Power Supplies 693In this case it acts like a battery of much smaller size.
19.10.6 Ampere-Hour Capacity
Deep cycle batteries are rated in ampere hours (Ah). An
Ah is a 1 A drain for 1 h, 10 A for 0.1 h, etc. It is calcu-
lated with the equation A × h. Drawing 20 A for 20 min
would be 20 A × 0.333 h, or 6.67 Ah. The accepted Ah
rating time period for batteries used in solar electric and
backup power systems and for nearly all deep cycle bat-
teries is the 20 hour rate. This is defined as the battery
being discharged to 10.5 V over a 20 h period while the
total actual Ah it supplies is measured.
Amp-hours are specified at a particular rate because
of the Peukert effect. The Peukert value is directly
related to the internal resistance of the battery. The
higher the internal resistance, the higher the losses
while charging and discharging, especially at higher
currents. The faster a battery is discharged, the lower
the Ah capacity. Conversely, if it is drained more
slowly, the Ah capacity is higher.
19.10.7 Battery Charging
Batteries can be charged by constant current or constant
voltage. When charged by the constant-current method,
care must be taken to eliminate the possibility of over-
charging; therefore, the condition of the battery should
be known before charging so that the charger can be
removed when the ampere-hour rate of the battery is met.
Charging with the constant voltage method reduces
the possibility of overcharging. With the constant
voltage method, charge current is high initially and
tapers off to a trickle charge when the battery is fully
charged. Two requirements must be met when using the
constant-voltage method:
- The charging voltage must be stable and set to 2.4 V
per cell for a lead-acid battery and 2.30 V per cell for
a gel cell battery. Gel cell open-circuit voltage is
2.12 V per cell. - A current-limiting circuit must be employed to limit
charge current when the battery is fully discharged.
A good battery charger charges in three steps. In the
first stage, charge current is at the maximum safe rate
the batteries will accept until the voltage rises to
80–90% of full charge level. Voltages at this stage typi-
cally range from 10.5–15 V. There is no correct voltage
for bulk or charge charging, but there may be limits on
the maximum current that the battery and/or wiring can
accept.
In the second stage, accept, the voltage remains
constant and current gradually tapers off as internal
resistance increases during charging. Voltages are typi-
cally 14.2–15.5 V.
After batteries reach full charge, the third stage
charging voltage is reduced to a lower level,
12.8–13.2 V, to reduce gassing and prolong battery life.
This is often referred to as a maintenance, float, or
trickle charge, since its main purpose is to keep an
already charged battery from discharging, Fig. 19-28.
An ideal charging state table is shown in Table 19-4.PWM, or pulse width modulation is sometimes used
as a float or trickle charge. In PWM chargers, the
controller circuit senses small voltage drops in the
battery and delivers short charging cycles (pulses) to the
battery. This may occur several hundred times per
minute and is called pulse width because the width of
the pulses varies from a few microseconds to several
seconds.
Most flooded batteries should be charged at no more
than the C/8 rate for any sustained period. C/8 is the
battery capacity at the 20 hour rate divided by 8. For a
220 Ah battery, this would equal 26 A. Gelled cells
should be charged at no more than the C/20 rate, or 5%
of their amp-hour capacity. AGM batteries can be
charged at up the C × 4 rate, or 400% of the capacity for
the bulk charge cycle.
Lead acid batteries require 15.5 V for 100% charge.
When the charging voltage reaches 2.583 V/cell,
charging should be stopped or reduced to a trickleFigure 19-28. Ideal charge curve.Table 19-3. Ideal Charging State
Cycle Voltage Current
Charge 12.0–14.3 Rising Maximum
Accept 14.4 Constant Falling
Float 13.5 Constant Small (<2% capacity)
Equalize 13.2–16.0 Rising Constant until 16.0 V