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Charging RV Batteries Using a Solar Panel

Your RV Battery can be charged through the alternator when driving, shore power or via a battery charger at home.  The Alternator is not a good option if you won’t be driving for long period of times and only offers a trickle charge to the RV batteries. Shore power will require hook ups on campsites and a battery charger requires a power source.

This is where Solar power comes in, solar power will allow you to charge your batteries. Depending on the size of the Solar Panel and sunlight conditions a Solar panel can Trickle charge or Fast charge your battery.  Solar panels are portable meaning you can charge your batteries wherever you are and may also save the starting battery from dying.  Even when maintaining your battery you don’t need to take out the battery.

A solar panel also provides DC current and most of the appliances in an RV run on 12v DC Current. This means after charging your RV batteries you can hook up the panels to some of your appliances directly that includes your laptop and phone.  You may also use an inverter in the set up and draw an AC current from the batteries while charging with DC power from the sun.

What Size Solar Panel to Charge my RV Battery

This will depend on the size of the battery. For a 100ah battery that is a deep cycle battery not the starter battery. Assuming you use 60 amp hours a day the battery will last for a day and a half.   100 Watt Solar Panel can around 6 amps during a peak sun hour or around 30 amps hours daily. So you need a 200 Watt panel or two 100 panels wired in Parallel.

Because of efficiency and sunlight conditions you may need to upgrade to a 300 watt system. Monocrystalline Solar Panels are the more preferred options as they have better efficiency and take up less space. Thin Film Panels which are lighter and more flexible don’t have great performance and output declines over time.  

How to Set Up a Solar Panel System RV Batteries

First of all you should never connect solar panels to your batteries directly. You need a charge controller to protect your batteries from overcharging.

  1. In order to start determine the location to install the charge controller. This should be as close the RV’s battery as possible. Make sure that this area is water tight and allow for as much air flow as possible.
  2. Ensure you choose the right wire gauge for your cables. At the end of the cables use alligator clips or ring terminals that are to be hooked up on the batteries terminals. The clips should be labeled positive or negative depending on where it the cable is hooked up on the panel.
  3. Access the battery and identify the negative and positive terminals. This is easy the negative terminal which is the ground cable will have a negative (-) and the power or positive cable will have plus (+) sign.
  4. When using a Charge Controller the battery should be the first thing you connect to the Controller. You should NEVER connect a Solar Panel to the Controller without a Battery. The charge controller should be clearly labeled for ‘BATTERY + and -‘ make SURE the polarity is correct.
  5. 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 panels. Make sure not to mix up the positive and negative Positive (+) is usually red, while negative (-) is usually black. If you are not sure you can check polarity using a multi meter.
  6. Make sure to select the correct profile for your battery on the charge controller whether Lithium, Gel or AGM batteries. More on this next.

3-Stage Charging Using a Charge Controller

A charge controller will allow you to set how you want your battery to be charge. You can pick from the 3-stages depending on the level of charge of your battery or your battery type.

The three stages are; bulk, absorption and float and are applicable to Wet, Gel and AGM Lead Acid batteries.

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.

Equalization

Some MPPT Chargers will routinely (once a month) apply an equalizing charge. This is the intentionally overcharging of Lead Acid batteries to remove sulfate crystals that build up on the plates over time commonly referred to as sulfation.

Setting your Charge Controller for Lithium Batteries

Lithium Batteries use different settings as they are charged differently.  Lithium batteries just use bulk mode until a target voltage is achieved. You may set for absorption mode for a couple of minutes before going to float. But it is possible to go straight from Bulk to float.

The basic is to ensure the bulk voltage is set at around 14.4V, 14.6 for the absorption stage and 13.5 or 13.6 for float.

But each battery should come with specifications from the manufacturer. Ensure you check the specified settings for your batteries by your manufacture.

An Example for the Renogy LFP Batteries:

Standard Charge shall consist of charging at 0.2C constant current rate until the battery reaches 14.6V.
The battery shall then be charged at a constant voltage of 14.6V while tapering the charge current.
Charging will terminate when the charging current has tapered to a 0.02CA. Charge Time is approximately 7 hours. Safe Charging consists of temperatures between 32 ºF and 113 ºF. Battery Standard Discharge is constant current of 0.2C to 10V.

Also make sure to turn off Temperature compensation and Equalization features as these settings are not required for lithium batteries.

Sizing a Charge Controller

Charge Controllers have a power rating measured Amps. This rating is the maximum amount of electrical current the controller can handle without failing. Solar panel current or battery current should not exceed the maximum power rating of the charge controller.

It is recommended to oversize the controller by around 20% as this design will run smoother and cooler. Always ensure that the controller enjoys enough air circulation not to overheat.

Using a Solar Generator

You can also consider a portable solar generator or a portable power station which is a large battery pack with both DC and AC outlets.  You can use it charge your campers batteries or run your AC appliances or power your DC lights.

It acts like a large power bank with inbuilt inverter and controller and can be recharged via solar panels and once fully charged you can use the solar panels to directly charge your devices such as phones and laptops and save up the power station for later.

Solar Trickle Charger

A solar trickle charger is a small 5 -10 watt solar panel that can be used as a maintainer to keep your batteries from dying. It charges the battery at the same rate it is discharging.

Whichever option you take make sure it the right one and properly sized. You can combine two or more options to make sure your system is running seamlessly and powering all your devices. RVing is all about freedom and solar ensures you have the freedom to enjoy the outdoors fully untethered from the grid. It also acts as an emergency backup and having it as a secondary option to your main source of power will provide you with a peace of mind.