AMATEUR RADIO
By GARRY CRATT, VK2YBX
Build this intelligent charger
for 12V gel batteries
Do you use 12V gel cells to power your transceiver
or other equipment? If so, you need to know the
best way to charge them so that they last as long
as possible. The "intelligent charger" described
here does all the right things to look after the
welfare of your 12V gel cells.
When it comes to charging sealed
lead acid batteries used in an
amateur station, most operators
use their standard 13.BVDC power
supply. There is a big drawback to
this method because, by definition,
a 13.BV DC power supply puts out a
constant 13.8 volts. Such a power
supply may deliver a very much
higher current to a discharged or
partially discharged battery than is
advisable.
In fact, gel cell or sealed lead
acid batteries are very fussy about
being over-charged, even at low
rates. They should not be charged
SOURCE
COMPENSATION
15
14
Charging current
+VIN
,------6-----6-----<
r + l - - - 0 1 3 VOLTAGE
SENSE
....,,...--1----012 CHARGE
ENABLE
POWER 7
INDICATE
9 OVER-CHARGE
INDICATE
OVER-CHARGE B
TERMINAL
Fig.1: block diagram of the UC3906 IC. It controls both the output voltage and
charging current to ensure optimum charging conditions.
66
SILICON CHIP
at a constant current or 'trickle
charged' unless the terminal
voltage of the battery is monitored
and the charging current is terminated immediately full charge is
reached. Voltage limited charging
is most suitable for this type of
battery.
The correct charging procedure
for sealed lead acid batteries is to
charge them at the optimum current, until a maximum voltage of
2.42 volts per cell is reached. This
equates to a voltage of 14.52V for a
"12V" battery comprising 6 cells.
At this level, the charging current
should drop to a level sufficient to
maintain the full charge voltage of
13.68V (2.28V per cell).
The "optimum" current depends
on the battery temperature, its
state of discharge and lastly, the
battery capacity which is expressed in ampere/hours. Typically
though, the recommended charging
current varies from a maximum of
C/5 to C/10 where C is the capacity
expressed in ampere hours at the
20-hour rate.
For example, a 4.5 amp/hour battery can deliver 225mA at the
20-hour rate. At C/5, the charging
rate would be 900mA. At C/10, the
charging rate would be 450mA.
Provided sealed lead acid batteries are carefully charged and
are not over-charged to the point
where significant gassing occurs,
they can have very long life in
standby or "floating" use where
they sit across a fixed power supply. They can last between 5 to 10
The parts are installed on a small PC board fitted with quick-connect lugs for the input and output terminals. It delivers
a maximum charging current of 500mA. A flag heatsink must he fitted to Qt for input voltages greater than 20V.
R1
0.5P. 5W
+
SUPPLY
INPUT
-i
01
1N5404
BATTERY
R3
1k
5
3
16
i-
R2
68k
1%
15
11
12
R4
22k
IC1
UC3906
1%
13
R5
10 270k 1%
R6
18k 1%
.,.
14
BCE
12V GEL BATTERY CHARGER
Fig.2: this circuit shows the UC3906 connected as a dual level float
charger. It's pin 16 output controls PNP power transistor Qt which
in turn handles the charging current.
years. However, designing a charger for such a standby application
is not an easy task, at least until
recently.
Fortunately, there is now an
"intelligent" battery charging chip
available, which has been designed
specifically for this purpose by
Unitrode Corporation of the USA.
The UC2906 and UC3906 battery
charger controllers contain all the
necessary circuitry to optimally
control the charge and hold cycle
for sealed lead acid batteries.
These integrated circuits monitor
and control both the output voltage
and current of the charger through
three separate charge states: a
high current bulk charge state, a
controlled over charge, and a precision float charge or standby state.
Optimum charging conditions are
maintained over an extended
temperature range with an internal
reference that tracks the nominal
temperature characteristics of the
lead acid cell.
Separate limit amplifiers regulate the output voltage and current levels in the charger by controlling the onboard driver. The
driver will supply up to 25mA of
base drive to an external power
transistor. Voltage and current
comparators are used to sense the
battery condition and provide inputs to the charge state logic.
In addition, a charge enable comparator with a trickle bias output
can be used to obtain a low current
turn-on mode for the charger. This
feature prevents high current
charging during abnormal conditions such as a shorted battery cell.
Other features include a supply
under-voltage sense circuit with a
logic output to indicate when input
JULY 1989
67
r------------._
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STATE 2
l
STATE 3
I
IT
!OCT
,..L,
}-vT
IMAX
STATE 1 : BULK CHARGE
STATE 2 : OVER•CHARGE
STATE 3 : FLOAT CHARGE
CHARGER OUTPUT CURRENT
Fig.3: the circuit starts off in state 1
(bullc charge) and then switches to
state 2 (over-charge) and finally to
state 3 (float charge).
power is present. In addition, the
over-charge state of the charger
can be externally monitored and
terminated using the over-charge
indicate output and the over-charge
terminate input.
Fig.1 shows the block diagram of
the UC3906. It comes in a standard
16-pin dual inline plastic package.
Maximum input voltage is 40 volts
DC.
A practical circuit
Fig.2 shows the UC3906 connected as a dual level float charger.
CHARGE
VOLTAGE
----::---.---:svoc
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IT
Registered address: 3/105 Hawthom Road
Caulfield North, 3161
Please send aWIA information package to:
NAME: ........................................................... .
ADDRESS: ..................................................... .
STATE
LEVEL
OUTPUT
oc
INDICATE
OUTPUT
oc
TERMINATE
INPUT
(C/S OUT)
~~] __ _
I
OFF
................................... POSTCODE ............... ..
L-------------.1
E001qii
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-
SILICON CHIP
Ail high currents are handled by
the external PNP pass transistor
Ql which is a readily available
plastic pack MJE2955. This circuit
uses the trickle bias output at pin
11 and the charge enable comparator at pin 13 to give the
charger a low current turn-on
mode.
The output current of the
charger is limited to a low level until the battery reaches a specified
voltage, preventing high current
charging if a battery cell happens
to be shorted. Of course, if you did
have a battery with a shorted cell,
you would have to discard it.
Fig.3 shows the various charge
states of the circuit. At switch-on,
the charger goes into state 1, the
high rate bulk charge state. In this
state, once the enable threshold has
been exceeded, the charger will
supply a peak current that is determined by the 250mV offset of the
current limit amplifier (monitoring
between pins 5 and 4) and the sensing resistor Rl.
Our circuit shows Rl with a
value of 0.50 so the peak current
value will be 500 milliamps.
To guarantee full recharge of the
battery, the charger's voltage loop
has an elevated reguhiting level
_J _______________ _
INPUT
SUPPLY
VOLTAGE
CHARGE
CURRENT
P.O. BOX300
CAULFIELD SOUTH
VICTORIA 3162
68
y
STATE 1
Learn more about the WIA and
Amateur Radio
Forward this coupon, or write to:
WIA EXECUTIVE OFFICE
- - v oc
- -v 12
- -v
L r- - v 31
~
STATE 1 ..
1.
I
i ~--.....,_J-_-STATE 2
I•
STATE 3 •
I.
STATE 1
Fig.4: this diagram shows a charge and discharge cycle for a
dual level float charger. Once the battery is charged, it is
maintained at a precise float voltage (Vf).
RESISTORS
D
D
D
D
D
D
No.
1
1
1
1
1
1
Value
270k0
68k0
22k0
18k0
1k0
0.50 SW
4-Band Code
not applicable
pot applicable
not applicable
not applicable
brown black red gold
not applicable
5-Band Code
red violet black orange black
blue grey black red black
red red black red black
brown grey black red black
brown black black brown brown
not applicable
Fig.5 (above): parts layout for the PC board. Fig.6 at
right shows the actual size PC pattern.
(Voc) during state 1 and state 2.
When the battery voltage reaches
95% of Voc, the charger enters the
over-charge state, state 2.
The charger stays in this state
until the "over-charge terminate"
pin goes high. The charger uses the
current sense amplifier to generate
this signal by sensing when the
charge current has tapered to a
specified level, Ioct. So pin 1 is connected to pin 8.
As an alternative, the over·charge terminate point could have
been controlled by an external
source, such as a timer, by using
the "over-charge indicate" signal
at pin 9.
If a load is applied to the batte'ry
and begins to discharge it, the
charger will contribute its full output to the load. If the battery drops
10% below the float level, the
charger will reset itself to state 1,
the bulk charge condition.
When the load is removed, a full
charge cycle will follow. A
graphical representation of a
charge and discharge cycle of the
dual level float charger is shown in
Fig.4.
When the charger is in the float
state, the battery is maintained at a
precise float voltage, Vf. The accuracy of this float state will maximise the standby life of the battery,
whilst the bulk charge and over
charge states guarantee rapid and
full recharge.
All of the voltage thresholds on
the UC3906 are derived from the internal reference. This reference
has a temperature coefficient that
tracks the temperature characteristic of the optimum charge and
hold levels for sealed lead acid batteries. This further guarantees that
proper charging occurs, even at
temperature extremes.
External supply
Because the series pass transistor, Ql, is a PNP type, the supply
input does not need to be very much
PARTS LIST
1 PCB, code SC 11107891,
77 x 41mm
4 quick connector lugs
Semiconductors
1 UC3906 battery charger
controller (IC1)
1 MJE2955 PNP transistor
(01)
1 1N5404 power diode (D1)
Capacitors
1 1 OOpF ceramic
Resistors (¼W unless stated)
1 270k0 1 %
1 18k0 1 %
1 68k0 1 %
1 1kO 5 %
1 22k0 1%
1 0 .50 5W
higher than the maximum output
voltage of the charger. This means
that the input voltage can range upwards from 15V DC, with 18-20V
being ideal. The higher the input
voltage to the circuit, the higher the
dissipation in Ql.
For the circuit as shown, which
delivers a maximum of 500mA, no
heatsink is required for Ql for input voltages up to a bout 20 volts
DC. For higher voltages, a flag heatsink will be required.
Construction
To provide a basis for experimentation, we have designed a small
printed circuit board measuring 76
x 40mm (coded SC 11107891). The
component layout diagram is Fig.5.
Our prototype board has been fitted
with quick-connect lugs for the input and output terminals but these
are optional.
Note that all the resistor values,
with the exception of Rl and R3,
should be 1 % tolerance to make
sure that the design targets for
over-charge voltage and float
voltage are obtained. For this
design, the float voltage is 13.8V
and the over-charge voltage is
14.56V.
The UC3906 is available from
VSI Electronics Pty Ltd, PO Box
578, Crows Nest, NSW 2065. Phone
(02) 439 8622.
~
JULY 1989
69
