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LABC1 • 6
plates on a regular basis or over an extended period of time, the charging
process will not be able to restore the battery to its former full potential. Time
to make a costly battery replacement!
Circuit Description
The LABC1 has been designed as a dependable workhorse to charge and
hold your 12 Volt lead acid batteries at their peak level, insuring a long life and
maximum performance. The charging procedure used when working with a
flooded ‘wet’ cell battery or one of the newer VRLA (Valve Regulated Lead
Acid – ‘Gel’ or ‘AGM’) batteries is the same. The battery being charged will
automatically set the LABC1 in one of two charging modes upon hookup. The
circuit design takes into account the battery’s current SOC (State Of Charge)
and adjusts the terminal voltage at J2 accordingly. The main charging circuit is
very simple because as we discussed before, the concept of lead acid
batteries has been around for centuries (give or take a few thousand years if
you don’t believe in the ‘Space Alien’ theory). The real secret to correctly
charging a lead acid battery system is to use a temperature compensated
voltage source that automatically varies its output in accordance with the
batteries SOC. ‘Frying’ your battery occurs when the charging unit fails to
sense that the electro-chemical rejuvenation (or charging) process has slowed
to the point that the higher voltage charging mode should end. Continual high
voltage charging will decrease the overall life of the battery.
Let’s take a closer look at the LABC1 schematic and see what’s happening.
The power supply inlet for the LABC1 is J1. The input voltage is immediately
presented to a full wave bridge rectifier consisting of diodes D1 to D4 and then
filtered by C1 to reduce the voltage ripple. Using a bridge configuration on the
voltage input allows the user more options to power their LABC1. The use of a
14 VAC or 20 VDC (positive tip) power supply will do nicely with any 12 Volt
lead acid battery. Varying your power supplies current capacity will allow you
to charge any type of lead acid battery without a problem. Most of the
standard cells require a charging current of 650mA or greater. For these
systems a 14 VAC (2 Amps or so) transformer will work very well. If your
application is to charge very small capacity batteries with a maximum charge
current of only a few hundred milliamps, using a 14 VAC @ 500mA ‘wall wart’
supply or a current limited bench-top power supply set for 20 VDC will avoid
excessive current draw that could damage a heavily discharged battery.
Internal heating from excessive charge current will also degrade your overall
battery life.
Moving on, VR1 is a voltage regulator that provides the precision terminal
voltage we need to charge the lead acid cells. Unlike a standard voltage
regulator that is designed for a fixed level output, VR1 lends itself well as a
variable voltage source. With a maximum current source capability of about
1.3 amps, VR1 gives the user the flexibility to charge even very large capacity