) of the battery has been discharged to the lower permissible limit.

In the design of the device, you can use domestic radio elements - the K140UD7 microcircuit (without changing the numbering of the connection pins, as well as the 521SAZ, 554SAZ microcircuits), the KT503B transistor, the KS139A zener diode, the KD102A diode, the relay - any one corresponding to the voltage of the battery battery and having a trip current of not more than 0,05, 0,125 energy capacity of the battery used. Resistors - any with a dissipation power of 0,25 or XNUMX watts. To limit the base current of the transistor, a resistor must be connected between the output of the microcircuit and the base.

The shortcoming noted at the end of the description can also have a positive role. If you work all the time with the same load, false alarms can occur from an emergency increase in current consumption (which indicates a load malfunction - short circuits, failure of elements, etc.). In this case, it is quite easy to identify the cause of disconnections - after automatic load disconnection, connect a current meter in parallel with the contacts of the power button. By the magnitude of the current, one can judge the serviceability of the load.

Author: J.Akkad

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