battery from an 11-volt solar array
(I don’t think it has more than one
amp of output current). Would it be
possible to charge two at once and
form a single 7.2 volt battery?
AYour power source requires
a charger with high efficiency and a range of input
variables — one that includes long periods of no power and
varying current. The output current is
proportional to the intensity of the
light falling on the solar cells, whereas the voltage remains more or less
constant (Figure 6). The ideal battery
charger for this situation would be a
low dropout regulator controlled by a
lithium-ion battery charge controller
With a discharged battery
connected to the charger, the circuit
operates as a constant current source.
When the current reaches 1A, it is
sensed by the 0.5-ohm resistor, which
produces 50 mV. This 50 mV is amplified by the op-amp to produce 1.27
volts, which is applied to the feedback
pin of the LM2941. Once the battery
voltage reaches 8.4V, the LM3420
takes over and begins to control the
feedback pin of the LM2941. The
LM3420 now regulates the voltage
across the battery, and the charger
becomes a constant-voltage charger.
When the sun goes down, the 1N5818
diode becomes reversed biased and
effectively disconnects the battery to
prevent it from discharging through
■ FIGURE 6
capacity? And how?
are wired in series
and charged from a
source. In extreme
the battery is discharged very deeply,
one of the cells can
charged — but it’s
more likely to occur
in NiCd batteries
than in gel cells.
Uneven battery cells lead to increased
sulfating and reduced battery life.
The solution to unequal battery
voltages is called equalization. This is
a charge that is about 10% higher than
normal full charge voltage — 14. 4
volts for a 12-volt battery — and is
applied for about two to 16 hours.
Flooded batteries should be placed on
an equalizing charge every 10 to 40
days. Gel cells should be equalized
only two to four times a year — at
most. The equalization process works
best if you separate the individual batteries and equalize them separately.
THE RUST TOLLS
QI have two questions about
1) We know that the battery life is
determined by the number of
charge/discharge cycles (200-500
cycles). What exactly happens inside
the battery after each charge/discharge
cycle that causes the remaining number
of cycles to be reduced by one count?
QI have an electric wheel chair
that uses two 12-volt gel
cells in series for 24 volts DC.
Over the years, I’ve noticed
that after the batteries have been discharged (unevenly), I can no longer get
a uniform charge out of the on-board
switching charger. One battery will
reach a high voltage and the other will
be about 0.5 volts lower. I suspect that
the internal battery resistance is rearing its ugly head. Why do the batteries
fail to balance out? Can they be balanced so they each will have the same
10V to 15V
+ 2 cells
■ FIGURE 7
February 2006 15