DIY Solar gel cell battery charger with a LM317
Recently I needed to use a solar panel to charge a 12 volt Gel Cell Battery. So I went on Amazon and bought a solar charge controller similar to this one. However there was a problem, this particular charge controller is intended for use with either a lead acid or an AGM battery. Not a Gel Cell battery.
The charging voltages of Gel Cell batteries are different than a lead acid battery. To charge a 12 volt Gel Cell battery, it is given a constant voltage of 13.4 volts, and the Gel Cell takes what available current it needs. This 13.4 volts for a Gel Cell is called the “Float Voltage”.
A lead Acid battery on the other hand, when properly charged has three stages. The first stage of properly charging a lead acid battery is the Bulk stage. This stage does 80% of the charging where the battery is charged at a constant current until the battery voltage goes up to 14.5 volts.
The second stage is the Absorption stage which does the remaining 20% of the charge of a lead acid battery by maintaining a constant 14.5 volts and decreasing the current until the battery is fully charged.
The last stage for charging a lead acid battery is the Float stage, where the voltage is reduced to 13.6 volts and held constant. The point of the float stage is to keep the lead acid battery in a fully charged state, which can be done indefinitely without hurting the lead acid battery.
So if I were to use the previously mentioned Charge Controller on my Gel Cell battery, it would charge the battery, but it would shorten its life due to the voltage to the gel cell being too high. So I decided to build my own Solar charger using a LM317 voltage regulator. Here is the Schematic diagram:
Solar Gel Cell Battery Charger using a LM-317