Solar Meshtastic Nodes: Built-in Solar Interface vs. MPPT? Which Solar Panel Specs Should I Choose?

This article uses the Heltec T114, powered by the nRF52 MCU , as a case study to to explore how to build an efficient solar-powered Meshtastic node. It's important to note that other Meshtastic hardware using the nRF52 chip exhibit similar power consumption, making the conclusions and calculation methods shared in this article equally valuable as a reference. This article is not limited to the Heltec T114 but applies to all similar hardware.

When we choose to build a solar-powered node with Heltec T114, we often face two problems:

1. Should I use the T114's built-in solar input interface directly, or should I use an MPPT controller in addition?

2. What solar panel power should I choose? What current and voltage should I choose?

The experiment showed that there was a significant difference in efficiency and applicability between the T114's built-in solar input interface and the MPPT controller.

More importantly, choosing the right size of solar panels for both methods requires scientific calculations, including consideration of equipment requirements and redundancy factors.

Efficiency: T114 default solar input interface vs MPPT controller

When building a solar power system, maximizing the output power of solar panels is a key consideration. The Heltec T114 provides a default solar input interface that supports direct connection to solar panels. However, this approach, lacking a dynamic optimization mechanism, has significant efficiency and performance limitations. In contrast, an external MPPT controller can significantly improve energy conversion efficiency and fully utilize the performance of solar panels through maximum power point tracking (MPPT) technology.

To facilitate understanding, we will compare the efficiency performance of the T114 default interface and the MPPT controller in detail below, and gradually demonstrate the calculation basis of these conclusions through data and formulas.

Default solar input interface

The default solar input interface of Heltec T114 is not an efficient energy conversion circuit, but a simple charging circuit:

· Fixed voltage requirement: At least 4.2V to start charging, recommended voltage range is 5V-6V.

· Input power limitation: The output of solar panels cannot be dynamically optimized, resulting in low utilization.

In actual testing :

· When using a 1W, 6V solar panel, the charging current is only 31mA.

· The charging power is calculated as follows:

P charge = V battery × 1 charge = 4.2V × 0.031A = 0.13W

· If the solar panel output power is 1W, the efficiency is:

Connect T114 to external MPPT

The MPPT controller dynamically adjusts the input voltage and current to achieve Maximum Power Point Tracking (MPPT), efficiently extracting the output power of the solar panel:

· Wide voltage adaptability range: usually supports 3V-18V, or even higher. I recommend choosing a 6V MPPT controller.

· Dynamic optimization: It can automatically adjust the relationship between voltage and current so that the solar panels always operate at the optimal power output point.

In actual testing:

· When using the same 1W, 6V solar panel, the charging current can reach 100-200mA.

· Take the middle value 150mA to calculate the charging power:
P charge = V battery × 1 charge = 4.2V × 0.15A = 0.63

· If the solar panel output power is 1W, the efficiency is:

Regarding MPPT wiring, you can refer to the following wiring diagram (you can ignore the purple sensor on the right).

The wiring diagram comes from heyitsyang's GitHub project W9ETC-Meshtastic-Solar-Node.

Efficiency comparison summary

2. How to calculate solar panel specifications

Let me first say the conclusion:

The following explains how this conclusion was reached. To equip the T114 with the appropriate solar panels, the following factors need to be considered:

1. Daily energy demand of the device: including the power consumption of the T114 itself and the power consumption of the load.

2. Sunshine time: local daily effective sunshine time.

3. Redundancy factor: Considers energy reserves in rainy weather or insufficient sunlight.

Calculate the daily energy requirements and solar panel specifications for the T114

To ensure the stable operation of the Heltec T114 under solar power supply, it is first necessary to determine the daily energy consumption of the equipment. Then, considering the power supply method and efficiency, the rated power and specifications of the solar panels should be calculated.

In some harsh environmental conditions, or when the system requires higher redundancy, the following parameters can be adjusted to ensure that the system can operate normally even if rainy weather lasts longer than expected or the rainy season becomes more frequent:

· Adjusted the standby current assumption to a higher range. Currently it is assumed to be 9 milliamperes (mA), but the average standby current of the nRF52 Series is typically between 9 mA and 15 mA.

· Adjust the assumed value for the average daily effective sunshine hours downward (for example, from 4 hours to a more conservative value).

· Increase the value of the redundancy factor K. For example, the current value is 1.5 , which can be increased as needed.

· Assuming lower system efficiency. According to preliminary experiments, the charging efficiency of the built-in solar interface is about 13% , while the efficiency of the MPPT charger (usually using the CN series IC) is about 63% .

Through these conservative assumptions and adjustments, the reliability of solar nodes can be effectively improved, ensuring that the system can still operate stably under extreme conditions.

1. Calculation of daily energy requirements for T114

The T114 has very low power consumption. Taking its average standby current of approximately as an example, and assuming that the battery voltage required for its operation is (typical value for the T114), its daily energy requirement can be calculated using the following formula:

E demand = V battery × 1 standby × T daily working time
Required battery standby daily working time
If the device runs 24 hours a day, substitute the relevant parameters:
E demand = 3.7V × 0.009A × 24h = 0.8Wh
Therefore, T114 requires about 0.8Wh per day  of electricity.

2. Calculation of solar panel output power requirements

To meet the daily energy needs of T114, the output power of the solar panels needs to match the local sunshine conditions. Assuming the local average daily effective sunshine time is hours, the output power of the solar panels can be calculated using the following formula:

in:

· K Redundancy: Redundancy coefficient, used to prevent rainy weather or insufficient sunlight. The recommended value is 1.5-2.

· E demand = 0.8Wh

· T sunshine hours = 4h

Assuming the redundancy factor is redundancy, substituting it into the formula yields:

Therefore, to meet the daily energy demand of T114, a solar panel with a minimum power of is required.

3. Adjust solar panel specifications based on different input methods

Different input methods (T114 default interface or MPPT controller) will affect the efficiency of the solar panel, so the power required by the solar panel needs to be calculated separately.

1. Using the T114 default interface
The Heltec T114 default solar input interface has a low efficiency of only about 13. To compensate for the efficiency loss, the solar panel needs to provide a higher power, which is calculated as follows:

Substitute the efficiency of 13 and the required power of 0.3W :

Recommended specifications : Select a rated power of 3W and a voltage of 6V solar panels to ensure adequate redundancy.

2. MPPT controller has
a higher efficiency, usually up to 63% . Compared with the default interface of T114, the required solar panel power is lower. The calculation formula is:

Substitute the efficiency 63  and the required power is 0.3W :

Recommended specification : Select the rated power of 1W , voltage is 6V Solar panels can meet the needs.

4. Solar Specification Comparison Summary

Through the above calculations, you can choose the appropriate solar panel and input method according to the actual usage scenario.

If you have high budget and efficiency requirements, it is recommended to give priority to the MPPT controller; if you pursue simple installation and low cost, the default interface of T114 can also meet basic needs.

3. Notes

When using the Heltec T114 for solar power, it's important to note the following key points to ensure system stability and efficiency. First, when selecting solar panels, it's recommended to consider a power redundancy factor, typically 1.5 to 2 times the required power. On rainy days or in winter, light conditions are often poor, and the actual output power may be far below the rated value. Redundancy can effectively address these situations, ensuring the device remains operational in low-light conditions. I designed my solar node using 3 times the required power; the more redundancy, the safer it is.

Secondly, be wary of false labeling. The nominal power of some solar panels on the market can differ significantly from their actual output. Therefore, after purchasing a solar panel, it's strongly recommended to use a multimeter to measure its current and power in full sunlight. This step can help confirm that the panel's actual performance meets expectations, thus preventing the use of inefficient products that could compromise the reliability of the entire system.

Additionally, to protect the battery, it's recommended to add a battery management system (BMS). A BMS automatically disconnects the circuit when the battery is overcharged or over-discharged, preventing damage. This is especially true during extended outdoor use, where the battery voltage may drop significantly at night or in low light conditions, causing degradation or even damage. Using a BMS can effectively extend the battery's lifespan.

Finally, pay special attention to the voltage range of the solar panel. The Heltec T114's default solar interface is designed for a voltage range of 5V to 6V. Excessively high input voltages (such as 12V) may damage the device. Especially when using higher-power solar panels, it's important to ensure the voltage meets the device's specifications. If you require solar panels with a wider voltage range, it's recommended to use an MPPT controller to optimize voltage and current, both protecting the device and improving charging efficiency.

In summary, through reasonable redundancy design, rigorous performance testing, the application of battery protection modules, and compliant voltage selection, the Heltec T114's solar power supply system can be ensured to operate efficiently and stably, meeting the device's power needs on both sunny and cloudy days.

Author: Hays Chan | Community blogger

Translator:Joe Tan | KEEPTEEN | meshtastic node repeater

Website: https//www.keepteen.com

Article link:Solar Meshtastic Nodes: Built-in Solar Interface vs. MPPT? Which Solar Panel Specs Should I Choose?