Battery Charger Regulator

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Most off-the-shelf car battery chargers
cannot not be left connected to the battery for long periods of time as
over-charging and consequent battery damage will occur. This add-on
circuit is placed in series with the battery being charged and is
powered by the battery itself. In effect, the circuit uses a
high-current Mosfet to control the charging current and it turns off
when the battery voltage reaches a preset threshold. Power for the
circuit is fed from the battery to 3-terminal regulator REG1 which
provides 8V.

LED1 indicates that the battery is connected and that power is
available. The 555 timer IC is configured as an astable oscillator
running at approximately 100kHz. It feeds a diode pump (D1 & D2) to
generate adequate gate voltage for Mosfet Q3, enabling it to turn on
with very little on resistance (typically 14 milliohms). With the Mosfet
turned on, current flows from the charger’s positive terminal so that
charging can proceed. The battery voltage is monitored by 10kO pot VR1.

Battery Charger Regulator Circuit

Battery Charger Regulator Circuit Diagram

When the wiper voltage exceeds the conduction voltage of zener diode
ZD1, transistor Q1 turns on and pulls pin 4 (reset) low to switch off
the 555 and remove gate drive to the Mosfet. This process is progressive
so that the cycle rapidly repeats itself as the battery charges.
Eventually, a point is reached when the battery approaches its charged
condition and the cycle slows right down. Transistor Q2 and LED2
function as a cycle indicator. When the battery is under charge, LED2
appears to be constantly on. When the battery is fully charged, LED2
briefly flicks off (charging) and returns to the on state (not charging)
for a longer period.

Author: Paul Walsh