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A Fast Charger

Turn a low-cost SMPS into a battery charger

A Fast Charger
Here’s how three Elektor readers came up with an idea for a fully automatic fast charger to keep the batteries in prime condition. This low-cost design uses an off-the-shelf switch-mode power supply coupled to an intelligent controller and can be easily adapted to charge other types of cell.
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Extra info / Update
• For fast chargeable 12 V lead acid batteries.
• 3.5 hour fast charge.
• Protected against short-circuit and reverse polarity connection.
• Optimised charging curve.
• Based on a modified low-cost industry standard SMPS.
• High efficiency from 76 %.
• Dead battery detection.
• The firmware supports 9 Ah cells, other cell capacities can be accommodated.
• Suitable for lead-acid and LiPo rechargeable cells.
Component list
R1,R7 = 18kOhm
R2 = 6.8kOhm
R3 = 2.7kOhm
R4 = 470Ohm
R5 = 1.5kOhm
R6 = 1kOhm
R8 = 12kOhm
R9 = 0.1Ohm 2W
R10 = 27kOhm
R11 = 1.2kOhm
P1 = 500Ohm, multiturn preset
(SMD 1206)
C1,C2 = 1µF 25V, electrolytic or multilayer
C3,C4 = 100nF
C5,C6 = 10nF
D1 = 50WQ03FN, Schottky, SMD TO252AA
D3 = 4-pin RGB LED, 5mm, common anode*
T1 = BC846, SMD SOT23
T2 = FDD5614, SMD TO252AA
IC1 = ATtiny24, SMD SO14
IC2 = LM78L05A, SMD SO8
K1 = 3-pin PCB plug, lead pitch 2.54mm (0.1 in.)
K2 = 2-pin PCB plug, lead pitch 2.54mm (0.1 in.)
K3 = 6-pin double row pinheader
SMPS 12V, 5A *
Silicon diode for SMPS, e.g. 1N4148*
Firmware* for IC1
* see article text
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