Lead-Acid (SLA) battery specifications generally prescribe a
sequence of three charging stages or modes for fast charging. This
the following form (4Ah Diamec
used as examples):
1. Charge at a current of 0.4C amps until the terminal voltage reaches
limit typically of 2.5V per cell (7.5V for a 6 volt and 15V for a 12
volt battery, but only for SLA AGM type batteries). C is the rated
battery capacity in Ampere-Hours (AH)
which should be marked on the battery case.
2. Continue to charge at that constant voltage until the current falls
to a value typically around 0.005C. At that point the battery is
essentially fully charged.
3. Charge the battery at a reduced float charge voltage of 2.3V per
cell (6.9V for a
6 volt and 13.8V for a 12 volt battery). This can be held indefinitely.
These specifications do not hold for other types of lead-acid batteries
such as wet cells. SLA batteries are generally of the Absorbed Glass
Mat (AGM) construction and can tolerate higher voltages and currents.
Effects of temperature need to be taken into account, and the
above numbers should be taken only as representative of any SLA AGM
battery and only at standard room temperature.
The chargers found in consumer equipment typically require a dc voltage
to be supplied and will charge the battery through a resistor for
current limiting. Sometimes
a zener diode controlled transistor switch will be used to prevent
exceeding the specified maximum voltage limit. These very simple
circuits are not optimal and do not
change to the third float charge stage when the battery is fully
charged. This may reduce the life of the battery if it is left on
charge too long.
The best way to approach the charging of the higher capacity lead-acid
batteries is to use an
efficient switch-mode power source that can be controlled either by a
dedicated chip or a microcontroller; the latter providing greater
flexibility to adapt to the charging process and to reconfigure itself
for different battery types and capacities (see e.g. Atmel
Application Note AVR450). A project of this
type will eventually be described
elsewhere on this site.
For the lower capacity SLA batteries, simpler analogue chargers can be
built with a handful of discrete
components to provide an
automatic fast charge, and can even be made
to operate in each of the three modes
without manual intervention. The
circuits shown below are designed to charge 6V SLA batteries with a
12-14V supply and consist of three sections, one for each of the
1. A constant current circuit set to give the specified maximum current
for the first charging stage.
2. A constant voltage circuit set to limit the voltage to the maximum
voltage for the second charging stage.
3. A circuit to drop the voltage limit when the current becomes
circuit uses forward biassed diodes in the base circuit of a
BJT to fix the voltage across an emitter resistor, and hence set the
current. Normally a zener diode would be used to provide a stable
However this circuit needs to be able to work down to quite low power
supply voltages, so the forward voltage drop of a diode is used, which
is nominally taken to be about 0.7V. One of the
diodes shown in the circuit is intended to cancel the base-emitter
voltage of the BJT and the second is to force the voltage across R2 to
diode voltage and the base-emitter voltage of the BJT have a strong
logarithmic dependency on the current, they nevertheless are not
circuit shown was tested with a dc sweep of the battery voltage (using gEDA tools and ngspice). The
battery is represented by VBattery which is an ideal voltage source,
but this is sufficient for comparative testing of the circuit. battery
voltage is along the x-axis and current is along the y-axis.
decrease in the controlled current observed in the plot below is due to
variations in the base current of Q1 affecting the current available to
Q2. When the voltage across Q1 and R2 drops below 1.4V the BJT and
starved of voltage and the current drops away sharply.
circuit is adapted from that given by Anantha
The controlled current is set by R2 and the base-emitter voltage of Q2,
typically about 0.7V. R1 must be chosen to provide the base current of
Q1 and also the collector current
of Q5 which in turn sets its precise base-emitter voltage. If Q1 and Q5
are operating in active mode, then a momentary increase in current
result in an increase in the base-emitter voltage of Q2 which will
in turn cause the base-emitter voltage of Q1 to decrease. This will
current back to a stable value. The advantage of using a
two BJT circuit of this form is that the current can be set more
precisely than the diode circuit and doesn't vary a great deal under
conditions. The strong temperature variations that will occur in the
power transistor Q1 will have little effect on the circuit operation.
Q2 is isolated from the changing voltage and temperature environment,
and is able to do the bulk of the work relatively stress free.
The value of R1
affects the range over which
the current is reasonably constant and needs to be chosen carefully. It
is important that R1 be connected to a constant supply voltage so that
variations in the current through it
The simulations showed a very similar dependency of controlled current
with battery voltage.
limiting in the second phase of charging can be achieved using a common
circuit in which the battery is placed in the emitter circuit of a BJT
that has its base voltage limited by a zener diode.
In this circuit an extra diode has been added to provide additional
drop. This will later be used as part of a strategy to reduce the
charge voltage limit for the third phase of charging.
The voltage limit on the battery is to be 7.5V. While current is
flowing into the battery, the diode will cancel the base-emitter
voltage of the BJT.
Therefore the zener would normally be chosen as 7.5V. However the base-emitter
voltage of the BJT will decrease
as the current through the BJT and the battery
voltage decrease is not matched by any changes in the diode
voltage, thus causing the turn off to stretch
up to higher battery voltages. This would allow the battery voltage to
exceed 7.5V in the
latter stages of the charging. As such we would select a lower value
zener diode to start turning off the BJT earlier.
We wish to
when the current into the battery has fallen to a preset value
in the second phase of charging. We can do
this by sensing the voltage across a resistor in series with the
and using this to
reduce the voltage limit allowed for charging. The voltage limiting
circuitry described above has two devices for voltage limiting, a zener
diode and a forward biassed diode. We can use the switch to short
circuit the diode and so reduce the voltage limit from the nominal 7.5V
to the float charge level 6.9V.
shown has the constant current circuit at the bottom. This allows the
collector resistor of Q6 to be taken to the power rail and so
provide a reasonably stable current to Q6.
A resistor bypassed with a diode has been added into the battery
circuit to provide a measurement of the charging current.
This provides base
drive to Q4 which will be turned on while the battery current is
during the first two charging phases. The base of Q3 will be pulled low
and Q3 will be off.
When the current has dropped to the preselected level to signal the end
of charging, the base voltage of Q4 will drop and Q4 will start to turn
off. The base voltage of Q3 will rise and it will switch on. This will
short circuit the diodes in series with the zener diode, further
pulling down the base of Q2 and forcing it to jump rapidly into
complete cutoff. From that time on, the battery characteristics take
over (these are not modelled here). Its terminal voltage will gradually
decrease to the point that Q2 will start to turn back on and allow
float charge current to flow. However this
current will not be enough to turn Q4 back on again, and the battery
voltage will stabilise at the lower float voltage. The capacitor C1 is
necessary to prevent premature switching with noise and to allow the
circuit to startup in the bulk charge phase.
are optional and provide a visual indication of charge termination.
circuit uses a power supply of 12V. However this is insufficient
to ensure that the current is maintained as the battery charges towards
the 7.5V limit. Q2 is pushed into cutoff prematurely causing the
current to droop after the battery voltage reaches about 6.5V. The
was generated with a slightly higher power supply of 14V.
shows a Spice dc sweep simulation of the battery voltage showing the
change in battery current
(magenta) up to the point where the switch circuit cuts in Beyond that
point the results are meaningless as the battery voltage never reaches
to those levels. The
blue curve shows how the voltage across the current sense resistor
varies as the current falls, while the red curve is the
collector-emitter voltage of Q3 impressed across the diode chain.
circuit as shown does work but not quite in the way that was hoped.
Firstly the current sense resistor, unless it is very small,
is dominated by the characteristics of the diode D5 and the turn-on
characteristics of Q4, and might as well be omitted. Thus the trigger
for switching is in fact somewhat higher than that desired and is
controlled only by the selection of the diode (which must be able to
handle the expected power dissipation). We need a better circuit for
this purpose, involving a small value resistor and a high gain
amplifier. This could be achieved with low cost operational amplifiers.
When the battery enters the final trickle charge phase the current is
small but non-zero. This results in a voltage across the base-emitter
of Q2 that can be upwards of 0.5V. Therefore the
zener diode does in fact need to be higher in value as shown, and only
two additional diodes used in the chain. The
variable and unknown voltage offsets in the BJTs can be eliminated also
by means of
The circuit exhibits hysteresis. As such the battery needs to
be discharged below a certain level in order for the circuit to begin
reliably in the high current bulk charging phase (a capacitor at the
base of Q3 can
help provided the battery is connected before power is applied). A push
button switch at the base of Q3 can also be used to manually reset the
circuit. This will raise the voltage cutout limit temporarily and allow
high current to flow. This current will in turn disable the switch.
operational amplifier chip (14 pin DIP) will
cost barely more than two small signal BJTs. In the above circuit three
or four BJTs can be replaced and a lower cost, conceptually simpler and
precise circuit will result.
Choose a current sensing resistor small enough that the voltage across
it at the highest current flow is negligible in the circuit. The value
0.7V is a suitable value but could be smaller. This will eliminate any
need for diode limiting. The circuit is to switch to float charge
when the current falls to the selected trigger point: for the example
Diamec batteries the
maximum current is to be 0.4C and the termination current is to be
An operational amplifier can be used, boosted with a power BJT, to set
a constant voltage between the BJT emitter and a more negative point.
This simple device can be used to establish both a constant current
circuit and a voltage limiting circuit to replace those used
in the original circuit above. If a comparator were to be used rather
operational amplifier then all four of the small signal BJTs may be
eliminated with the four comparators in a quad package. Note that it is
worthwhile including diode protection for the LM324 positive power in
case the supply is reversed.
amplifier chosen is the LM324 which is a low cost quad opamp device
that can be used with a single supply. In the circuit
shown the BJT of the constant current circuit is replaced with the
operational amplifier X3 to ensure that the voltage across R2 is
precisely that of the diode D4. The diode voltage is unaffected by
changes in the heavy battery current as it is isolated by X3. The
resistor R1 is selected to set the voltage across the diode D4 to about
isolates the diode chain D1 and D2 from the base current
of Q2 and allows a more precise setting of the limiting battery
voltage. Resistor R3 is chosen to set the voltage across D2 to about
resistor R5 provides a measurement of the charging current.
This is amplified by X1 used as a comparator which turns Q3 off during
bulk charging and turns it on when the current has dropped to the
the associated resistor chain provide a reference voltage to define the
voltage across R5 at which bulk charge termination occurs. The value of
R10 can be adapted to set this point.
to remove R5 and use R2 as a measure of current. This does not work
because Q1 moves into deep saturation as the battery current drops and
as X3 responds by trying to force Q1 to pull more current. The result is that
the base current of Q1 increases in R2 and the voltage across it does
not fall low enough.
is still some uncertainty in the circuit as the voltages across
the diodes will depend on the diode choice and the current through
them. However we have effectively removed the variability of these
voltages. The circuit can be tuned by adjusting the diode currents,
although it is not necessary to have very great precision for this
the previous discrete component circuit, when the current
reaches the termination
point the diode D2 is shorted and the circuit switches off all current
to the battery until the latter settles to below 6.8V, after which it
float charging phase.
shows a Spice dc sweep simulation of the battery voltage showing the
change in battery current
(red) and the voltage across the diode D2 (blue). It can be seen to
hold the desired voltage limit very closely as the current reduces to
the termination point. The slight rounding in the battery current curve
is due to Q2 moving from saturation to cutoff as the battery voltage
increases. This can be eliminated by increasing the power supply
voltage beyond 12V.
final graph for
the operational amplifier based charger shows
measurements taken over time of a small 6V SLA battery being charged
from an initial terminal voltage of 6.3V. The curves clearly show the
voltage rising to about 7.3V while the current is essentially constant
at around 850mA. The voltage remains at 7.3V while the current drops
the termination point at which the circuit will switch to a float
mode. The actual termination current was set somewhat high at 94mA for
illustrative purposes. Note that the terminal voltage doesn't drop
instantaneously when the current is switched off. Ultimately the system
settles to a float charge of about 5mA at a terminal
voltage of 6.6V. The vertical scale represents battery terminal voltage
in volts, while the current in amps is scaled up by a factor of five
for better visibility.
In practice the
circuit didn't achieve the desired 0.4C (1.6A) bulk charging current,
but that should only require a reduction in the current setting