Exponential charger for small batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells
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The proposed charger for small-sized batteries (NiCd and similar) charges with a decreasing exponential current change. This allows you to get the most complete charge and thereby extend the life of the batteries.
To explain the exponentiality of the charge process, we use the equivalent circuit (Fig. 1).
Connect the capacitor C in series with the resistor R to a constant voltage source Uin. We assume that at the initial moment the capacitor is completely discharged. Then the capacitor will be gradually charged through the resistor R to the value Uin and the voltage across the resistor R will change in accordance with the curve shown in Fig. 2.
A diagram of a charger that implements the described principle is shown in Fig. 3.
The rechargeable battery G1 is connected to the current-setting capacitor C1 through a buffer stage on a field-effect transistor VT1, which has a large input resistance and does not load the capacitor. Resistor R1 sets the initial charge current. Capacitor C1 and resistor R5 are timing elements. Resistor R4 limits the accidental short-circuit current of the battery.
Devices PA1 and PV1 serve to control the charging process. After inserting the battery into the charger (the SA3 toggle switch is open and the supply voltage is not supplied), its initial voltage is measured using PV1. NiCd batteries have a "memory effect", therefore, before charging, they should be discharged to approximately 1 V. To do this, the SA2 toggle switch closes and the voltage on the battery is monitored using the PV1 device. After reaching a voltage of 1 V on G1, SA2 opens and SA3 closes.
Resistor R1 with the SA1 toggle switch closed sets the initial charge current equal to (0,3 ... 0,5) C (C is the nominal capacity of the battery). In this mode, G1 is charged for 3...4 hours. Further, the SA1 toggle switch opens, and the further charge goes in an exponential mode. The battery is not recharged, so you can safely leave it in the charger for a long time, for example, overnight.
The resistance R5 and capacitance C1 are selected so that the time constant T of the exponential circuit is:
Т=С1R5=20...000 (s).
In terms of hours, this is 5 ... 6 hours. In the described device, the capacitance of the capacitor is 4700 uF, and the resistance of the resistor is 5,1 MΩ. Due to the fact that the input resistance of the field-effect transistor VT1 is very large, the resistance R5 can reach 10 ... 15. MOhm.
The power supply for the charger must have an output power of at least 10 W at a voltage of 9 V. It does not need to install a filter capacitor after the rectifier.
Authors: A.Partin, L.Partina, Yekaterinburg
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