This paper describes the operation and application of the
UC3906 Sealed Lead-Acid Battery Charger. This IC provides reductions in the cost and design effort of implementing optimal charge and hold cycles for lead-acid batteries.
Described are the design and operation of several charging circuits using this IC. The charger designs use current
and voltage sensing combined with sequenced current
and voltage control to maximize battery capacity and life
for various applications. The presented material provides
insight into expected improvements in battery performance with respect to these specific charging methods.
Also presented are uses of the many auxiliary functions
included on this part. The unique combination of features
on this control IC has made it practical to create charge
and hold cycles that truly get the most out of a battery.
Battery technology has come a long way in recent years.
Driven by the reduction of size and power requirements of
processing functions, batteries now are used to provide
portability and failsafe protection to a new generation of
electronic systems. Although a number of battery technologies have evolved, the lead-acid cell remains the workhorse of the industry due to its combination of prolonged
standby and cycle life with a high energy storage capacity.
The makers of uninterruptible power supplies, portable
equipment, and any system that requires failsafe protection are taking advantage of the improvements in this technology to provide secondary power sources to their products, for example, the sealed cell, using a trapped or gelled
electrolyte, has eliminated the positional sensitivity and
greatly reduced the dehydration problem.
The charging methods used to replenish or maintain the
charge on a lead-acid battery have a significant effect on
the performance of the cells. Building an optimum charger,
one that gets the most out of a battery, is not a trivial task.
Making sure that a battery undergoes the proper charge
and hold cycle requires precision sensing and control of
both voltage and current, logic to sequence the charger
through its cycle, and temperature corrections — added to
the charger’s control and sensing circuits — to allow
proper charging at any temperature. In the past this has
required a significant number of components, and a substantial design effort as well. The UC3906 Sealed Lead-
FIGURE 1. The UC3906 Sealed Lead-Acid Battery Charger combines precision voltage and current sensing with voltage and current control to realize optimum battery charge cycles. Internal charge state logic sequences the device
through charging cycles. Voltage control and sensing is referenced to an internal voltage that specially tracks the
temperature characteristics of lead-acid cells.
Acid Battery Charger has all the control and sensing functions necessary to optimize cell capacity and life in a wide
range of battery applications.
The block diagram for the UC3906 is shown in figure 1.
Separate voltage loop and current limit amplifiers regulate
the output voltage and current levels in the charger by controlling the onboard driver The driver will supply 25mA of
base drive to an external pass element. Voltage and current sense comparators are used to sense the battery condition and respond with logic inputs to the charge state
logic. The charge enable comparator on this IC can be
used to remotely disable the charger The comparator’s
25mA trickle bias output is active high when the driver is
disabled. These features can be combined to implement
a low current turn-on mode in a charger, preventing high
current charging during abnormal conditions such as a
shorted or reversed battery.
A very important feature of the UC3906 is its precision
reference. The reference voltage is specially temperature
compensated to track the temperature characteristics of
lead-acid cells. The IC operates with very low supply current, only 1.7mA, minimizing on-chip dissipation and permitting the accurate sensing of the operating environmental temperature. In addition, the IC includes a supply
under-voltage sensing circuit, used to initialize charging
cycles at power on. This circuit also drives a logic output to
indicate when input power is present. The UC3906 is specified for operation over the commercial temperature range
of 0°C to 70°C. For operation over extended temperatures,
-40°C to 70°C the UC2906 is available.
During the charge cycle of a typical lead-acid cell, lead sulfate, PbSO4, is converted to lead on the battery’s negative
plate and lead dioxide on the battery’s positive plate. Once
the majority of the lead sulfate has been converted, overcharge reactions begin. The typical result of over-charge is
the generation of hydrogen and oxygen gas. In unsealed
batteries this results in the immediate loss of water. In
sealed cells, at moderate charge rates, the majority of the
hydrogen and oxygen recombine before dehydration
occurs. In either type of cell, prolonged charging rates significantly above C/500, will result in dehydration, accelerated grid corrosion, and reduced service life.
The onset of the over-charge reaction will depend on the
rate of charge. At charge rates of >C/5, less than 80% of
the cell’s previously discharged capacity will be returned
as the over-charge reaction begins. For over-charge to
coincide with 100% return of capacity, charge rates must
typically be reduced to less than C/100. Also, to accept
higher rates the battery voltage must be allowed to
increase as over-charge is approached. Figure 2 illustrates
this phenomenon, showing cell voltage vs. percent return
of previously discharged capacity for a variety of charge
rates. The over-charge reaction begins at the point where
the cell voltage rises sharply, and becomes excessive
when the curves level out and start down again.
Capacity and life are critical battery parameters that are
strongly affected by charging methods. Capacity, C, refers
to the number of ampere-hours that a charged battery is
rated to supply at a given discharge rate. A battery’s rated
capacity is generally used as the unit for expressing
charge and discharge current rates, i.e., a 2.5 amp-hour
battery charging at 500mA is said to be charging at a C/5
rate. Battery life performance is measured in one of two
ways; cycle life or stand-by life. Cycle life refers to the number of charge and discharge cycles that a battery can go
through before its capacity is reduced to some threshold
level. Standby life, or float life, is simply a measure of how
long the battery can be maintained in a fully charged state
and be able to provide proper service when called upon.
The measure which actually indicates useful life expectancy in a given application will depend on the particulars
of the application. In general, both aspects of battery life
will be important.
FIGURE 2. Depending on the charge rate, over-charge reactions begin, (indicated by the sharp rise in battery voltage), well below 100% return of capacity.
(Reprinted with the permission of Gates Energy Products. Inc.)
Once a battery is fully charged, the best way to maintain
the charge is to apply a constant voltage to the battery. This
burdens the charging circuit with supplying the correct
float charge level; large enough to compensate for self-discharge, and not too large to result in battery degradation
from excessive overcharging. With the proper float charge,
sealed lead-acid batteries are expected to give standby
service for 6 to 10 years. Errors of just five percent in a float
charger’s characteristics can halve this expected life.
This charger, called a dual level float charger, has three
states, a high current bulk charge state, an over-charge
state, and a float state. A charge cycle begins with the
charger in the bulk charge state. In this state the charger
acts like a current source providing a constant charge rate
at IMAX. The charger monitors the battery voltage and as it
reaches a transition threshold, VIZ, the charger begins its
over-charge cycle. During the over-charge, the charger
regulates the battery at an elevated voltage, Voc, until the
charge rate drops to a specified transition current, lock.
When the current tapers to Iocr, with the battery at the elevated level, the capacity of the cell should be at nearly
100%. At this point the charger turns into a voltage regulator with a precisely defined output voltage, VF. The output voltage of the charger in this third state sets the float
level for the battery.
To compound the above concerns, the voltage characteristics of a lead-acid cell have a pronounced negative
temperature dependence, approximately -4.0mV/°C per
2V cell. In other words, a charger that works perfectly at
25°C may not maintain or provide a full charge at 0°C and
conversely may drastically over-charge a battery at
+50°C. To function properly at temperature extremes a
charger must have some form of compensation to track the
battery temperature coefficient.
With the UC3906, this charge and hold cycle can be implemented with a minimum of external parts and design effort.
A complete charger is shown in figure 4. Also shown are
the design equations to be used to calculate the element
values for a specific application. All of the programming of
the voltage and current levels of the charger are determined by the appropriate selection the external resistors
To provide reasonable re-charge times with a full 100%
return of capacity, a charge cycle must adapt to the state
of charge and the temperature of the battery. In sealed, or
recombinate, cells, following a high current charge to
return the bulk of the expended capacity, a controlled overcharge should take place. For unsealed cells the overcharge reaction must be minimized. After the over-charge,
or at the onset of over-charge, the charger should convert
to a precise float condition.
Operation of this charger is best understood by tracing a
charge cycle. The bulk charge state, the beginning, is initiated by either of two conditions. One is the cycling on of the
input supply to the charger; the other is a low voltage condition on the battery that occurs while the charger is in the
float state. The under-voltage sensing circuit on the
UC3906 measures the input supply to the IC. When the
input supply drops below about 4.5V the sensing circuit
forces the two state logic latches (see figure 1) into the bulk
charge condition (L1 reset and L2 set). This circuit also disables the driver output during the under-voltage condition.
To enter the bulk charge state while power is on, the
charger must first be in the float state (both latches set). The
input to the charge state logic coming from the voltage
sense comparator reports on the battery voltage. If the battery voltage goes low this input will reset L1 and the bulk
charge state will be initiated.
A state diagram for a sealed lead-acid battery charger that
would meet the above requirements is shown in figure 3.
With L1 reset, the state level output is always active low.
While this pin is low the divider resistor, RB is shunted by
resistor Rc, raising the regulating level of the voltage loop.
If we assume that the battery is in need of charge, the voltage amplifier will be in its stops trying to turn on the driver
to force the battery voltage up. In this condition the voltage
amplifier output will be over-ridden by the current limit
amplifier. The current limit amplifier will control the driver,
regulating the output current to a constant level. During this
FIGURE 3. The dual level float charger has three charge states. A constant
current bulk charge returns 70-90% of capacity to the battery with the remaining
capacity returned during an elevated (constant) voltage over-charge. The float
charge state maintains a precision voltage across the battery to optimize
stand-by Iife.
time the voltage at the internal, non-inverting, input to the
voltage sense comparator is equal to 0.95 times the internal
reference voltage. As the battery is charged its voltage will
rise; when the scaled battery voltage at PIN 13, the inverting input to the sense comparator, reaches 0.95Vref the
sense comparator output will go low. This will reset the second latch and the over-charge state will be entered. At this
time the over-charge indicator output will go low. Other
than this there is no externally observable change in the
charger Internally, the starting of the over-charge state
arms the set input of the first latch – assuming no reset signal is present -– so that when the over-charge terminate
input goes high, the charger can enter the float state.
charge current is less than Ioc~, (25mVIRs) the open collector output of the comparator will be off. When this transition current is reached, as the charge rate tapers in the
over-charge state, the off condition of the comparator output will allow an internal 1OyA pull-up current at PIN 8 to pull
that point high. A capacitor can be added from ground to
this point to provide a delay to the over-charge-terminate
function, preventing the charger from prematurely entering the float state if the charging current temporarily drops
due to system noise or whatever. When the voltage at PIN
8 reaches its 1V threshold, latch L1 will be set, setting L2 as
well, and the charger will be in the float state. At this point
the state level output will be off, effectively eliminating Rc
from the divider and lowering the regulating level of the voltage loop to VF.
In the over-charge state, the charger will continue to supply
the maximum current. As the battery voltage reaches the
elevated regulating level, Voc, the voltage amplifier will
take command of the driver, regulating the output voltage
at a constant level. The voltage at PIN 13 will now be equal
to the internal reference voltage. The battery is completing
its charge cycle and the charge acceptance will start to
taper off.
In the float state the charger will maintain VF across the
battery, supplying currents of zero to IMAX as required. In
addition, the setting of L1 switches the voltage sense comparator’s reference level from 0.95 to 0.90 times the internal
reference. If the battery is now discharged to a voltage level
10% below the float level, the sense comparator output will
reset L1 and the charge cycle will begin anew.
As configured in figure 4, the current sense comparator
continuously monitors the charge rate by sensing the voltage across Rs. The output of the comparator is connected to the over-charge terminate input. Whenever the
The float voltage VF, as well as Voc and the transition voltages, are proportional to the internal reference on the
UC3906. This reference has a temperature coefficient of
FIGURE 4. Using a few external parts and following simple design equations the UC3906 can be configured as a dual level float charger.
-3.9mV/°C. This temperature dependence matches the
recommended compensation of most battery manufacturers. The importance of the control of the charger’s voltage levels is reflected in the tight specification of the tolerance of the UC3906’s reference and its change with temperature, as shown in figure 5.
An alternative method for controlling the over-charge state
is to use the over-charge indicate output, PIN 9, to initiate
an external timer. At the onset of the over-charge cycle the
over-charge indicate pin will go low. A timer triggered by
this signal could then activate the over-charge terminate
input, PIN 8, after a timed over-charge has taken place.
This method is particularly attractive in systems with a centralized system controller where the controller can provide
the timing function and automatically be aware of the state
of charge of the battery.
The float, VF, and over-charge, Voc, voltages are set by
the internal reference and the external resistor network,
RA, Rs, and Rc as shown in figure 4. For the dual level float
charger the ranges at 25°C for VF and Voc are typically
2.3V-2.40V and 2.4V-2.7V, respectively. The float charge
level will normally be specified very precisely by the battery
manufacturer, little variation exists among most battery
suppliers. The over-charge level, Voc, is not as critical and
will vary as a function of the charge rate used. The absolute
value of the divider resistors can be made large, a divider
current of 5Ofi will sacrifice less than 0.5% in accuracy
due to input bias current offsets.
FIGURE 5. The specially temperature compensated reference on the UC3906
is tightly specified over 0 to 70°C (-40 to 70°C for the UC2906). to allow proper
charge and hold characteristics at all temperatures.
Besides simply charging batteries, the UC3906 can be
used to add many related auxiliary functions to the charger
that would otherwise have to be added discretely. The
enable comparator and its trickle bias output can be used
in a number of different ways. The modification of the state
diagram in figure 2 to establish a low current turn-on mode
IMAX, Iocr, Voc, and VF can all be set independently. IMAX,
the bulk charge rate can usually be set as high as the available power source will allow, or the pass device can handle. Battery manufacturers recommend charge rates in the
C/20 to C/3 range, although some claim rates up to and
beyond 2C are OK if protection against excessive overcharging is included. Iocr, the over-charge terminate
threshold, should be chosen to correspond, as close as
possible, to 100% recharge. The proper value will depend
on the over-charge voltage (Voc) used and on the cell’s
charge current tapering characteristics at Voc.
IMAX and locr are determined by the offset voltages built
into the current limit amplifier and current sense comparator respectively, and-the resistor(s) used to sense current.
The offsets have a fixed ratio of 250mV/25mV. If ratios other
than ten are necessary separate current sensing resistors
or a current sense network, must be used. The penalty one
pays in doing this is increased input-to-output differential
requirements on the charger during high current charging. Examples of this are shown in figure 6.
FIGURE 6. Although the ratio of input offset voltages on the current limit and
current sense stages IS fixed at 10. other ratios for l~~xllom are easily obtained.
Note that a penalty for ratios greater than 10 is increased voltage drop across
the sensing network at h4X.
of the charger (see figure 7) is easily done. By reducing the
output current of the charger when the battery voltage is
below a programmable threshold, the charging system
protects against: One, high current charging of a string
with a shorted cell that could result in excessive outgassing
from the remaining cells in the string. Two, dumping charge
into a battery that has been hooked up backwards. Three,
excessive power dissipation in the charger’s pass element.
As shown in figure 7, the enable comparator input taps off
the battery sensing divider, When the battery voltage is
below the resulting threshold, VT, the driver on the
UC3906 is disabled and the trickle bias output goes high.
A resistor, RT, connected to the battery from this output
can then be used to set a trickle current, (I 25mA) to the
battery to help the charger discriminate between severely
discharged cells and damaged, or improperly connected,
path, the divider in the figure is referenced to the open collector power indicate output, PIN 7, instead of ground.
Connected in this manner the divider string will be in series
with essentially an open when input power is removed.
When power is present, the open collector device will be
on, holding the divider string end at nearly ground. The
saturation voltage of the open collector output is specified
to be less than 50mV with a load current of 5Ocp\.
In applications where the charger is integral to the system,
i.e. always connected to the battery, and the load currents
on the battery are very small, it may be necessary to absolutely minimize the load on the battery presented by the
charger when input power is removed. There are two simple precautions that, when taken, will remove essentially all
reverse current into the charging circuit. In figure 8 the
diode in series with the pass element will prevent any
reverse current through this path. The sense divider
should still be referenced directly to the battery to maintain
accurate control of voltage. To eliminate this discharge
This scheme uses a relay between the battery and its load
that is controlled by Q1 and the presence of voltage across
the load. When primary power is available Q1 is on via D5.
The battery is charging, or charged, and the trickle bias
output at PIN 11 is off. When input power is removed, C2
provides enough hold-up time at the load to let Q1 turn off,
and the relay to close as current flows through R1. The battery is now providing power to the load and, through D1,
power to the charger. The charger current draw will typically be less than 2mA. As the battery discharges, the
UC3906 will continue to monitor its voltage. When the vol-
Figure 9 illustrates the use of the enable comparator and
its output to build over-discharge protection into a charger.
Over-discharging a lead-acid cell, like over-charging, can
severely shorten the service life of the cell. The circuit monitors the discharging of the battery and disconnects all load
from the battery when its voltage reaches a specified cutoff
point. The load will remain disconnected from the battery
until input power is returned and the battery recharged.
FIGURE 7. The charge enable comparator, with its trickle bias output, can be used to build protection into the charger. The current foldback at low battery voltages
prevents high current charging of batteries with shorted cells, or improperly connected batteries, and also protects the pass element from excessive power dissipation.
tage reaches the cut-off level, set by the divider network,
R5-R8, the trickle bias output, PIN 11, will go high. Q1 will
turn back on and the relay current will collapse opening its
contacts. As the load voltage drops, capacitor Cl supplies
power to the UC3906 to keep Q1 on. Once the input to the
charger has collapsed the power indicate pin, as shown in
figure 8, will open the divider string. The battery will remain
open-circuited until input power is returned. At that time the
battery will begin to recharge.
FIGURE 8. By using a diode in series with the pass element, and referencing
the divider string to the power indicate pin. pin 7, reverse current into the
charger, (when the charger is tied to the battery with no input power), can
be eliminated.
When large series strings of batteries are to be charged, a
dual step current charger has certain advantages over the
float charger of figures 3 and 4. A state diagram and circuit
implementation of this type of charger is shown in figure 10.
The voltage across a large series string is not as predictable as a common 3 or 6 cell string. In standby service
varying self discharge rates can significantly alter the state
of charge of individual cells in the string if a constant float
voltage is used. The elevated voltage, low current holding
state of the dual step current charger maintains full and
equal charge on the cells. The holding, or trickle current,
IH, will typically be on the order of 0.005C to 0.0005C.
To give adequate and accurate recharge this charger has
a bulk charge state with temperature compensated transition thresholds, VI*, and Vzl. Instead of entering an elevated voltage over-charge, upon reaching VI2 the charger
switches to a constant current holding state. The holding
current will maintain the battery voltage at a slightly elevated level but not high enough to cause significant overcharging. If the battery current increases, the charger will
attempt to hold the battery at the V F level as shown in the
state diagram. This may happen if the battery temperature
increases significantly, increasing the self-discharge rate
beyond the holding current. Also, immediately following
the transition from the bulk to float states, the battery will
only be 80% to 90% charged and the battery voltage will
drop to the VF level for some period of time until full charging is achieved.
In this charger the current sense comparator is used to regulate the holding current. The level of holding current is
determined by the sensing resistor, PsH. The other series
FIGURE 9. Using the enable comparator to monitor the battery voltage a precise discharge cut-off voltage can be set.
When the battery reaches the cut-off threshold the trickle bial output switches off the load switch relay and the battery is
left open circuited until input power IS returned.
resistor, RE, is necessary for the current sense comparator
to regulate the holding current. Its value is selected by
dividing the value of IH into the minimum input to output
differential that is expected between the battery and the
input supply. If the supply variation is very large, or the
holding current large, (> 25mA), then an external buffering
element may be required at the output of the current sense
There are four factors to consider when choosing a pass
device. These are:
1. The pass device must have sufficient current and power
handling capability to accommodate the desired maximum charging rate at the maximum input to output
The operating supply voltage into the UC3906 should be
kept less than 45V. However, the IC can be adapted to
charge a battery string of greater than 45V. To charge a
large series string of cells with the dual step current
charger the ground pin on the UC3906 can be referenced
to a tap point on the battery string as shown in figure 11.
Since the charger is regulating current into the batteries,
the cells will all receive equal charge. The only offset results
from the bias current of the UC3906 and the divider string
current adding to the current charging the battery cells
below the tap point. RB can be added to subtract the bulk
of this current improving the ability of the charger to control
the low level currents. The voltage trip points using this
technique will be based on the sum of the cell voltages on
the high side of the tap.
2. The device must have a high enough current gain at the
maximum charge rate to keep the drive current required
to less than 25mA.
3. The type of device used, (PNP, NPN, or FET), and its
configuration, may be dictated by the minimum input to
output differential at which the charger must operate.
4. The open loop gain of both the voltage and the current
control loops are dependent on the pass element and its
Figure 12 contains a number of possible driver configurations with some rough break points on applicable current
ranges as well as the resulting minimum input to output differentials. Also included in this figure are equations for the
dissipation that results on the UC3906 die, equations for a
resistor, Ro, that can be added to minimize this dissipation, and expressions for the open loop gains of both the
voltage and current loops.
FIGURE 10. A dual step current charger has some advantages when large series strings must be charged. This type of charger maintains constant current during
normal charging that results in equal charge distribution among battery cells.
As reflected in the gain expressions in figure 12, the open
loop voltage gains of both the voltage and current control
loops are dependent on the impedance, Zc at the compensation pin. Both loops can be stabilized by adjusting
the value of this impedance. Using the expressions given,
one can go through a detailed analysis of the loops to predict respective gain and phase margins. In doing so one
must not forget to account for all the poles in the open loop
expressions. In the common emitter driver examples, 1
and 3, the equivalent load impedance at the output of the
charger directly affects loop characteristics. In addition, a
pole, or poles, will be added to the loop response due to
the roll-off of the pass device’s current gain, Beta. This
effect will occur at approximately the rated unity gain frequency of the device divided by its low frequency current
gain. The transconductance terms for the voltage and current limit amplifiers, (1/1.3K and 1/300 respectively), will
start to roll off at about 500KHZ. As a rule of thumb, it is wise
to kill the loop gain well below the point that any of these,
not-so-predictable poles, enter the picture.
FIGURE 11. A dual step current charger can be configured to operate with
input supplies of greater than 45V by using a tap on the battery to reference
the UC3906. The charger uses the voltage across the upper portion of the
battery to sense charging transition points. To minimize charging current
offsets, Rs can be added to cancel the UC3906 bras and divider currents.
the 0.22pF value will be required to roll off the large open
loop gain that results from the Beta squared term in the
gain expression. Series resistance should be less than 1K,
and may range as low as 100 ohms and still be effective.
If you prefer not to go through a BODE analysis of the loops
to pick a compensation value, and you recognize the fact
that battery chargers do not require anything close to optimum dynamic response, then loop stability can be assured by simply oversizing the value of the capacitor used
at the compensation pin. In some cases it may be necessary to add a resistor in series with the compensation
capacitor to put a zero in the response. Typical values for
the compensation capacitor will range from 1000pF to
0.22pF depending on the pass device and its configuration. With composite common emitter configurations, such
as example 3 in figure 12, compensation values closer to
The power dissipated by the UC3906 requires attention
since the thermal resistance, (100°C/Watt) of the DIP
package can result in significant differences in temperature between the UC3906 die and the surrounding air,
(battery), temperature. Different driver/pass element configurations result in varying amounts of dissipation at the
UC3906. The dissipation can be reduced by adding external dropping resistors in series with the UC3906 driver,
FIGURE 12. There are a large number of possible driver/pass element configurations, a few are summarized here. The trade-offs are between current gain, input to output
differential. and in some cases, power dissipation on the UC3906. When dissipation is a problem it can be reduced by adding a resistor in series with the UC3906 driver.
(see figure 12). These resistors will then share the power
with the die. The charger parameters most affected by increased driver dissipation are the transition thresholds,
(VI2 and V& since the charger is, by design, supplying its
maximum current at these points. The current levels will not
be affected since the input offset voltages on the current
amplifier and sense comparator have very little temperature dependence. Also, the stand-by float level on the
charger will still track ambient temperature accurately
since, normally, very little current is required of the charger
during this condition.
To estimate the effects of dissipation on the charger’s voltage levels, calculate the power dissipated by the IC at any
given point, multiply this value by the thermal resistance of
the package, and then multiply this product by -3.9mV/°C
and the proper external divider ratio. In most cases, the
effect can be ignored, while in others the charger design
must be tweaked to account for die dissipation by adjusting charger parameters at critical points of the charge
Input supply voltage . . . . . . . . . . . . 9.0V to 13V
Operating temperature range . . . . . 0°C to 70°C
Start-up trickle current (IT) . . . . . . . 10mA o/IN = 10V)
Start-up voltage (VT) . . . . . . . . . . . . 5.1V
Bulk charge rate (IMAX) . . . . . . . . . . 500mA (C/5)
Bulk to OC transition voltage (VIZ) . . 7.125V
OC voltage (Voc) . . . . . . . . . . . 7.5V
OC terminate current (IOCT) . . . . . . .50mA (C/50)
Float voltage (VF) . . . . . . . . . . . . . . 7.0V
Float to Bulk transition
voltage (V3,) . . . . . . . . . . . . . . . . 6.3V
Temperature coefficient on
voltage levels . . . . . . . . . . . . . . . -12mV/°C
Reverse current at charger output
with the input supply at 0.0V . . . . 15fi
In order to achieve the low input to output differential,
(1.5V) the charger was designed with a PNP pass device
that can operate in its saturation region under low input
supply conditions. The series diode, required to meet the
reverse current specification, accounts for 1.0V of the 1.5V
minimum differential. Keeping the reverse current under
~,uA also requires the divider string to be disconnected
when input power is removed. This is accomplished, as
discussed earlier, by using the input power indicate pin to
reference the divider string.
In figure 13 the schematic is shown for a dual level, float
charger designed for use with a 6V, 2.5amp-hour, sealed
lead-acid battery. The specifications, at 25°C, for this
charger are listed below.
FIGURE 13. This dual level float charger was designed for a 6V (three 2V cells) 2.5AH battery. A separate “fully
charged” indicator was added for visual indication of charge completion.
The driver on the UC3906 shunts the drive current from the
pass device to ground. The 470ohm resistor added
between PIN 15 and ground keeps the die dissipation to
less than 100mW under worst case conditions, assuming
a minimum forward current gain in the pass element of 35
at 500mA.
The charger in figure 13 includes a circuit to detect full
charge and gives a visual indication of charge completion
with an LED. This circuit turns on the LED when the battery
enters the float state. Entering of the float state is detected
by sensing when the state level output turns-off.
Figures 14-16 are plots of charge cycles of the circuit at
three temperatures, 25°C, 50°C and 0°C. The plots show
battery voltage, charge rate, and percent return of previously discharged capacity. This last parameter is the integral of the charge current over the time of the charge cycle,
divided by the total charge volume removed since the last
full charge. For all of these curves the previous discharge
was an 80% discharge, (2amp-hours), at a C/10, (250mA),
rate. The discharges were preceded by an over-night
charge at 25°C.
FIGURE 14. The nearly ideal characteristics of the dual level float charger are
illustrated in these curves. The over-charge state IS entered at about 80% return
of capacity and float charging begins at just over 100% return.
The less than 100% return of capacity evident in the
charge cycle at 0°C is the result of the battery’s reduced
capacity at this temperature. The tapering of the charge
current in the over-charge state still indicates that the cells
are being returned to a full state of charge.
Eagle-Picher Industries, Inc., Battery Notes #200,
#205A, #206, #207, #208.
2. Gates Energy Products, Inc., Battery Application
Manual, 1982.
3. Panasonic, Sealed Lead-Acid Batteries Technical
4. Yuasa Battery Co., Ltd., NP series maintenance-free rechargeable battery Application Manual.
FIGURE 15. At elevated temperatures the maximum capacity of lead-acid
cells is increased allowing greater charge acceptance. To prevent excessive
over-charging though, the charging voltage levels are reduced.
FIGURE 16. At lower temperatures the capacity of lead-acid cells is reduced as
reflected by the less-than-100% return of capacity in this 0°C charge cycle, illustrating the need for elevated charging voltages to maximize returned capacity.
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