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CHAPTER 5: Putting Your Solar System Together

Marine Solar Wiring & Installation

Learn how to wire, protect, connect, and install a marine solar system correctly: from the solar panels to the controller and battery bank.

A properly sized solar system can still underperform if the wiring, connections, fusing, or installation are not done correctly. This chapter covers the practical considerations that affect system performance, reliability, and safety.
​Before installing anything, understand the two primary sections of the solar system:

1. Solar panels → solar controller
2. Solar controller → battery bank

The wiring between these components must be sized to carry the expected current while minimizing voltage drop.

Key takeaway
The right solar panels and controller are only part of the system. The wiring must also be capable of carrying the power the system is designed to produce.

5.1. Wiring Multiple Solar Panels: Series vs. Parallel

​Solar panels can be wired in series, parallel, or a combination of both.

Wiring Solar Panels in Series

When panels are connected in series:

PROS

• The amperage from the solar array to the controller is lower than in a parallel configuration so the wire size can be smaller.

CONS

• The voltage from the solar array to the controller is higher than in a parallel configuration so the controller operates less efficiently to bring the voltage down to what the battery bank requires.
• If one panel is shaded, the output of the entire solar array is significantly reduced.

Wiring Solar Panels in Parallel

When panels are connected in parallel:

PROS

• The voltage from the solar array to the controller is lower than in a series configuration so the controller operates more efficiently to bring the voltage down to what the battery bank requires. (More power stored)
• If one panel is shaded, the higher output of the other panels is not hindered. (If the panels have proper diodes built in)

CONS

• The amperage from the solar array to the controller is higher than in a series configuration so the wire size must be larger.

☑️ For many marine installations, CMP generally favors parallel wiring, particularly where shading is a concern. However, the best configuration depends on the panel arrangement, wire run, controller specifications, and shading conditions.

CMP Tip:
If in doubt and you have limited shading, try both configurations to see which provides the maximum watt hours per day.

Hybrid configurations

A hybrid arrangement can combine the advantages of both approaches.
For example, two panels in series combined with another two-panel series string in parallel. This can be useful when panels are installed in different areas of a boat with different shading conditions.

5.2. Choosing the Right Marine Solar Wire

Solar Wire Specifications

Marine solar wiring needs to handle current reliably in a harsh environment.

For marine installations, we recommend:

• Tinned marine wire to reduce corrosion.
• High-strand-count wire for flexibility and resistance to vibration and fatigue.
• UV-resistant solar cable for wiring exposed to sunlight.
• Proper wire sizing based on current, wire length, and acceptable voltage drop.

Why Voltage Drop Matters

Voltage is lost as current travels through a wire. Excessive voltage drop reduces the amount of power that reaches the controller or battery.

Wire size therefore depends not only on the amount of solar power but also on how far the power has to travel.

CMP's wire-sizing charts can be used to determine an appropriate wire gauge for a specific installation.

Don't undersize the wire just because the system works on paper.
The wiring must be sized for the actual current and distance of the installation.

5.3. MC4 Solar Connectors

Most CMPower solar panels come with MC4 connectors pre-installed. These male and female connectors provide a secure, waterproof connection and make solar panel wiring simple.

For marine installations, CMPower recommends applying dielectric grease inside the connectors to help protect the connection from moisture and corrosion.

Connecting Solar Panels with MC4 Connectors

To connect two or more solar panels in parallel, use MC4 T-branch connectors. These combine the panel connections before the wiring runs to the solar controller.
Depending on the system, 2-to-1, 3-to-1, and 4-to-1 MC4 branch connectors are available.

Although MC4 connectors are highly reliable, they can occasionally fail because of corrosion or a poor connection. If a solar system suddenly stops producing power, inspecting the MC4 connections is a good troubleshooting step.

Installing MC4 Connectors

MC4 connectors can be installed in the field, but a specialized MC4 crimping tool is required. The metal contact is crimped onto the stripped solar cable, inserted into the connector housing, and secured by tightening the connector barrel. An internal gasket seals around the cable to create a waterproof connection.

For CMPower solar kits, this step is already done for you. Kits include 50- or 60-foot cable coils with male and female MC4 connectors pre-installed. Cutting the coil in half provides two approximately 25- or 30-foot cable runs, ready to connect the solar panels to the controller.

This plug-and-play approach eliminates the need for field crimping and simplifies installation.

Key takeaway: MC4 connectors provide a reliable, waterproof connection for marine solar wiring. Properly installed connectors and protection against corrosion are important for long-term reliability.

5.4. Turning a Solar System On and Off

• We recommend inserting a switch or breaker in the positive wire between the solar array and the controller to turn the solar system on and off.
• Why? To prevent the alternator and shore power charge controller from prematurely going into float mode.
• The alternator may sense a higher than actual battery voltage due to the solar controller voltage and thus sense the battery bank is in a higher state of charge and prematurely go into float mode reducing the output of the alternator.
• If this occurs, use the switch to shut down the solar system and gain the full output power from the alternator or shore charger.
• This only occurs with some charging devices under certain conditions.

Note: Any DC on/off switch with sufficient rated capacity will do.

5.5. Wiring the Solar Controller to the Battery Bank

• Wiring – Position the Controller close to the battery bank.
• Unless there is a battery monitor, the negative wire from the controller should attach to the negative bus or the negative terminal on the battery. The positive wire should go directly to the battery.
• Wire Size – Adequate size wire is critical.
  Solar array watts / boat voltage = amps going through the wire. 240 watts / 12V = 20A


• Fuse – Per ABYC standards, the positive wire leading to the battery bank should be fused roughly 10 amps above the rated capacity of the solar controller.


• Load function – The LOAD function available on some controllers is not used for marine applications. Ignore it. LOAD is primarily used to control lighting systems.


• Temperature sensor – The temperature sensor should be attached to the side or top of a battery. If the battery heats up, the controller cuts back on the power supplied. A temperature sensor is not necessary for LiFePO4 batteries or for small systems with solar arrays under 150 watts.


• Connection sequence – The solar controller is powered by the battery bank, not the solar panels. Thus, the solar controller should be attached to the battery bank first and then to the solar panel array.

Always check to be sure the + and – wires are the correct polarity before inserting into the controller.

5.6. Wiring a Solar Controller with a Battery Monitor

If your boat has a battery monitor with a shunt, the solar controller should be wired through the shunt so the monitor can accurately measure all power entering and leaving the battery bank.
A shunt acts like a measuring point between the battery bank and the boat's electrical system. It allows the battery monitor to track the current flowing into and out of the batteries.

Connecting the Solar Controller

Without a battery monitor, the controller's negative wire would normally connect directly to the battery bank's negative terminal.

When a shunt is installed:

• Connect the solar controller's negative wire to the system/charging side of the shunt, rather than directly to the battery negative terminal.
• Connect the controller's positive wire to the positive battery bank terminal, with the appropriate fuse.
• This ensures the solar charging current passes through the shunt and is included in the battery monitor's measurements.

If the controller's negative connection bypasses the shunt and connects directly to the battery, the monitor will not measure that charging current correctly.

Key takeaway: When a battery monitor and shunt are installed, route the solar controller's negative connection through the shunt so the monitor can accurately track solar energy going into the battery bank.

5.7. Solar System Fuse Considerations

​Proper fusing protects the solar wiring, controller, and battery bank from excessive current if a component or wire develops a short circuit. Tom recommends considering three key fuse locations in a marine solar system.

☑️ Fuse Each Solar Panel

When an array has more than two solar panels, install an inline fuse on the positive wire from each panel before the panels are combined through an MC4 T-branch connector.
A 10-amp inline fuse is typically used with CMPower panels. MC4-compatible inline fuse holders make this connection simple and plug-and-play.
If one panel develops a short circuit—for example, from physical damage or another fault—the fuse can disconnect that panel without allowing the fault to affect the other panels in the array.

☑️ Fuse the Solar Controller

Install a fuse between the solar controller and the battery bank. CMP recommends sizing this fuse approximately 10 amps above the controller's rated capacity.
This fuse protects the positive wiring between the controller and battery bank.

☑️ Use a T-Class Fuse with LiFePO₄ Batteries

LiFePO₄ batteries can deliver very high amounts of current during a short circuit. CMPower recommends using a T-Class fuse as part of the battery protection system.
T-Class fuses are heavy-duty, high-interrupt-capacity fuses designed to safely disconnect the battery in the event of a serious short circuit.

A properly protected solar system should consider fusing at the solar panel array, solar controller, and battery bank. The appropriate fuse type and rating depend on the equipment and system configuration.

5.8. Solar System Troubleshooting

When a solar system is not producing the expected power, start with the simplest possibilities: battery connection, fuses, wiring, MC4 connections, and battery state of charge.

Meter shows voltage but zero amps from the solar panel

• The controller is likely not getting power from the battery.
• Check the battery connection and the fuse between the battery and the controller.
• Measure the voltage of the battery wires at the controller.
• If all looks good, measure the voltage and amperage at the panel pigtails.
• Reboot the controller.

A useful troubleshooting clue:
A controller can appear to be working because the solar panel is providing enough voltage to power its display, while a blown fuse or poor battery connection prevents the system from actually charging the batteries.

The solar panels are not producing the full amount of rated power

• The power output of a solar panel decreases as the panel heats up.
• The batteries may be at full charge so the controller is cutting back the solar power.
• There may be a loose or defective connection in the MC4 connectors or the wiring between the solar panels and the solar controller.

When troubleshooting:
Check the battery connection and fuses first, then inspect the wiring and MC4 connections before assuming the solar panels or controller have failed.

When You Need More Help
If these basic checks do not identify the problem, use CMPower's solar system troubleshooting guide here for a more detailed step-by-step checklist.

5.9. Mounting Marine Solar Panels

​Choosing the right mounting method depends on the type of solar panel, the installation surface, and how the boat will be used. CMPower solar panels can be installed using several practical approaches, including J-channel mounting for walk-on panels, heat-dissipating materials beneath panels, and bolted canvas mounting.

Mounting Semi-Rigid Walk-on Panels with a “J” Channel

For walk-on semi-rigid solar panels installed on a boat deck, a J-channel mounting system can provide a secure installation while allowing the panel to remain walkable.

The J-channel supports and secures the edges of the panel without requiring the panel surface itself to be penetrated. This approach is particularly useful when installing solar panels in areas where crew members need to walk on or around the panels.

Heat Dissipation for Flexible and Semi-rigid Walkable Solar Panels

Solar panels can become extremely hot in direct sunlight. It can reach 150°F or more on a warm day.

When installing flexible or semi-rigid panels directly over a canvas bimini or boat deck, placing an appropriate material between the panel and the mounting surface can help manage heat.

Two materials discussed in the CMP Solar Design Seminar are:

• Reflectix® insulation -- useful beneath flexible panels installed over canvas, particularly in hot climates. The reflective bubble material helps reduce radiant heat transferred to the canvas and cockpit area.
• Twin-wall polycarbonate -- available in 4 mm, 6 mm, and 8 mm thicknesses and can be used beneath semi-rigid panels to create separation from the deck and allow better heat management.

Mounting Flexible Panels to Canvas

Flexible solar panels can also be secured directly to a canvas bimini or similar fabric surface using a bolt-and-washer mounting method.

CMPower panels include grommets around the perimeter. The mounting method uses these existing grommets to secure the panel without modifying the panel itself.

The basic installation involves:

1. Create a clean hole in the canvas using a hot tool to prevent excessive fraying.
2. Insert a 10-24 bolt through the canvas and secure it with a washer.
3. Position the solar panel grommet over the bolt.
4. Add the appropriate washer and nylon lock nut.
5. Tighten the assembly securely.
6. Apply adhesive/sealant around the mounting point if desired for additional protection.

This method also makes the panel relatively easy to remove when necessary: simply remove the lock nut and lift the panel away.

Choose the Mounting Method for Your Boat

There is no single mounting method that works for every boat. Consider:

• Walk-on deck: J-channel or another low-profile mounting system.
• Canvas bimini: Bolt-and-grommet mounting or another canvas-compatible mounting method.
• Hot climates: Consider a heat-dissipating or insulating layer beneath the panel.
• Permanent deck installation: Choose a mounting method appropriate for the deck material and installation requirements.

The goal is to secure the panel properly while considering walkability, heat, ventilation, durability, and ease of maintenance.

5.10. Wiring a Battery Bank: Series vs. Parallel

Once you have determined the right battery bank size for your boat, the next step is deciding how the batteries should be connected.
Batteries can generally be wired in parallel or series, depending on the voltage and power requirements of your system.

Wiring Batteries in Parallel

When identical batteries are wired in parallel, the system voltage remains the same, while the available battery capacity and current capability increase.

For example, four 12V, 100 Ah batteries wired in parallel remain a 12V battery bank, but their capacity is combined.

This configuration is useful when a boat requires more stored energy or higher available current while continuing to operate on a 12V system. Multiple batteries in parallel can also provide the current capacity needed for high-demand loads such as engine starting or other large DC loads.

Wiring Batteries in Series

When batteries are wired in series, the voltage increases, while the amp-hour capacity remains the same as a single battery in the series string.

For example, four 12V, 100 Ah batteries wired in series create a 48V battery bank with 100 Ah of capacity.

Series wiring is commonly used when a higher-voltage battery system is required, such as in electric propulsion or other higher-voltage marine applications.

Key Takeaway

• Parallel wiring: Increases available capacity and current while maintaining the same voltage.
• Series wiring: Increases system voltage while maintaining the amp-hour capacity of the battery string.

Always follow the battery manufacturer's specifications regarding the maximum number of batteries that can be connected in series or parallel. For the CMPower battery shown in this example, the BMS supports up to 10 batteries in parallel and 4 batteries in series.

Important: Before connecting batteries together, make sure their state of charge is within 5% of each other, as indicated in the installation guidance.

5.11. Managing Multiple Battery Types Onboard

The Dilemma of Multiple Battery Types Onboard

Many boats have more than one battery system onboard. For example, you may have a lead-acid or AGM battery for engine starting, thrusters, or generator starting, while upgrading the house bank to LiFePO₄.

This creates an important wiring and charging consideration.

The Rule: Never combine different types or sizes of batteries.

Lead-acid, AGM, and LiFePO₄ batteries have different charging and discharge characteristics. For this reason, different battery types or sizes should not be directly combined into the same battery bank.

Instead, they should be kept electrically separated while using an appropriate charging method for each battery.

✅ Option 1: Battery Isolator

Charging Multiple Batteries While Keeping Them Separated

A battery isolator allows multiple batteries to be charged from the same charging source while preventing the batteries from being directly connected to one another.

For example, an alternator can charge a lead-acid starting battery and a LiFePO₄ house bank through an isolator without allowing the batteries to directly interact.

Advantages:

• Keeps different battery banks electrically separated.
• Allows multiple batteries to be charged independently.

Disadvantage:

• An isolator provides only one charging profile, which may not be ideal when charging batteries with different charging requirements.

Important: A battery combiner is not the same as an isolator. Avoid using a combiner to directly connect different battery types or sizes.

✅ Option 2: DC-to-DC Charging

The Recommended Solution for a LiFePO₄ House Bank

A DC-to-DC charger is an effective solution when you want to keep a conventional starting battery while adding a LiFePO₄ house bank.
The alternator first charges the starting battery. Power can then be supplied through a DC-to-DC charger to the LiFePO₄ house bank.
The DC-to-DC charger keeps the battery banks isolated and adjusts the incoming power to provide the appropriate charging profile for the house battery.
This allows each battery system to operate independently while still using the alternator as a charging source.

Advantages:

• Keeps different battery types electrically isolated.
• Supports different charging profiles.
• Easy to install and monitor.
• Helps protect the alternator.
• Multiple DC-to-DC chargers can be wired in parallel when additional charging capacity is needed.

For boats with a lead-acid or AGM starting battery and a LiFePO₄ house bank, this is generally the preferred approach.

✅ Option 3: Dual-Output Solar Controller

Charging Separate Start and House Batteries with Solar

Solar can also be used to charge separate battery banks.
A dual-output solar controller can maintain a starting battery while directing the primary charging capacity to the house battery bank. This allows a lead-acid starting battery and LiFePO₄ house bank to remain separate while both receive solar charging.

✅ Option 4: Single Battery Bank

Use One Primary House Bank with an Emergency Battery

Another approach is to use a sufficiently capable LiFePO₄ house bank as the primary battery system for major onboard loads, including house loads and other high-demand equipment.
A separate battery can then be retained for emergency engine starting.

In this configuration, a battery switch can be used to access the emergency battery if the primary battery bank becomes unavailable.
This approach simplifies the main power system while maintaining a backup starting option.

Alternator Protection and Sudden Battery Shutdown

One important consideration when connecting a LiFePO₄ battery bank to an alternator is what happens if the battery management system (BMS) suddenly disconnects the battery.
A sudden battery shutdown can create a voltage spike that may travel through the charging system and potentially damage sensitive alternator components, including its internal diodes.

Using an Alternator Protection Device

Alternator protection devices are designed to absorb or suppress these voltage spikes.
These devices can provide additional protection when an alternator is connected directly to a battery bank that could be suddenly disconnected by its BMS. Several manufacturers offer alternator protection solutions, including Sterling Power, Balmar, and Victron.
In a direct alternator-to-LiFePO₄ battery connection, an alternator protection device can be inexpensive insurance against damaging voltage spikes.

DC-to-DC Charging Can Provide Additional Protection

However, the system design can eliminate the need for a separate alternator protection device.
When using a DC-to-DC charger between the starting battery and the LiFePO₄ house bank, the starting battery remains connected to the alternator. If the LiFePO₄ battery bank shuts down, the starting battery can help absorb the resulting spike.
The DC-to-DC charger also isolates the house battery from the alternator and helps prevent the voltage spike from passing back through the charging system.

Key takeaway: A direct alternator-to-LiFePO₄ battery connection may benefit from a dedicated alternator protection device. In a properly configured DC-to-DC charging system with a separate starting battery, the system itself provides additional isolation and protection.

5.12. LiFePO₄ Battery Installation Considerations

​Installing a LiFePO₄ battery bank involves more than simply replacing an existing battery. Before installation, consider battery location, charging equipment, cable configuration, circuit protection, and how multiple batteries are connected.

• Batteries can be mounted on bottom or any side.
• Batteries should be mounted in a moderate temperature environment.
       – not in the engine room unless well ventilated.
• Charging parameters for charging sources may need to be adjusted.
       – max voltage, min voltage cut off, etc.
• Alternator loading – Efficient charging of LiFePO4 batteries may result in overheating of the alternator or additional strain on the alternator belt.
• Battery monitor - Peukert constant should be set to 1.04 (1.25 for lead acid).
• Ideally, all battery cables should be the same length so the resistance is equal.
• A T-class fuse should be installed directly to the battery terminal per ABYC.
• All positive wires should be fused near the battery bank per ABYC.
• All batteries should be within 5% State of Charge before connecting together.
  All batteries at 80% to 100% SOC when connecting is ideal.
• Lead acid or AGM batteries should not be connected with LiFePO4 batteries.
• An alternator protector is cheap insurance to protect alternator diodes.

Final Installation Reminders

Before completing the installation:

• Do not directly combine different battery chemistries.
• Verify that charging equipment is configured for the battery type.
• Consider alternator protection based on your charging configuration.
• Keep parallel battery cables balanced whenever practical.
• Fuse positive conductors appropriately near the battery bank.
• Use suitable high-interrupt protection for LiFePO₄ battery systems.
• Match battery state of charge before connecting batteries in parallel.

A properly planned installation helps the battery bank charge and discharge evenly while protecting the batteries, charging equipment, and the rest of the boat's electrical system.

5.13. LiFePO₄ Battery Operating Considerations

​Installing a LiFePO₄ battery bank is only part of the process. Understanding how batteries balance, how the BMS behaves over time, and how temperature affects charging can help you operate and maintain the system properly.

Battery Balancing

Multiple batteries wired in parallel will get out of balance after several hundred cycles.
Why?

• Each battery has an internal resistance.
• The battery with the lowest internal resistance will charge and discharge faster than the batteries with higher internal resistance.

BMS Calibration

A battery BMS often requires recalibration after several hundred cycles; especially when batteries are wired in parallel.

• To recalibrate a battery, simply fully discharge and fully charge it.
• New batteries may require two full charge/discharge cycles to calibrate the BMS.

It has been observed that more cycles on a parallel battery bank result in faster balancing and less requirement for calibration.

Sleeping BMS

A BMS may go to sleep after a period of inactivity to conserve battery power.
Three ways to wake up a BMS:

• Apply a charge (not a good option on a boat)
• Apply a discharge
• Press a reset button on the top of the battery

Operating Temperature

For operation in cold harsh environments a battery can be equipped with a blanket heater.
The heater is activated when the battery is below operating temperature and a charge is applied. Once the battery is warmed to a safe operating temperature the BMS is activated and the charge is applied to the cells.

5.14. Parameters for Charging LiFePO4 Batteries

​If a “LiFePO4” setting is not available on your controller, use the “USER” function to program and set the following parameters:

• Absorption voltage: 14.0 volts (acceptable range is 13.6V to 14.4V,  28.0V to 28.8V) 
• Absorption Time: The recommended setting is .3 to .5 hours per 100ah of LiFePO4 battery
  (for example: for 2 -100ah batteries select .6 to 1 hour). 
• Float Voltage:  13.5 volts (13.6 volts or lower is acceptable for LiFePO4 batteries although float is not necessary) 
• Equalization voltage: Do not equalize LiFePO4, turn off the equalize function, but in case it ever runs a cycle or equalization can’t be turned off, set to 14.0 volts or less.
• Temperature Compensation:  LiFePO4 batteries do not need temperature compensation. 
• Low Temperature cut-off:  -5 degrees C, 20 degrees F

5.15. Things to Unlearn When Moving from Lead Acid to LiFePO4 Marine Batteries

• Constant Voltage – Voltage does not reflect the State of Charge.
• Battery Charging – Charging to full is not necessary.  95% of power rating is available, not just top 50%.
• Battery Charging – Short absorption stage, no equalization, float stage is not necessary.
• State of Charge – 50% State of Charge is fine. 45% of power is still available.
• Faster more efficient charging – More power is stored faster.
• Discharging – Constant discharge voltage so appliances run more efficiently resulting in decreased power usage.
• Maintenance – No toxicity. No water level to check, no heat to be concerned about.
• Easy Winter Storage – Simply disconnect until next season.
• Easy Monitoring – Bluetooth app displays SoC and power remaining anytime from Smartphone or tablet.

Chapter 5 Summary

Wiring & Installation: Putting Your Solar System Together

A properly designed marine solar system depends on more than choosing the right panels, controller, and batteries. All components must be correctly wired, protected, configured, installed, and maintained to ensure safe and reliable operation.

In this chapter, you learned how to:

• Wire solar panels and battery banks in series and parallel to achieve the required voltage, capacity, and current.
• Select appropriate wiring configurations for your solar array, battery bank, and charge controller.
• Use MC4 connectors, fuses, switches, battery monitors, and other system protection devices correctly.
• Keep different battery chemistries and battery banks electrically separated and understand when to use battery isolators, DC-to-DC chargers, or dual-output solar controllers.
• Protect alternators when charging LiFePO₄ batteries, including understanding the role of alternator protection devices and DC-to-DC charging.
• Install LiFePO₄ batteries correctly, including proper mounting, cable configuration, circuit protection, charging parameters, battery state of charge, and temperature considerations.
• Maintain balanced battery banks by matching cable lengths and state of charge when connecting batteries in parallel.
• Understand LiFePO₄ battery operating considerations, including battery balancing, BMS calibration, BMS sleep mode, and cold-temperature charging.
• Troubleshoot common wiring, charging, battery, and installation problems.
• Choose practical mounting methods for different boat surfaces and solar panel types.

With the system properly wired, protected, installed, and maintained, your solar and battery system can safely deliver the power needed to keep your boat's batteries charged and your electrical system running reliably.