Designing a Portable LiFePO Solar Power System for Field Radio
An analysis of a portable LiFePO₄ and MPPT-based solar power system for field radio, including power budgeting, energy flow, and seasonal performance considerations.
Designing a Portable LiFePO₄ Solar Power System for Field Radio: Real-World Performance and System Design
Introduction: From Battery Packs to Energy Systems
Most discussions around portable power tend to revolve around one number: battery capacity. But once you take a system into the field - especially for amateur radio - that perspective quickly becomes insufficient.
The real question is more practical:
Can the system sustain operation through continuous energy flow, not just stored energy?
To explore this, I built a compact solar-powered setup designed for real field use. The system is intentionally simple:
- a 12V 6Ah LiFePO₄ battery (EREMIT)
- a 100W foldable solar panel
- a Victron BlueSolar MPPT 75/15 charge controller
- Anderson Powerpole connectors for modularity
The goal was not to maximize runtime from a battery, but to achieve something more useful: energy autonomy, particularly for operating an Icom IC-705 at full 10W output in the field.
Understanding the System: Two Ways to Operate
What makes this setup interesting is that it can be used in two fundamentally different ways. Both are technically valid, but they serve very different purposes.
The simplest configuration is a direct connection between the solar panel and the battery:
graph LR
A[Solar Panel] --> B[LiFePO4 Battery]
At first glance, this seems overly simplistic, but the EREMIT battery makes it possible. Its internal Battery Management System (BMS) handles the critical safety functions: it stops charging when the battery is full, protects against overvoltage and temperature, and keeps the internal cells balanced.
A key enabler is its relatively wide input tolerance. The BMS can accept charging voltages up to approximately 30V, which allows certain portable “12V-class” solar panels to be connected directly.
This mode is useful when simplicity is the priority - lightweight setups, emergency charging, or situations where minimizing components matters more than control.
However, it comes with clear limitations. There is no voltage regulation under load, no optimization of solar output, and no visibility into system behavior. It also relies entirely on the solar panel staying within safe voltage limits. In practice, this is best understood as a charging shortcut, not a complete power system.
From Charging Setup to Power System
The configuration that defines real-world usability introduces the MPPT controller:
graph LR
A[Solar Panel] -->|DC Input| B[MPPT Controller]
B -->C[LiFePO4 Battery]
B -->D[IC-705]
With this addition, the system transitions from a simple energy path to a managed power system.
The solar panel is no longer passively connected - it is actively optimized. The battery acts as a stable energy buffer, smoothing out fluctuations. Most importantly, the load sees a consistent and reliable voltage, even as conditions change.
At the center of this is the Victron BlueSolar MPPT 75/15. It continuously tracks the panel’s maximum power point, extracting more usable energy under real-world conditions where sunlight is rarely constant. At the same time, it manages the charging process through a three-stage profile (bulk, absorption, float), ensuring efficient and safe operation.
This control layer is what allows the system to charge and supply power simultaneously - a requirement for any meaningful field use.
Battery Behavior: Small Capacity, Strategic Role
On paper, a 6Ah battery is modest, offering roughly 76Wh of energy. In a traditional setup, that would be a limitation.
In this system, it isn’t.
The battery’s role is not to serve as the primary energy source, but as a buffer between generation and consumption. LiFePO₄ chemistry is particularly well suited for this: it provides a stable voltage around 13.2V across most of its discharge curve, supports high cycle life, and handles load variations without significant voltage sag.
One important nuance is that voltage alone is not a reliable indicator of state of charge. Unlike lead-acid batteries, LiFePO₄ maintains a nearly flat voltage profile until it is close to empty.
What the Solar Panel Actually Delivers
Under real conditions - spring weather in Germany, around 18°C with clear skies - the panel performed close to expectations. I observed charging currents of nearly 6A, which is close to its theoretical output.
This has a significant impact on system behavior. During daylight hours, the battery charges quickly and often remains near full capacity. Instead of slowly draining, the system behaves more like a solar-powered supply with a battery buffer.
Estimating Real Power Consumption
To understand system limits, it helps to look at actual consumption.
The Icom IC-705 draws approximately 0.4A in receive mode and up to 2.8A when transmitting at 10W. In a realistic operating scenario - roughly 20% transmit and 80% receive - the average power consumption is about 11 to 12 watts.
With a usable battery capacity of around 5.4Ah, this results in approximately 6 hours of continuous operation without solar input.
Expanding the System: More Than Just a Radio Power Supply
To make the setup more practical, I added a simple Y-split cable at the output:
graph TD
A[Solar panel] --> B[MPPT Controller]
B --> C[Y-Split]
B --> D[Battery]
C --> E[12V Socket]
E --> F[USB Adapter]
F --> G[Phone]
C --> H[IC-705]
This allows the system to power the radio while simultaneously charging additional devices such as a smartphone.
In practice, this transforms the setup into a general-purpose field power source.
Charging a phone typically adds another 5–10W of load, increasing total consumption to roughly 17–22 watts. As a result, battery-only runtime drops to around 3.5 to 4.5 hours. Under good sunlight conditions, however, this additional load is often offset by solar input.
Scaling the System: Adapting Capacity to Use Case
If longer runtime is needed, the system can be expanded by adding more batteries in parallel:
12V 6Ah + 12V 6Ah → 12V 12Ah
This increases capacity while maintaining the same system voltage. Doubling the battery capacity effectively doubles runtime, making it possible to extend operation through the night.
While series configurations (e.g., 24V systems) are technically possible, they are not particularly useful here. Most devices - including the radio—expect 12V, so higher-voltage systems would require additional conversion stages.
For this setup, parallel expansion remains the most practical option.
Why the System Works in Practice
The real advantage of this system becomes clear when looking at energy flow.
Even under moderate sunlight - around 30W of real output—the system can cover the average load of the radio. In these conditions, the battery is no longer being depleted during the day. Instead, it remains stable or charges further.
This creates a state of energy equilibrium, where solar input matches or exceeds consumption. It is this condition that enables sustained field operation.
Seasonal Reality: When Solar Is Not Enough
The behavior described so far assumes favorable conditions - reasonable daylight duration and sufficient solar intensity. This is often true in spring and summer, but it does not hold year-round.
In winter, several factors change simultaneously:
- Daylight hours are significantly shorter
- The sun remains at a lower angle, reducing panel efficiency
- Cloud cover is more frequent
- Overall solar irradiance drops
Under these conditions, the panel may no longer be able to sustain the system load during the day. Instead of maintaining energy equilibrium, the system gradually shifts back toward battery-dependent operation.
This has direct consequences. The battery may not fully recharge before sunset, and available runtime becomes more limited.
Design Implication: Capacity Becomes Critical
In low-sun scenarios, the role of the battery changes fundamentally.
Instead of acting primarily as a buffer, it becomes the primary energy reserve.
This is where a larger battery becomes valuable. Increasing capacity allows more energy to be stored during short daylight windows and provides a buffer for extended periods of low solar input.
In practical terms, adding batteries in parallel is the most effective way to adapt the system for winter use.
This introduces a clear trade-off:
- In summer, a small battery is sufficient and keeps the system lightweight
- In winter, a larger battery improves reliability but increases weight and complexity
Powering the IC-705 Correctly
The Icom IC-705 supports both a removable battery pack and external DC input.
In this setup, the LiFePO₄ battery functions as an external power source, not as a replacement battery. This allows the radio to operate at its full 10W transmit power while maintaining a stable supply voltage.
Practical Details That Matter
Using Anderson Powerpole connectors simplifies the system significantly. They are widely adopted in amateur radio, provide low resistance, and make it easy to reconfigure connections in the field.
There are still limitations. A single 6Ah battery will not sustain long overnight operation under heavy load, and once the battery is full, excess solar energy cannot be used. Cable quality and sizing also influence performance, particularly at higher currents.
Conclusion: A Different Way to Think About Portable Power
This setup demonstrates a shift in perspective.
Instead of focusing on maximizing stored energy, it is more effective to design around continuous energy availability. With sufficient solar input, even a small battery becomes viable, enabling lightweight and flexible field systems.
Final Thought
Once solar production exceeds your average consumption, your system is no longer defined by battery capacity - but by sunlight availability.