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Application Note
Choosing the Correct Solar Battery Charger for Your
Solar Application

Mike Emanuel
ABSTRACT
With the introduction of the widespread availability of solar panels as a power source, there is becoming an
increasing need to be able to flexibly charge batteries with a solar input source. Different topologies are needed
to meet the requirements of the different input sources and/or batteries. Several battery chargers (together will
be referred to as Solar Battery Chargers throughout the remainder of this document) use Maximum Power Point
Tracking (MPPT) algorithms to extract the maximum power from a solar panel and to charge a battery. These
devices cover a wide range of battery voltages as well as feature different topologies to accommodate these
input voltages and charge voltages.

Table of Contents
1 Introduction.............................................................................................................................................................................2
2 How MPPT and VINDPM Works on Solar Battery Chargers............................................................................................... 2
2.1 Buck MPPT........................................................................................................................................................................ 2
2.2 Boost MPPT....................................................................................................................................................................... 3
2.3 Buck-Boost MPPT..............................................................................................................................................................3
3 Summary................................................................................................................................................................................. 4
4 References.............................................................................................................................................................................. 5

List of Figures
Figure 1-1. Sample I-V and P-V Curve........................................................................................................................................ 2
Figure 2-1. Solar Buck Charger................................................................................................................................................... 3
Figure 2-2. Solar Boost Charger..................................................................................................................................................3
Figure 2-3. Solar Buck-Boost Charger.........................................................................................................................................4

List of Tables
Table 3-1. Feature Comparison of Solar Battery Chargers..........................................................................................................4

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1 Introduction
The output voltage of a solar panel is tightly linked to the current drawn from the solar panel. If too much current
is drawn from the solar panel the output of the solar panel will crash. The key to successful solar panel utilization
is to find what is called the Maximum Power Point (MPP). At the MPP the maximum amount of power available
from the solar panel is delivered [1], [2], [3]. Figure 1-1 shows Current vs. Voltage and Power vs. Voltage curves.
Note how there is a clear MPP of approximately 7W and 13.5V for the Power vs. Voltage curve. Increasing or
decreasing the panel voltage beyond this point lowers the panel output power.
0.7 10.5
Sample Solar Output Current
Sample Solar Output Power
0.6 9

0.5 7.5
Output Current (A)

Output Power (W)

0.4 6

0.3 4.5

0.2 3

0.1 1.5

0 0
0 2 4 6 8 10 12 14 16
Output Voltage (V)

Figure 1-1. Sample I-V and P-V Curve

Different switch mode topologies are used to accommodate the different input and output voltages as well as
allow for the higher efficiencies of a switching charger. In this application note, the buck, boost, and buck-boost
topologies are discussed.
2 How MPPT and VINDPM Works on Solar Battery Chargers
To extract the MPP from a solar panel, a MPPT algorithm is used. One good way is to use the Fractional
Open Circuit Voltage (FOCV) technique. In this method, the solar battery charger input voltage is regulated to
a percentage of the open circuit voltage (OCV) of the solar panel. This OCV is the output voltage of the solar
panel under a no load condition [4]. During normal sunlight conditions this ratio, also known as a K-factor, is
typically between 75% to 85%. Another method is to regulate the input voltage to a fixed value. All of the Solar
Battery Chargers presented here work by using one of these two algorithms. The following sections detail how
specifically each charger achieves MPPT operation.
Using MPPT requires input voltage regulation, also known as VINDPM (Input Voltage Dynamic Power
Management). VINDPM activates when the battery charger requires more output power than the input can
handle which starts to lower the input voltage. By selecting the appropriate VINDPM setting, the charge current
will reduce under VINDPM to prioritize the system load and to prevent the input voltage from dropping below the
VINDPM value. The VINDPM setting is determined by external resistor divider or I2C setting. The combination of
choosing the desired K-factor from the OCV and regulating with VINDPM to the target input voltage presents a
clear MPPT solution.
2.1 Buck MPPT
In a buck converter the input voltage is always greater than the output voltage. Please see Figure 2-1
demonstrating the buck topology in grey. A simple way to program VINDPM in a buck charger is to use a
resistor divider such as R3 and R4 in Figure 2-1. A reference voltage is targeted at MPPSET. The goal is to
program the resistor divider such that the MPPSET reference voltage is met when the input is at the VINDPM
threshold.
The BQ24650 is a 26-V 10-A buck charge controller with MPPT through the MPPSET pin. The target reference
voltage for MPPSET is 1.2 V. The charger will pull only the current from the solar panel that keeps the input
voltage set at the desired MPPT voltage and the MPPSET voltage at 1.2 V. The BQ24650 is standalone, but
comes with two STATx pins to detail the status of the device.

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www.ti.com How MPPT and VINDPM Works on Solar Battery Chargers
CVCC

RVCC
RSNS

R2
R3
VCC
MPPSET HIDRV

R1
BQ24650

R4
VFB
SRN SRP LODRV

BUCK

Figure 2-1. Solar Buck Charger

2.2 Boost MPPT

In a boost converter the input voltage is always less than the output voltage. Please see Figure 2-2
demonstrating the boost topology in gray. A simple way to program VINDPM in a boost charger is to user a
resistor divider such as the VOC_SAMP divider shown with ROC2 and ROC1 in Figure 2-2. First, the converter is
disabled and then the VOC of the solar panel is sampled at the input voltage. A reference voltage is measured at
VOC_SAMP in the VOC condition and this is the VINDPM setting until the next sample.
The BQ25504, BQ25505, and BQ25570 are 100-mA boost chargers with MPPT via VOC_SAMP pin. In this
family of devices, the input voltage is sampled once every 16 seconds (typical) by disabling the converter. The
converter then regulates the input voltage to the desired percentage of the OCV. The BQ25505 and BQ25570
provide options for an 80% MPPT (or K-factor) for solar devices. For both of these devices and the BQ25504,
the K-factor can be set by adjusting the VOC_SAMP divider.
CSTOR
ROC2 ROC1

VOC_SAMP VSTOR VBAT_SEC

L LBOOST

BOOST
VIN_DC BQ25505

Figure 2-2. Solar Boost Charger

2.3 Buck-Boost MPPT

In a buck-boost converter the input voltage can be greater than, less than, or equal to the output voltage. Please
see Figure 2-3 demonstrating the buck-boost topology in grey. A host-controlled charger presents more options
to program MPPT. A robust way to implement MPPT is to program the desired K-factor via I2C as shown by SDA
and SCL pins in Figure 2-3. The charger will periodically disable charging and measure the input voltage, also
known as the OCV of the solar panel. Next, the charger multiplies the OCV by the K-factor and will hold the input
to this value as VINDPM if the panel is overloaded.
The BQ25798 is a 18.8-V 5-A I2C buck-boost charger with MPPT. The BQ25798 is well suited for environments
that change temperature because as the panel cools or heats the BQ25798 will change the input voltage
regulation accordingly without having to set a fixed OCV. The I2C capability also gives flexibility to change the
K-factor on the fly.

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SW1 SW2

VBUS SYS
CSYS
CVBUS
Q1 Q4

BAT
BATFET
Q2 BUCK- Q3 CBAT
BOOST

SCL
ADC

SDA
BQ25798

Figure 2-3. Solar Buck-Boost Charger

3 Summary
There are many options to choose for Solar Battery Chargers. Buck, boost, and buck-boost converter topologies
are accessible as well as a wide range of charge currents. Each battery charger works fixing the MPP Voltage or
by measuring the unloaded input voltage (or OCV) and regulating the input voltage at a fixed ratio of the OCV.
For detailed features and operation, see Table 3-1 for a comparison of the solar battery chargers.
Table 3-1. Feature Comparison of Solar Battery Chargers
Device BQ24650 BQ25798 BQ25504, BQ25505, BQ25570
Input Voltage (max) 28 V 24 V 3 V (BQ25504) and 5.1 V
(BQ25505 and BQ25570)
Battery Voltage (max) 26 V 18.8 V 5.25 V (BQ25504) and 5.5 V
(BQ25505 and BQ25570)
Charge Current (max) 10 A 5A 0.1 A
Topology Buck Buck-Boost Boost
Chemistry Lead Acid, Li-Ion/Li-Polymer, Li-Ion/Li-Polymer, Lithium Li-Ion/Li-Polymer, SuperCap
Lithium Phosphate/LiFePO4 Phosphate/LiFePO4
Interface Standalone (RC-Settable) I2C Standalone (RC-Settable)
How to Program MPPT Resistor Programmable I2C Programmable Resistor Programmable
Type of MPPT Fixed MPP Voltage FOCV FOCV

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4 References
1. S. Negi, A. Maity, A. Patra, and M. Sharad, Adaptive Fractional Open Circuit Voltage Method for Maximum
Power Point Tracking in a Photovoltaic Panel, 2019 32nd International Conference on VLSI Design and 2019
18th International Conference on Embedded Systems (VLSID), Delhi, India, 2019, pp. 482-487.
2. Texas Instruments, Implementing a Simple Maximum Power Point Tracking (MPPT) Algorithm application
note.
3. Texas Instruments, Maximum Power Point Tracking With the bq24650 Charger application note.
4. T. Esram and P. L. Chapman, Comparison of Photovoltaic Array Maximum Power Point Tracking
Techniques, in IEEE Transactions on Energy Conversion, vol. 22, no. 2, pp. 439-449, June 2007.

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