12V Powered 12V Lead Acid Battery Charger with Indicator


Some of you might wonder
why a charger is needed at all, to charge a 12 Volt battery from a 12
Volt source! Well, firstly the “12 Volt” source will typically vary
anywhere from 11 Volt to 15 Volt, and then a battery needs a controlled
charge current and voltage, which cannot result from connecting it
directly to a voltage source. The charger described here is intended for
charging small 12 Volt lead acid batteries, such as the gelled or AGM
batteries of capacities between about 2 and 10 Ah, using a car’s
electrical system as power source, regardless of whether the car engine
is running or not. I built this charger many years ago, I think I was
still in school back then. On request of a reader of my web site, I’m
publishing it now, despite being a rather crude circuit.

The outside of the charger

The outside of the charger

It works, it is uncritical to build, and uses only easy-to-find
parts, so it has something in its favor. The downside is mainly the low
efficiency: This charger wastes about as much power as it puts into the
battery. The charger consists of two stages: The first is a capacitive
voltage doubler, which uses a 555 timer IC driving a pair of transistors
connected as emitter followers, which in turn drive the voltage doubler
proper. The doubler has power resistors built in, which limit the
charging current. The second stage is a voltage regulator, using a 7815
regulator IC. Its output is applied to the battery via a diode, which
prevents reverse current and also lowers the voltage a bit.

The inside of the charger

The inside of the charger

The resulting charge voltage is about 14.4V, which is fine for charging a gelled or AGM
battery to full charge, but is too high as a trickle charger, so don’t
leave this charger permanently connected to a battery. If you would like
to do just that, then add a second diode in series with D3! There is a LED
connected as a charge indicator. It will light when the charge current
is higher than about 150mA. The maximum charge current will be roughly
400mA. There is an auxiliary output, that provides about 20V at no load
(depending on input voltage), and comes down as the load increases. I
included this for charging 12V, 4Ah NiCd packs, which require just a
limited current but not a limited voltage for charging.

The schematic of the charger

The schematic of the charger

Note that if the charge output is short-circuited, the overcurrent
protection of U2 will kick in, but the current is still high enough to
damage the diodes, if it lasts. So, don’t short the output! If instead
you short the auxiliary output, the fuse should blow. I built this
charger into a little homemade aluminum sheet enclosure, using dead-bug
construction style. Not very tidy, but it works. Note the long leads on
the power resistors. They are necessary, because with shorter leads the
resistors will unsolder themselves, as they get pretty hot! The
transistors and the regulator IC are bolted to the case, which serves as
heat sink. The transistors don’t heat up very much, but the IC does.

Source: Homo Ludens


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