20 W Low-Cost Lead-acid Battery Charger

Charging Requirements for Lead-Acid Batteries

Lead-acid batteries have existed for a very long time and might be considered out of date, but they are still used in many applications worldwide. This is not only because they are more cost-effective than other battery types. They are also very reliable, robust, have a decent lifetime, and their characteristics are well known. Recharging batteries usually need a dedicated charger circuit for the used chemistry. For lead-acid batteries, this circuit has to fulfill the following three requirements: First of all the battery has to be charged with constant current. After reaching the typical cell voltage, the charging process has to switch into providing a constant controlled voltage, and at the very end, charge retention is required.

In Figure 1, two charging profiles for lead-acid batteries are shown. The profile with solid lines is more complex to implement, but the time to fully charge the battery is quicker and it is also more beneficial for the life span of the battery. In contrast, the dotted profile can be realized more cost-effective. Thus the dotted profile was chosen for TIDesign PMP10081.

 

Figure 1: Two charging profiles for lead-acid batteries

 

TI's TIDesign PMP10081

Texas Instruments’ TIDesign PMP10081 provides a simple and low-cost solution for charging a six-cell lead-acid battery and meeting all three requirements mentioned above. The original application behind PMP10081 is to buffer and supply industrial emergency exit lighting with a lead-acid battery. The battery charger was tested with a resistive load, an electronic load, and a real battery. For testing this design under real conditions, a lead-acid battery with six cells (YUASA NP 5-12, 12V, 5 Ah, AGM) was used.

 

Figure 2: Schematic of TIDesign PMP10081, which is a SEPIC switch mode power supply modified for lead-acid battery charging

 

As lead-acid batteries have a quite simple charging profile, the necessary curves for voltage and current can be easily reproduced. This can be accomplished by adding an auxiliary sensing circuit to a switch-mode power supply and connecting it to the feedback loop of the control IC. How this can be done is shown in Figure 2. The controller for this design is Texas Instrument’s TPS40210. It is a low-cost non-synchronous Boost controller IC with current-mode control. In PMP10081, it is configured as Single Ended Primary Inductor Converter (SEPIC) allowing the battery to be charged from lower, equal and higher supply voltages than the maximum battery voltage. The input voltage can be in a range of 9 V and 36 V and will be converted to a maximum output voltage of 13.8 V, with the maximum adjustable charge current being limited to 1.5 A.

 

Meeting Charging Requirements with the TLC272

A slight modification of the controller’s feedback loop for generating two error signals enable the Switch-mode power supply (SMPS) to operate as a lead-acid battery charger with the requirements mentioned above. The SEPIC power supply is extended by an additional dual operational amplifier circuit with TI’s TLC272. One amplifier stage is configured as a unity gain buffer for the output voltage signal. The other amplifier circuit is used for sensing the current through the output inductor. Because the average current signal through the output inductor is equal to the output current, the second amplifier circuit is configured as an integrator and the signal is provided to the feedback network of the control IC. Both amplifier outputs are connected via diode-ORing to the feedback network of the SEPIC charger. Thus, one of the two signals is always dominant depending on the active charge status of the battery.

When switched on with a discharged battery connected, the charger circuit starts with providing a constant current which can be set and adjusted by resistor R17 as shown in Figure 2. This is because the external current sense signal overwrites the output voltage signal due to the diode-ORing. With a higher charge of the battery comes an increase in voltage. When the limit of 13.8 V is reached, the charging process will be voltage controlled as the output voltage is now the dominant signal. Under this condition, the output current decreases continuously until the battery is fully charged. A small trickling current will make sure that the battery does not self-discharge. For testing under real conditions with the YUASA NP 5-12, the charging current was limited to 1 A, which is 20% of the battery’s maximum capacity. After the battery is fully charged, the SEPIC power supply provides a very small load current of 5 mA for charge retention. That is the reason why a battery will neither be damaged nor self-discharge, in case it is not disconnected from the charger.

The design can easily be adjusted for batteries with fewer or more cells by changing the feedback resistor divider (resistors R18/R19) and the charge current limit (resistor R17). As the high side resistor R18 also affects the error amplifier gain, it is highly recommended to apply the modification to the low side resistor R19. Depending on the new output voltage FET and diode of the design might have to be replaced as well.

 

Testing with a Resistive/Electronic Load vs. Testing with a Real Battery

If a power supply is working with a resistive or electronic load, this does not automatically mean it will do the same with a lead-acid battery. A real battery has similar characteristics to a capacitor and therefore behaves a little bit different than the other two loads when connected to a power supply. In the case of PMP10081, the feedback loop has to be slightly tuned-up when a lead-acid battery is connected as a load, though it is working properly with the two aforementioned loads. For charging a battery, the constant current feedback network needs good dynamic behavior in contrast to the voltage control loop.

For PMP10081, the current loop achieves a bandwidth of approximately 7 kHz while the constant voltage feedback path has to be considerably slower. Its bandwidth with a resistive load is 296 Hz. With the YUASA NP 5-12 lead-acid battery connected to the output, the bandwidth of the voltage control loop is in a range between 60.9 Hz and 3.9 Hz for a decreasing charge current from 900 mA to 70 mA. The current loop’s behavior and bandwidth stay the same for all tested load conditions. A phase margin of more than 60° is maintained under all specified and tested conditions.

 

Figure 3: Frequency response for voltage-controlled and current-controlled charging. Red/blue graphs show the current controlled loop, purple/orange graphs show the voltage-controlled loop.

 

Low-Cost Lead-Acid Battery Chargers are Possible

PMP10081 shows how a very low-cost lead-acid battery charger solution can be built. This is accomplished by adding an operational amplifier circuit and a slight modification of the power supply’s feedback network. With these adjustments, the battery charger circuit fulfills the general requirements of constant current and constant voltage charging, depending on the status of the battery. Besides that, the SEPIC power supply is also able to provide charge retention. The design was built and tested. All necessary data of PMP10081 like schematic, BOM, Gerber files and test reports can be found online in TI’s reference design database TIDesigns.

 

About the Authors

Bernd Geck is heading the European Power Supply Design Services Team based in Freising, Germany. He holds a Diploma in Electrical Engineering from the University of Applied Sciences in Ulm and has more than two decades experience in electronic hardware design with a focus on power management and RF. He started the EMEA Power Supply Design Services in October 2006. Right now, he and his team are generating +200 designs per year. In addition, he educated hundreds of customers in power supply design via various seminars and workshops across Europe and Northern America. Bernd was elected as Member of Technical Staff in 2011.

Markus Zehendner is a Systems Applications Engineer in TI’s EMEA Power Supply Design Services Group since 2014. He holds a Bachelor in Electrical Engineering and a Master in Electrical and Microsystems Engineering from the Technical University of Applied Sciences in Regensburg. His design activity includes reference designs of isolated and non-isolated DC/DC converters for all application segments.

 

This article originally appeared in the Bodo’s Power Systems magazine.