For optimum charging of lead-acid batteries a 3-step technique is often used, consisting of an initial constant-current (CC) charge, then a high constant-voltage (CV) top-off, and finally a lower CV trickle-charge.
But a simpler two-step technique can also work fairly well, albeit perhaps not charging the battery quite as full, which is to start with a CC until the battery reaches a specific voltage, indicating it is near a full charge (≈90%), and then revert to a CV mode at a lower, trickle-charge voltage to avoid overcharge while topping off and then maintaining the battery charge.
For low battery charging currents a linear regulator works well, but for higher charge currents a switch-mode power supply (SMPS) is preferred for better efficiency and minimal heat sink requirements.
Discussed here is a simple switch-mode circuit that uses hysteretic (bang-bang) feedback to control the charging current. A hysteretic design is inherently stable and thus requires no compensation of the control loop as linear feedback loops do. This reduces the design complexity of the circuit, and eliminates possible stability problems when changing from the CC to the CV mode while charging the low and varying load impedance of a battery.
Hysteretic converters generally have more output ripple and their switching frequency varies with load, but that's not normally a concern for battery charging.
The circuit efficiency is high at around 90% so a heat sink should not be needed for the MOSFET.
The circuit (below) uses one LM339 quad comparator chip to perform all the feedback and control functions, along with one CD4049 buffer IC, one P-MOSFET, one Schottky diode, a TL431 reference, and several passive components.
During the initial CC charge, U1 rapidly switches P-MOSFET M1 in a PWM fashion to generate the desired output current with inductor L1 smoothing the voltage pulses from the MOSFET and converting them to a current.
The U2, Tl431 reference provides an accurate and stable 2.5V which is used to establish both the CC and CV values.
Its voltage is reduced to about 130mV at the U1(+) input for the CC reference, which is compared to the voltage generated across the current shunt resistor, R1.
When the peak battery charge current generates an R1 voltage exceeding U1's (+) reference voltage, U1 turns off M1.
This causes R7 to generate about 30mV of hysteresis voltage at U1's input, so M1 stays off until the battery current from the inductor has dropped about 20%, at which point U1 turns MI back on.
This is the hysteretic feedback sequence that controls the battery charge current in the CC mode.
U4 through U9 are six CMOS inverters in a single IC package that are paralleled to act as a poor-man's gate driver for M1.
Thc circuit maintains a CC charge until the battery voltage reaches its maximum charge voltage of ≈14.4V.
At that point U11 changes state and latches high through positive feedback from U12, changing to the CV trickle-charge mode.
This removes R9 from ground and changes the resistor divider voltage at U10's (-) input, such that the battery has to drop to ≈13.6V before U10 starts the charging.
This is the maintenance CV mode to top off the battery and keep it charged.
The charger will revert to the high-voltage, CC charge rate if a battery is attached with a voltage of <13V.
The initial charge current (blue trace) controlled by U1 is ≈6A average until the output battery voltage (yellow trace) rises to ≈14.4V.
This triggers U11 to latch to the trickle-charge mode, reducing the charging voltage to ≈13.6V. This causes U10 to pull the buffer input signal low, turning off M1 (red trace) and stopping the output current.
(The simulated battery consists of capacitor CBatt in parallel with resistor RBatt. The capacitor size is such that the simulated charge time is only a couple of milliseconds. The actual charge time will be much longer of course, depending upon the battery size and state of charge.)
U10 starts switching when the battery voltage drops to ≈13.6V, generating the trickle charge current (as determined by simulated battery resistor, RBat).
(R14 provides hysteresis for U10's oscillations during trickle-charge).
The 14.4V CC charge trip point and the 13.6V CV trickle charge voltages used in the simulation are typical values for a 12.6V lead acid battery.
These can be adjusted, if needed, by changing the values of R4, R8, and R9 (R11 is just to insure that U11 trips before U10 when the maximum charge voltage is reached).
R4 and R8 must be selected first to determine the CV trickle charge voltage (to give a Verr of 2.5V at U10's (-) input).
Then you select the value of R9 (which forms a voltage divider consisting of R4+R5 in parallel with R8 and R9) to determine the charge voltage at which it changes from the CC mode to the CV mode (again for a Verr voltage of 2.5V).
The charging current can be changed by using a different value for the shunt resistor, R1.
To make the current adjustable, you could replace R12 with a 10kΩ pot.
The inductor must be sized to handle the peak ripple current without saturating, which is about 7A in the simulation. One rated for 8A should be sufficient.
The input voltage should be ≥15V with a maximum of about 25V.
This means a standard 12.6Vac output transformer into a full-wave bridge Schottky rectifier should work as a supply.
It does require sufficient filtering so that the low point on the ripple voltage is not much lower than 15V (at least 5mF of filter capacitance per amp of current for a 12.6V transformer).
Note that the transformer power rating should be at least 40% (1.4) above the charger's CC limit times 14.4V (86.4W * 1.4 = 121W for a 6A CC) to avoid overheating the transformer.
The DC current is taken at the peak AC output by the rectifiers, which is 1.4 times the RMS voltage, requiring 1.4 times the transformer power for a given transformer current.
Another good supply (V1) for the circuit would be a 15V, 6A SMPS adapter, which are efficient and relatively low cost.
Note that proper grounding and decoupling are very critical for this current due to the high current, fast rise-time signals. Building the circuit on a board with a ground plane is best. Otherwise use a single-point type ground with the common of R1 being that point.
Connect C1 from the source of M1 to this point, also D2's anode, and all the common points for the rest of the circuit.
Building this on a plug-in type breadboard will likely result in erratic and unsatisfactory operation of the circuit.
Ending Caveat: This circuit was designed and simulated but not built, so it is likely that the built circuit will not perform exactly as the simulation shows due to component tolerances, layout parasitics, etc., and may require some tweaking to obtain proper operation. So anyone building this circuit should be aware of that and have the ability to understand the circuit operation and make any required changes.
Lead-Acid Battery Charger, 12V 6A, Using Hysteretic SMPS Circuit With Auto Trickle-Charge
Simple, efficient, lead-acid battery charger auto switches from CC to CV (trickle-charge) mode