12Vdc mobile battery charger circuit diagram can delivers up to 20 V output from the 12V car power supply, allowing a constant current charging to NiCad battery unit around 18 V max.
V1 forms a square wave oscillator, D1 and D2, coupling this square to supply 12-V battery to reach more than 20 VDC. If this is not necessary, S1 can be left open. Q1 form a current regulator to determine the charge level of rechargeable batteries. R4 is taken from a table or can be activated with a rotary selector switch. The following table is the value of R4 for charging various battery types :
| Cell Size | Amp/hr Rate | R4 value (14 hr rate) |
| N | 150 mA | 120 ohms (a .25-watt |
| AA | 500 mA | 47 ohms @ .5 watt |
| C | 1500 mA | 12 ohms © .5 watt |
| D | 1500 mA | 12 ohms @ .5 watt |
| D (High capacity) | 4000 mA | 3.3 ohms @ 2 watt |
Parts list :
R1 : 390K
R2 : 2.2M
R3 : 2.2K
R4 : 10 ohm
C1 : 1uF
C2, C3 : 220uF
D1,D2 : 1N4002
Q1 : 2N3055T
U1 : TDA2003
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