It is a 12 V SLA (Sealed Lead Acid) battery charger circuit

ref : http://www.talkingelectronics.com/projects/BatteryCharger-12vSLA/BatteryCharger-12vSLA.html

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This circuit was specifically designed to recharge alkaline cells. The unusual connection of the transistor in each charging unit will cause it to oscillate, on and off, thus transferring the charge accumulated in the capacitor to the cell. The orange LED will blink for around 5 times a second for a 1.37V cell. For a totally discharged cell the blinking is faster but it will decrease until it will come to a stop when the cell is charged. You may leave the cell in the charger as it will trickle charge and keep it at around 1.6V. To set the correct voltage you have to connect a fresh, unused cell and adjust the trimmer until oscillations set in, then go back a little until no oscillation is present and the circuit is ready to operate. You should use only the specified transistors, LED colors, zener voltage and power rating because they will set the final voltage across the cell. A simple 9V charging circuit was also included: it will charge up to around 9.3V and then keep it on a trickle charge: the green LED will be off while charging and will be fully on when the battery is close to its final voltage.

A 2.5VA transformer will easily charge up to 4 cells at the same time although 2 only are shown in the schematic. In order to minimize interference from one circuit to the other they have nothing in common except the transformer and, in order to show a balanced load to the transformer, half of the charging units will use the positive sinewave and the other half the negative sinewave. Make sure to use high beta transistors such as BC337-25 or better BC337-40. Given the dispersion of the transistor parameters it might happen that oscillations do not take place. Use a slightly higher zener voltage: 7.5V instead of 6.8 or a green led in place of the orange ones.

All types of alkaline cells can be recharged: it will take 1 day for a discharged AA cell or 9V battery and up to several days for a large D type cell. The best practice is not to discharge completely the cell or battery but rather to give a short charge every so often although admittedly this is not easy to achieve. Do not attempt to recharge a totally discharged cell or a cell showing even the slightest sign of damage.

I tried successfully to recharge NiMH cells as well. Although the charging profile for these cells is quite different from alkaline cells, the circuit seems to work fine provided you do not leave them in the charger forever, because of the possibility of overcharging especially for the smaller batteries.

The mains transformer must be suited for the voltage available in each country: usually 230Vac or 115Vac.

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Presently we have a variety of cells or batteries to choose from; lead acid, zinc carbon, Nickel cadmium and Nickel metal hydride, lithium, silver oxide and what not!Cell is a storage form of electrical energy. When more cells are used, it is known as battery, but these words are used interchangeably. These are used where portability of the equipment is needed and some times as a back up for the mains voltage. Each these cells have their own characteristics, like cell voltage, current capability, storage capacity and rechargeability.

Good old Zinc carbon cells have 1.5 volts and if two cells are fixed in series, they show 3V. These are the most common cells powering most of the portable devices and are available in various sizes known variously as AAA, AA, BB, CC etc. This nomenclature denotes the capacity and dimensions of each cell but all these cells show a voltage of 1.5 volts. These are not normally rechargeable. Alkaline batteries last longer.

Lead acid cells are most widely used for high power applications such as in automobiles, UPS systems, emergency lighting systems where weight of the battery is no concern but the capacity required is high. Sealed lead acid batteries or maintenance free batteries are another form of the same where the electrolyte is within the moistened separators. Lead acid batteries can deliver high currents and recharge cycles are high. Each cell voltage is 2 volts and in a typical car battery, six of such cells are connected in series in a single container to give 12 V.

Nickel cadmium cells (Ni. Cad) show a voltage of 1.2 volts, which means that two cells in series show a voltage of 2.4 volts. These are rechargeable. They suffer from a memory effect. If they are recharged after only partially discharged, they tend to recharge only to that partial extent rather than to their full capacity.

Nickel metal hydride cells (Ni. Mh) also show a voltage of 1.2 volts and are rechargeable. They show certain better characteristics than nickel cadmium cells. They are environmentally friendly and do not show the memory effect ofNi-Cad cells.

Lithium ion cells are the fast emerging technologies. These have higher energy density and most of the present day cell phones, laptops; handy-cams are powered by these cells. They should be always used with protection circuit. Charge them only with specified charger. They have a cell voltage of’3.6 volts

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