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12V DC Trickle Charger
Method · June 2019
DOI: 10.13140/RG.2.2.21832.37127
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Justin Jose P
College of Engineering Trivandrum
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12V DC Trickle Charger
Trickle charging means charging a fully charged battery at a
rate equal to its self-discharge rate, thus enabling the battery to
remain at its fully charged level; this state occurs almost
exclusively when the battery is not loaded, as trickle charging
will not keep a battery charged if current is being drawn by a
load. A battery under continuous float voltage charging is said
to be float-charging.
A 12Ah battery should tolerate a constant trickle current of 100
mA, but battery manufactures recommend float charge for long
term storage. The difference is that a float charger limits the
voltage to 13.8 volts for lead-acid batteries of six cells.
A bridge rectifier will give a peak voltage of about 17.5 volts ;
with a full wave bridge rectifier, the dc voltage will be about 16
volts peak. Referring to figure the battery is float charged to 13.8volts, the voltage at Q1 collector will be 14.3 v which
means the zener(D2) must be 13.8V. R1 shunts any zener leakage to ground to prevent Q1 from turning on prematurely.
R2 limits the peak current to less than 1 ampere: (16-14.3)/4=0.425 amp and the average current- 0.135 amp.
When battery is fully charged, all the currents flows through Q1 and its power dissipation is : 14.3*0.135=1.9watts.
D3 is necessary to protect Q1 in case the generator charging circuit tries to charge the battery to more than 13.8 volts.
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