How to Charge Gel Battery with Solar Panels

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Charging gel batteries with solar panels is one of the best ways to use renewable energy in an off grid or grid tied home. If you have never used this method before, the recharging process is actually easy.

The basic steps are as follows. Connect the charge controller to the battery first. Plug the charge controller wires into the solar panels and leave it there until the battery is charged.

What You Need

Step 1. Position the Charge Controller

Solar controllers do not have weatherproofing, so they are best installed indoors. Find a safe place if you have to install the controller outdoors. There should be a roof over it in case of a downpour.

Place the controller on a piece of wood, PVC or any non-conductive material. Do not mount the controller on anything metallic.

Step 2. Connect the Battery Cables onto the Battery

If your gel battery already has cables, skip to the next step. Otherwise, connect the wires into the battery. The cables are usually red (positive) and black (negative).

Note that the colors may be different with your battery. Refer to the instructions to find out which is positive and negative, and how to connect each wire. Most of the time the battery wires will already be connected though.

Step 3. Connect the Battery Cables to the Charge Controller

Important: connect the battery to the charge controller before you plug the battery into the solar panel. if you connect the battery to the solar panel first, this could result in a sudden power charge and damage the components.

Hold the ends of the battery cables and insert them into the corresponding charge controller ports. You do not need to use MC4 connectors. Just put the battery wires into the ports and secure them with a screwdriver.

Use a #16 or #10 gauge wire for 12V batteries. Here is a guide for other wire sizes. Charge controllers have a positive and negative port for the battery wiring. Make sure you insert the cables into the appropriate slots.

Step 4. Put MC4 Connectors on the Charge Controller Wires

Take the cables running from the solar controllers and put male and female MC4 connectors on them. You need to install the connectors so you can plug these into solar panel cables.

Connect the input cable bare ends in the same manner you did with the output wiring. Just insert them into the proper ports. Fasten it with a screwdriver.

MC4 connectors are included in solar panel kits. You can buy these from any solar supplier. Refer to the instructions on how to properly insert them into wires.

Step 5. Connect the MC4 Connectors to the Solar Panels

If you followed the step above, the charge controller wires will have MC4 connectors. Match these male and female connectors with the MC4 connectors on the solar panel.

Connect the male charge controller connector to the female MC4 solar panel connector.
Connect the female charge controller connector to the male MC4 solar panel connector.

Snap the connectors together. You will hear a clicking sound which indicates they are secure.

Step 6. Test the System

Now you have to test the charge controller load output. If the connections are correct, the controller display will turn on and indicate it is running.

Solar controller display screens vary, with some offering more features than others. Others are bare bones but newer ones even have mobile apps so you can monitor the gel battery charge from your phone.

If the display does not turn on or an error message appears, double check the connectors. There might be something loose or the positive / negative connectors not properly configured.

Step 7. Charge the Battery

If everything looks fine, you can leave the system and let the battery charge. You can unplug the cables once the battery is charged.

How Long Does It Take to Charge a Gel Battery?

It depends on the solar panel output, how much sunlight is present and how the depleted the battery is. The solar controller display provides information on how much charge has gone into the battery.

A 250W solar panel can charge a 100ah gel battery in 5 hours with clear skies.
To recharge a 300ah gel battery bank in 5 hours, you will need at least 4 x 300W solar panels.

The formula is solar panel watts x sun hours = watt output

Gel battery capacity are in amp hours, so you need to multiply amps by the battery voltage to find its wattage.

Example: 200ah x 12V = 2400

A 200ah 12V battery is 2400 watts. If you have two 300W solar panels they can recharge the battery in 5 hours.

This assumes the weather is ideal, without any clouds, drizzle etc. You can look at the charge controller and see the effect of cloudy skies on the battery charge.

If the sky is clear and the sun is out, the battery charges quickly. If clouds suddenly form and block the sun, the charging slows down. A sudden downpour and the charge could stop.

The number of sun hours is only one factor. The panel efficiency is another. The higher the PV module efficiency rating, the more sunlight gets converted into electricity. This is what you want to see when charging batteries.

What Charge Controller Should You Buy?

Divide the solar panel watts by their voltage to get the amps. This is the charge controller size you should get.

if you have a 24V 300W solar panel:

300 / 24 = 12.5

You need a 12.5A charge controller. Most are sold in 10A, 20A, 30A, 50A and 60A, so you should buy a 20A controller.

If you bought an all in one solar panel kit, a charge controller will be included, so no need to figure out what size to buy.

But if you only bought solar panels, check the back panel or the instructions. You will find its specifications including the watts.

Keep in mind that solar panels produce higher voltage than their nominal rating. A 12V solar panel can reach 18 volts when the conditions are ideal. A 24V panel might go up to 36 volts. That is why you need the right charge controller size so the battery does not get overcharged .

You can buy a larger controller than what your batteries need, but never smaller. As for features, look for those with overcharging and overloading safeguards. A display that provides essential charging information is also ideal.

MPPT vs. PWM Controllers

There are two types of charge controllers, PWM (Pulse Width Modulation) and MPPT )Maximum Power Point Tracking). Both have the same purpose, ensure the current does not overcharge or overload the battery. However, an MPPT controller is more efficient.

MPPT charge controllers do a better job of moving energy into the battery. PWM controllers work fine with small batteries. But when you charge large or multiple gel batteries, MPPT controllers are better.

An MPPT charge controller makes necessary adjustments so the battery gets the highest possible power without overloading it. With a PWM, the draw is limited to 14.4V. If the solar panel generates anything higher than that, it is discarded. With an MPPT, the power goes into the battery.

Tips and Warnings

  • If you purchased a solar panel kit, all the wires, cables, connectors, charge controller and solar panels will be included. This is the easiest approach if you are new to solar power.
  • If you buy each component separately, make sure they are compatible with each other.
  • Mark the wires so you do not confuse the positive with the negative. Bundled solar wires have different colors to avoid confusion.
  • Gel batteries need maintenance. Follow the instructions given to ensure it lasts as long as possible.
  • Be certain you ground all electrical components.
  • Gel batteries should not be confused with AGM. AGM cost more but are more efficient. Gel batteries do require more maintenance but easier on the pockets.


The steps here are applicable for almost any solar power system. However there might be some slight differences with yours, so refer to the instructions that came with your kit.