Solar Panel Efficiency Meter

Field Testing of Prototype Systems in Extreme Environments
When testing a device or product in the field, resources, material limitations, debugging, and time pressures are challenging. However, failure is the mother of success in field testing, and the experience gained in field testing also helps designers gain a comprehensive understanding of the system. We are testing a prototype system of a solar-powered 12V lead-acid car battery charging circuit. We want to use this car battery to power LED lights, audio systems, and water pumps for camping showers and mist systems. We want to prove that the charging circuit will not be damaged and will work properly. Our test environment is the hot Nevada desert in the United States. There is no water, no shade, no electricity, nothing. In such conditions, the only hope of survival is to transport everything you need in a freight truck for a few days and then transport it back without missing anything. In every way, this is a perfect test environment to determine whether the system is sufficient to provide comfortable living conditions.

Validate your design with real-life
testing For field tests such as this, the consequence of failure is a very bad feeling. It is generally accepted that the charging system must be monitored to prevent the battery from being completely drained. Therefore, appropriate measurements must be taken and collected in real time to maintain and restore battery life. The equipment used includes a DC1688 demonstration board, which uses Linear Technology's LT3652 solar cell charge controller, which is connected to a Solec 70W (Solec SQ-70) solar panel with a maximum voltage of 17V and current of 4.45A ¹

¹ Solec SQ-70 Solar Panel Features:
Rated Power: 75W at 25oC
Maximum Voltage: 17.00V
Maximum Current: 4.45A
Short Circuit Current: 4.75A
Open Circuit Voltage: 21.40V
Dimensions: 46.8" x 20.9" x 1.5"
Weight: 16.5 lbs / 7.5 kg

The question is, if we start with a fully charged battery and keep it charged during the day, will the Solec 70W solar panel provide enough power to charge the battery to run the LED lights all night, run the 12V water pump for the shower, play a few hours of iPod music through the 12V car audio amplifier, and recharge other external devices?

A key part of this type of testing is testing the efficiency of the charging circuit. Does the charging circuit maximize the battery charge? Does the charging circuit maximize the use of sunlight and solar panels? Although it is not difficult to measure efficiency using output power/input power x 100, it is not practical to carry 4 multimeters and a calculator to the desert to constantly take readings and measurements. However, we can use an "efficiency meter" developed in-house. Using two Linear Technology demonstration boards, a microcontroller and an ADC, an efficiency meter can be constructed to measure and calculate efficiency in real time and finally display the results on an LCD display. No multimeters, no calculators, and most importantly, no computers are required. The system can work independently with only a 9V battery and internal embedded programming, which makes the system very portable. Real-time efficiency data is extremely useful in optimizing solar panel charging systems, for example, it takes the guesswork out of positioning the solar panel, and it can also be used to determine when charging efficiency is the best during the day and properly budget battery power usage.

Linear Technology's demo board DC590B includes a PIC microcontroller that interfaces with the DC571A demo board. The DC571A demo board consists of an 8-channel 24-bit differential ADC LTC2418. Only 4 channels of the ADC are used, two of which measure V _IN and V _{OUT , and the other two channels utilize the LTC6103 current sense amplifier on the DC1116A demo board to measure I IN} and I _OUT as voltages , since voltage is proportional to current. The DC1116A demo board is slightly modified, with resistors replaced to achieve a 0.1V/1A ratio.

It is easy to connect the LTC2418 (ADC DC571A) demo board to the DC590B's microcontroller interface, then record the code on the ADC's 4 differential channels, take the instantaneous value of the measured voltage, convert, calculate the efficiency and display the result on the LCD. The end result is a simple and clear system that displays Vs (solar panel voltage), I _IN (input current from the battery charger), Vb (battery voltage), I _OUT (output current from the battery charger to the 12V lead-acid battery) and the calculated efficiency (Vb x I _OUT ) / (V _s x I _IN ) x 100.

Operating each channel as a differential voltage measurement channel means that the ground paths for V _IN and V _OUT need to be tied together. In my design, channels 3, 5, 7, and 9 are tied to ground. Channels 2 and 6 are Vs and Vb. Channels 4 and 8 are IIN and IOUT. This ADC has an internal reference voltage limit of 2.5V, so I had to use a voltage divider on the Vs and Vb channels to stay within the maximum input range of 20V. The resistor dividers on the channels maximize the resolution, and the internal code used to recalculate the true voltage value uses negligible error correction.

The final step was to make the system portable enough to run on a 9V alkaline battery for easy carry-on, but flexible enough to use a 12V lead-acid battery. The DC823B demo board uses the LTM4600 step-down µModule® regulator, which has a maximum input voltage range of 20V and regulates the 5V voltage provided to the entire solar panel battery charger system (Figure 1). Connect a 9V alkaline battery to the V _IN terminal of the demo board with a 5V output select jumper. The LTM4600's BURST mode is more efficient at light loads, helping to extend battery life. We beta-tested the final product from design to prototype system by working on it in the field.

Figure 1: A solar panel efficiency meter that displays results in real time on an LCD display; from left to right:
DC823B Demo Board (LTM4600 - Step-Down µModule Regulator)
DC590B Demo Board PIC Microcontroller
DC571A Demo Board (LTC2418 - 24-Bit, 8-Channel ADC LTC2418)
DC1116A Demo Board (LTC6103 - Current Sense Amplifier)

Real-time efficiency
Living in the desert for a few days is not an easy task unless there is some comfort provided by battery power. We used a 12V water pump to combat the heat with a shower and misting system (Figure 2). Evenings were pleasant, too, with a string of LED lights lighting the camp. We used a cordless drill that could be recharged for continuous use while we built a shade shelter to block out the sun (see Figure 3). We even had a few festive nights playing iPod music through an amplifier and several speakers. The real-time efficiency meter helped us estimate power consumption throughout the day, not only helping us orient the solar panels, but also helping us determine how long the equipment would last before the batteries ran out. Camping was still challenging because we had to deal with sandstorms that blocked the sun, which prevented the solar panels from charging the batteries and forced us to limit power usage. In the end, however, the system worked as designed, the batteries never ran out, and the designers withstood the test of field testing without any loss.

Figure 2: Rechargeable tools were used to build the shade shelter and shower enclosure using a 12V water pump.

Figure 3: System configuration for maximum efficiency in the desert:
A. Solec SQ-70 75W solar panel
B. 12V lead-acid car battery (two batteries in a green box; one for charging, the other for working)
C. Charger and battery for a Ryobi cordless drill
D. Water pump for shower and cooling mist system
E. French press coffee maker for morning pick-me-up
F. Speakers used as legs for a table with salvaged wood planks