Charge Controller Selection and Wiring

Why You Need a Charge Controller

Solar panels produce unregulated voltage - a nominally "12 V" panel can output up to 22 V open-circuit. A charge controller protects the battery from overcharge, manages the multi-stage charge profile (bulk, absorption, float), and in many cases provides a protected load output to prevent deep discharge.

PWM vs. MPPT

PWM (Pulse Width Modulation)

Simple and cheap. The controller connects the panel more or less directly to the battery and chops the current. Efficient only when the panel's operating voltage is close to the battery voltage. Fine for small, well-matched systems where cost matters more than harvest efficiency.

MPPT (Maximum Power Point Tracking)

Continuously sweeps the panel's voltage-current curve to find the operating point that yields maximum power output. Typically delivers 10 - 30% more energy than PWM, especially in cool or partly-cloudy conditions, or when panel voltage is significantly higher than battery voltage. Required for larger panels; recommended for any challenging installation.

Recommendations for LoRa Mesh Nodes

Small systems: 5 W panel + 3.7 V LiPo or small LiFePO4

A dedicated solar LiPo charging board with built-in MPPT is the simplest and cheapest approach. Examples:

• CN3791-based boards (widely available on AliExpress, ~$3 - 8)
• Waveshare Solar Power Manager (~$12 - 15)

These handle cell-level charging directly and fit neatly into a small enclosure alongside the node.

Medium systems: 10 - 20 W panel + 12 V LiFePO4

A dedicated MPPT controller is warranted. Good options at modest cost:

• Renogy Wanderer 10A - inexpensive, reliable, widely available
• Victron SmartSolar 75/10 - premium, Bluetooth monitoring via the VictronConnect app, excellent LiFePO4 support. Higher cost but extremely reliable for permanent installs.

Wiring Sequence

Always follow this order to protect the controller:

1. Connect battery to controller first
2. Connect panel to controller
3. Connect load to controller

Disconnect in reverse order: load → panel → battery.

LiFePO4 Charge Profile

LiFePO4 uses a different charge profile from lead-acid. Key voltages per cell:

• Absorption voltage: 3.65 V/cell
• Float voltage: 3.60 V/cell

For a 4-cell (12 V nominal) pack: absorption = 14.6 V, float = 14.4 V.

Warning: Most cheap PWM controllers are factory-calibrated for lead-acid (14.4 - 14.8 V absorption). Using lead-acid settings on LiFePO4 will overcharge and damage the cells. Verify that your controller has a LiFePO4 mode or use a controller specifically designed for LiFePO4.

Load Output and Low-Voltage Disconnect (LVD)

Many charge controllers have a dedicated "load" terminal that automatically disconnects the load when the battery drops below a programmable voltage. Connect your node to this terminal rather than directly to the battery. Set LVD to 3.0 V/cell:

• 12 V system (4S LiFePO4): LVD = 12.0 V

This prevents deep discharge, which is the primary cause of premature LiFePO4 cell death.

Fusing

Always fuse the battery connection as close to the battery terminal as practical. A short circuit without a fuse can dump hundreds of amps through wiring and cause a fire. Use automotive blade fuses. Size the fuse to approximately 2× the expected maximum current draw of the system.

Example: a node drawing 500 mA peak - use a 2 A fuse on the battery lead.