Understanding your solar battery – a bit of electrochemistry



If your eyes roll back when you start thinking of high school chemistry you may want to skip to the next section, but for those who need a primer on batteries, then read on here. A battery has three main parts: the electrodes, electrolyte and separator. There are always at least two electrodes in a battery, one positive terminal and one negative terminal. The positive terminal is called a cathode (you can remember this is by spelling it as ca+hode). The negative terminal is called the anode. The electrolyte is the liquid that the electrodes are placed in. It allows for charge to flow between the cathode and anode. The separator keeps the anode and cathode from being directly linked. This means that electrons have to go through the wire in order to complete the electrochemical reaction.


Lead-acid batteries consist of a series of lead plates (electrodes) in dilute sulfuric acid solution (electrolyte). When discharging, oxygen atoms from lead oxide (PbO2) in the positive plate react with hydrogen atoms from sulfuric acid (H2SO4) in the electrolyte. As you probably guess, these are going to produce water (H2O). Meanwhile, lead sulfate (PbSO4) is also produced at the cathode and anode. So in general we can say that during discharge sulfate ions leave the electrolyte and water is produced. During charging the reaction goes in the opposite direction producing lead oxide at the cathode. Hydrogen can be evolved when overcharging, creating a risk of explosion as hydrogen is flammable. Proper handling and care of batteries is therefore crucial.

General care and maintenance: how to get the most out of your solar batteries


The goal of battery care and maintenance is to improve you battery performance and life. Battery life is a highly variable property that depends on all kinds of factors such as storage temperature and depth of discharge (DOD).

Renogy MPPT Charge Controller will also protect the batteries from overcharging, over-discharging, overload, and short circuit.


About 80% of failures are caused by sulfation, a process where sulfur crystals form on the battery’s lead plates and prevent chemical reactions from happening. Sulfation occurs when the battery has a low charge or electrolyte level.Due to the dangers of sulfation it is very important to monitor, maintain and control these two factors in flooded batteries. To do this you will need distilled water, a digital voltmeter, a temperature compensating hydrometer and proper safety gear.


Remember, you cannot and do not need to check the fluid level and the specific gravity in AGM and gel batteries. So the first two steps only apply to flooded batteries.


How to check the fluid level


You only do this for unsealed batteries (FLA)–these are the flooded lead acid batteries. Open your battery cap and look inside. Distilled water should be added into the cells so that no metal lead surfaces are visible. Most batteries will have a “fill line” indicating where the electrolyte level should be. The maximum fluid levelis approximately 1/2″ below the cap. Do not overfill your batteries, you don’t want them to spill!


How to check the charge level


Determine the state of charge or depth of discharge (DOD) by checking the specific gravity and voltage of the battery. The specific gravity table below will help you determine the state of charge of your battery. If your batteries are 6V instead of 12V then simply divide the voltages by two. Similarly, double the voltage values for a 24V system.


How to charge batteries


All off-grid solar homeowners should have a general idea about the battery charging cycle. There are three charging phases: float, bulk and absorption. You only need to know the basics about these phases:

Float charging, also called trickle charging, consists of charging the battery at the same rate that it is discharging. This just keeps the batteries fully charged.

Bulk charging occurs when you start to charge a discharged battery. Voltage rises towards the maximum permissible level.

Absorption charging follows the bulk step. Voltage is held constant at the maximum level and current starts to decrease until the battery is fully charged.


If you install an adjustable charge controller you will have to set your float, bulk and absorption charging voltages. Ensure that these are the same as those that the battery manufacturer recommends. For solar electricity systems, the time in which there is charging from the solar panels can be too short to go through full bulk and absorption phases. If this is the case, you can get away with setting both phases at the same voltage. For specific charging voltages please contact your manufacturer or refer to your battery’s data sheet.


How to clean batteries


Battery terminals need to be regularly cleaned with a mixture of baking soda and distilled water using a battery terminal cleaner brush. Afterward rinse the terminals with water, ensure that all connections are tight and coat the metal components with a commercial sealant or a high temperature grease. Of course make sure to remove the clamps (negative first) before cleaning.


How to replace batteries


When replacing old batteries keep in mind that your batteries’ performance could be harmed through ‘mixing’. When old and new batteries are used together, the newer batteries will quickly degrade to the quality of the aged ones. For this reason, mixing old and new batteries is a huge waste of your money. Avoid this by properly maintaining your batteries so that they all have good life spans.


How to use batteries safely


As mentioned before, lead-acid batteries produce hydrogen which is flammable in the presence of oxygen. In fact, Saturn V rockets used hydrogen and oxygen as fuel in their upper stages. To prevent rocket fuel from building up in your battery bank, connect the box to the outdoors with vent pipes and make sure the system is well vented. Some systems also use fans to help vent the gasses out.


If you’ve got a battery backup system that kicks in when there is a power outage, you need it to work after several months, or even years, of inactivity. In order to make sure it will work, the batteries need to stay fully charged. If you let your battery just sit there, it will slowly lose its charge. The charging mode that keeps your batteries topped off is called trickle charging. So to take care of a backup battery bank, its best to get AGM batteries, which we discuss below, because they require practically no maintenance, and trickle charge them off a small solar panel or other electricity source. If you use a good setup, your solar batteries can last 8 years.


Batteries are dangerous


Proper safety precautions must be taken whenever you are near your battery bank. Use thick gloves and protective eyewear and remove all metal items. The last thing you want is to be burned by acid or electrocuted. Just in case there is an acid leak, make sure you have baking soda and water near the batteries. These can be used to neutralize the acid.