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Charging a Boat Battery with a Solar Panel

The modern boat requires electrical power to run many applications on it including the fish finder, the pump and for propulsion. A boat can run on two batteries one for trolling and the other one for cranking i.e starting the engine.  

After a long day on the boat you will need to recharge your batteries. While your cranking battery can be recharged by the alternator this is not the same for the trolling deep cycle battery or bank.

An onboard charger would mean that you may have to drag your battery or boat to somewhere close to power. This may not be possible in some cases maybe you are out in the wilderness or the storage compartment of your boat doesn’t allow. You may consider a portable charger but this are only good for maintaining a battery and not fully charging a deep cycle battery.

A Solar panel allows you to charge from anyway where you have access to sunlight. But size is important and just like portable chargers and onboard chargers the size of the panel will determine whether it will fully recharge your battery or just maintain it.

What Size Solar Panel to Charge your Marine Battery

For a small boat a 20 Watt Solar Panel will be sufficient for trickle charging your battery. This is to prevent your battery from dying from self-discharge. Solar panels produce DC power that acts as a charger topping up the battery.  

If you are looking to recharge the battery a 70 watt charger will most likely do the job however it might be to slow in bad weather or cloudy conditions and you should consider a higher Wattage to get faster charging.

A 12V 100 watt Monocrystalline Solar panel that is efficient (around 20% conversion rate) will ensure you get 5-8 amps a day like you would a regular charger and recharge your batteries in under 8 hours.  This can be adequate for a small boat with a single 12v deep cycle battery.

Charging a Boat Battery Bank

If there are more than two batteries on the boat (that is the cranking battery and two deep cycle batteries) you may need to double up on the panel. The batteries can be connected in parallel to maintain the voltage and charge them as one bank.

Depending on the number of batteries you want to charge or maintain you may have a two, three or even 4 battery bank. Most batteries are connected in Series which adds up the voltage to 24V or 36V in some cases.

These may reduce the charging time from the solar panels and you will need to add them to compensate depending on how fast you want to charge your batteries.

Charge Regulator

In whatever set up and wattage of the Solar panel it is important to always have a charge controller. A Charge Regulator or a Charge Controller will ensure the battery is protected from Overcharging. The controller also prevents reverse charging or the solar panel discharging the battery at night or when not in the sun.  

Three Stage Charging

The voltage and current relationship is referred to the battery charge cycle and is broken down in three stages. The three stages are; bulk, absorption and float and are applicable to Wet, Gel and AGM Lead Acid batteries. A charge controller will allow you to set how you want your battery to be charged.

You can set from the 3-stages depending on the level of charge of your battery or your battery type.

Bulk Charging

Bulk charging is the normal battery charging that provides a fast charge. To provide a fast charge sufficient sunlight must be available and the battery must have been deeply discharged.

Absorption Charging

The Absorption stage is where the battery slows down the amps going in the battery once a certain voltage is reached.  It is conditioning mode to step down the charging but allow the panel to complete charging.

Float Charging

At the float stage the battery is full and is only being topped off with a small charge to compensate for the idle discharge.

A 10A PWM Charge Controller will do the job. A PWM Charge Controller will work for AGM, SLA and Gel Batteries. You will need a more specialized charge controller for your lithium batteries as they behave differently from Lead Acid batteries in terms of the stages of charging. 

Wiring

You will need the following wiring to ensure charge is delivered from the Solar Panel to your battery;

  • DC to SAE Connector
  • SAE Alligator Clips to attach to battery.
  • An extension cable.

Ensure that you have long cables (up to 6ft) according to the boat size to ensure that battery is connected to the Solar panel and the Solar Panel is mounted at the best location ensure maximum sunlight absorption. You will also need to mount the charge controller away from any water exposure as the most charge controllers are not waterproof.

Depending on your battery you may also find that Cables with Ring clips may suit you better as they don’t easily detach from the battery. This can lead to great disappointment if you leave the alligator clips intact only to find them detached later and battery not charged.

Remember also always connect the battery before connecting the solar panel and when disconnecting disconnect the solar panel first and then the battery.

Mounting the Panels

Solar panels are traditionally mounted perpendicularly and facing south for maximum output. But since a boat is always moving it recommended mounting the panels horizontally.

You can choose to mount them permanently or hook them up temporarily. You can place the panels on the deck, radar arches or on the T-tops and biminis.

Whether you are storing you boat or batteries in a cabin ensure you have long cables to mount the solar panels where there is sufficient sunlight.

How to hook up your boat batteries to a Solar Panel

The following are the steps to follow when connecting your motorcycle battery to a solar panel.

  1. Choose where to Charge from

With Solar panels you have the option of charging the deep cycle batteries while they are on the boat or remove the batteries to charge them from your RV or cabin.

2. Clean the Terminals

Wipe off any grease or dirt and ensure the batteries terminals are clean.

3. Connect the Cables

Make the connection using the alligator clips or the O-ring terminals by connecting the red (positive) cable to the red terminal first. Then connect the black (negative) to the negative terminal

4. Connect the Charge Regulator

Make sure the first thing you connect is the Controller. You should NEVER connect a Solar Panel to the Controller without a Battery. The charge controller should be clearly labeled for Positive and Negative (‘BATTERY + and -‘). This is where you hook up the batteries.  

5. Connect the Solar Panel to the Charge Controller

Once you have connected the battery now connect the charge controller to the solar panels. The charge controller should also be labeled for a ‘PV INPUT + and -‘. This is where you hook up the cables from the panels.

6. Check the Indicator

The controller will have an indicator to show that it is receiving charge from the Panels. Once connected you confirm you can leave.

7. Disconnecting

When disconnecting start with the Panels first and then disconnect the batteries from the controller. Remember to Grease the terminals to prevent corrosion.