QUESTIONS & ANSWERS
adequate to maintain the batteries.
There are two ways to regulate
the voltage: one is a shunt which
bypasses the battery when it is fully
charged. The problem is that the
regulator will have to dissipate 30
watts at times when the battery is
fully charged. The other way is a
series transistor to turn off the
current when the battery is fully
charged. I like this better because the
power dissipation is less.
In Figure 4, I use an op-amp to
compare the battery voltage with a
fixed reference. When the divided
battery voltage rises to equal the
fixed reference, the series transistor
(Q1) will be shut off. The positive
feedback provides hysteresis so that
the charge — once shut off — will not
start until the voltage drops about 0.2
volts. The LED (D1) lights to show
that the system is charging. The
diode in series with Q1 prevents the
negative lead of the solar panel from
going to ground through the internal
diode of Q1.
To calibrate the charger, connect
a fully charged battery ( 13. 8 VDC)
and adjust R4 until the LED just goes
out.
BTW, I have used a smaller solar
panel (four 3V cells) and connected it
directly to the battery (two 12V truck
batteries). The batteries stayed up for
three years until I sold the truck.
■ FIGURE 5.
charge up to 12 at one time
individually as some may not be
present at times while in use. (I have
heard charging Li-Ion cells requires
different circuits than do NiMH or
NiCAD) The charging station will
permanently be in a vehicle and
powered off 12 VDC to 14. 4 VDC
(vehicle batteries). A slow charge rate
is acceptable if necessary.
— Geoff Mayberry
LITHIUM-ION
BATTERY CHARGER
AThe lithium-ion cell is charged from a current source with a voltage limit. Service life is reduced by
charging to higher voltages; 4. 2 volts
is the max. Since your application has
the cell in the charger most of the
time, charging to a lower voltage for
longer service life will probably be
desirable. A 4.0 volt limit will give
82% capacity, and charge time will
be several hours.
QI’m working on a project hat I need a schematic for and hope you can help me.
First, I need a circuit to charge
a 4. 2 VDC 900 mAh rechargeable
Li-Ion prismatic cell (All Electronics
Corp.; cat# LBATT- 60; $2.50 ea.
They are listed as Iomega
p/n31021100).
I’m building rechargeable
lighting devices that will drop into a
recharge station and will likely spend
most of their lives on it, so
overcharging is a big issue. Each unit
will contain one of the Li-Ion packs,
and the charger should be able to
LITHIUM-ION CHARGER PARTS LIST
PART DESCRIPTION
R1, R6 1.3 OHMS, 5%, 1/4W
R2 10K, 5%, 1/4W
R3 3.9K, 5%, 1/4W
R4, R9 1K, 5%, 1/4W
R5 75K, 5%, 1/4W
R7 680 OHMS, 5%, 1/4W
R8 2 OHMS, 5%, 1/4W
Q1 P-MOS, 20V, 24A, LOGIC LEVEL
Q2 PNP, 40V,100 mA
D1 DIODE, 40V, 100 mA
IC1 DUAL, RAIL/RAIL OP-AMP
IC2 4V REFERENCE, TO-92
IC3 8V REGULATOR, 40V IN, 1A
C1 1,000 µF, 35V, 20%
C2 0.1 µF, 10%, 50V CERAMIC
■ FIGURE 6.
MOUSER PART #
594-5073NW1R300J
271-10K-RC
271-3.9K-RC
271-1K-RC
271-75K-RC
271-680-RC
594-5073NW2R000J
512-NDP6020P
512-3906TA
78-1N4148
863-MC33202PG
595-LM4040C41ILP
511-L7808ACV
647-UHE1V102MHD
810-FK18X7R1H104K
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