The Battery Charge Process

Introduction

Batteries are complex mechanisms that can even fool the experts at
times, so it comes as no surprise that non-technical people have a
hard time understanding the charge process. Ask a typical crowd of
battery users when their batteries are full charged and at least ten
answers will surface.

In both Living on 12 Volts with Ample Power, and Wiring 12 Volts for
Ample Power the authors explain that a battery is fully charged when
the voltage is about 14.4 Volts and current through the battery has
declined to less than 2% of the capacity of the battery in Amp-
hours ...2 Amps for a 100 Ah battery.

That information is substantially correct, however, a more intuitive
feel for the charge process is necessary, not only to understand when
the battery is full, but also to know when the battery is not
behaving normally. It is the intent of this application note to
provide enough information about the charge process so that the
average user can judge how well the batteries are charging.

The Bulk Charge Step

When a charge source is first applied to a well discharged battery,
charge current begins to flow, typically at the maximum rate of the
charge source. If a true 40 Amp charger is connected to an 8D battery
which is completely discharged, about 40 Amps of charge current would
flow for some period of time. Because most of the charge is delivered
at the maximum charger rate, the first step of the charge cycle is
called the bulk charge step. NOTE: During the bulk step, battery
voltage will steadily rise.

The Start of the Absorption Step

At the instant battery voltage has risen to the maximum allowable
voltage of the charge source, current through the battery begins to
decline. This simultaneous event of reaching maximum voltage and the
start of current decline marks the beginning of the absorption step.

For instance, if the 40 Amp charger is set to 14.4 Volts, then when
battery voltage has risen to 14.4 Volts, the charger will now hold
the voltage constant. Current through the battery will begin to
decline. NOTE: The charger, (or alternator), is not limiting the
current at this point. The battery is `absorbing' all it can at the
voltage setpoint.

The End of the Absorption Step

The absorption step should continue until current through the battery
declines to about 2% of battery capacity in Amp-hours as mentioned
above. Without knowing what the current is through the battery, you
can't know when it's full. Just because that fancy charger, (or
inverter/charger), has kicked out to float is no sign that the
battery is full ...there is no charger on the market that measures
battery current!

It's a given, then, that you need to measure battery current to know
when the battery is full. With a battery current meter, you can
discover some very interesting details about the charge process. For
instance, you can discover that once the charger voltage limit is
reached, battery current begins to decline. If the current decline is
rapid, either the batteries are nearly full, or they are NO GOOD! If
the current decline is slow, then either the charge source has more
output than the batteries can reasonably absorb, or the batteries are
NO GOOD! Here's where Amp-hour instrumentation is particularly
valuable.

Given enough time at the absorption voltage, charge current will
decline to a steady-state value, that is, a low current that either
stays constant, or declines very little. At the point where charge
current has gone as low as it is going to, then the batteries are
truly full. While 2% of Ah rating is close, good batteries will reach
a steady state current at less than 1% of Ah rating.

The Float Step

Once a battery is full, a lower voltage should be applied that will
maintain the full charge. Depending on the type of battery, (liquid,
gel), and the age of the battery, 13.4 - 13.8 Volts is appropriate as
a float voltage.

Temperature Compensation

The voltage given above are good only at F, (C). For high
temperatures, voltage will be less. It is important to charge
batteries with temperature compensation. To learn more about this
aspect of charging, refer to page 70 in the revised edition of Wiring
12 Volts for Ample Power.

A Very Common Problem

Your batteries are only four months old. You discharge them until
their voltage is less than 11 Volts and then crank up the engine. The
alternator brings up the voltage to 14.4 Volts very quickly, but the
current begins to decline immediately and in a few minutes is down to
a few Amps. You:

*suspect your voltage regulator and immediately call the factory and
ask for a replacement to be sent out; OR
*realize that something has happened to the batteries because the
alternator and regulator are operating as expected.

Conditioning Batteries

How do batteries that are only four months old die? Perhaps they
weren't broken in properly; maybe they sat deeply discharged for a
few days or more; perhaps they were allowed to self-discharge over
the last four months ...there's plenty of ways to murder batteries.

All batteries that refuse to accept a charge are not necessarily
ready for the scrap heap. Often, a deep discharge followed by a slow
charge will recover lost capacity and charge acceptance. For more
information, refer to Wiring 12 Volts for Ample Power.