Batteries are essential and expensive elements in off-grid installations. However, their operation and maintenance are not well known/not well understood by the general public. This tutorial, therefore, has several objectives:
Batteries are often the most expensive and most fragile constituents of an electrical conversion system. Hence, it is important to take care of them through proper use and monitoring.
Lead acid batteries are very fragile. They are sensitive to overcharging, partial charging, deep discharges, excessively rapid charges, and to temperatures above 20°C. All these factors can lead to premature aging, mainly due to a combination of lack of technical knowledge, poorly- sized systems and erroneous use by a person. If one does not control these factors, the batteries will quickly be damaged.
The damage will result in reduced battery life and, in some cases, there could be irreparable deterioration of batteries. Batteries will last longer when used properly, and so their replacement will be less frequent. In the long run, one can make considerable savings. Another interesting aspect is that the conversion system will be more efficient if the batteries are in a good condition. The better the batteries’ condition, the more efficient the installation will be.
In this tutorial, we will learn how to properly use and maintain lead-acid batteries.
To understand the causes of battery failure, it is important to understand the chemical reactions at work inside it.
Reaction during discharge: During discharge, the following chemical reaction takes place:
The units of the batteries are indicated as abbreviations which are not always easy to understand. Here is a summary table of the units associated with the batteries :<td 50Ah C20battery means a battery of 50Ah capacity with 20h discharge C100battery: 90Ah (capacity of 90Ah with a discharge in 100h).>-
|Capacity (Ah)||The amount of current that a battery can store or release, usually specified in Ah for a given discharge rate.||A 10 Ah battery can produce 5 Amperes (A) for 2 hours (h).|
|Tension (V)||Battery voltage level. It must be compatible with the connected devices.||Lead-acid batteries are made up of units delivering 2.1 Volts (V) and connecting these units in series makes it possible to reach the generally desired voltage. For example, six units connected in series deliver 12 V. To create 24 V or 48 V systems, 12 V batteries are, in turn, connected in series.|
|Energy (Wh)||The product of multiplication of the capacity by the voltage.||A 200Ah 24V battery will have an energy of 4800 Watts hour (Wh).|
|Discharge rate, Cxx||Expressed as a unit of C10, C20or C100, it indicates the capacity of a battery according to its rate of discharge.||Cold Cracks Amps (CCA)||This is the maximum extractable current from a battery over a short period when starting the engine, for example.||CCA 420A 5 sec indication means the battery can deliver 420A for 5 sec.|
|SOC (State of Charge)||State of charge of a battery, which indicates the amount of electricity remaining.||SOC = 50 %: the battery’s charge is 50%.|
|DOD (Depth of Discharge)||State of discharge of a battery, or the amount of electricity consumed.||DOD + SOC = 100% .|
|Number of cycles||For a battery, a cycle represents a discharge followed by a charge. The number of cycles of a battery depends on the depth of discharge or amount of electricity consumed.||The higher the DOD, the lower the cycle life, the same battery can have.
There are several types of and technologies for lead batteries, each adapted to a particular use, environment and constraints. Understanding the differences is essential to choosing and maintaining your battery correctly. This part summarizes the main categories of lead acid batteries and their characteristics.
A starter battery is intended to provide high current for a very short time. It is designed to start an engine (for example a vehicle or a generator). Starter batteries are sometimes called "car battery", "truck battery" or "thin plate battery".
View inside a starter battery .
The name of these batteries comes from their first use: powering the motor of electric vehicles such as forklifts. They are generally equipped with "thick or tubular plates" which allows them to withstand fairly deep discharges and have a long lifespan. They are well suited for use in solar photovoltaics.
These batteries are used in emergency power supplies, in particular for computer or telecommunication systems. They are designed so as to be constantly recharged and to be discharged only infrequently.
These batteries are intended for use in photovoltaic solar installations. They are designed to withstand a high number of cycles (since they will be discharged every night and recharged every morning), and their depth of discharge is generally good but can vary greatly from one model to another. Service batteries have almost the same characteristics as solar batteries.
An open battery is a battery with liquid electrolyte equipped with plugs allowing to fill it. Open batteries are not watertight: the liquid inside evaporates little by little, so it is necessary to check its level regularly and top it up if necessary with distilled water.
|Delivers current in cold temperatures ( Their CCA rating indicates the battery has sufficient power to crank an engine in very cold temperatures)||Risk of non-homogeneity of the electrolyte if little used = premature aging|
|Withstands overloads and overheating (one can add liquid if it evaporates)||Release of hydrogen and, therefore, risk of explosion if environment not ventilated|
|Low cost||Not conducive to cold, risk of electrolyte freezing|
|Strong self-discharge (10-12% per month) if not used regularly.|
|Leaks possible if there is tilting or shaking/vibrations|
A sealed battery is a liquid electrolyte battery equipped with a system to prevent the evaporation of the water contained in the electrolyte, by gas recombination. These batteries do not require maintenance, and are often called VRLA for "Valve Regulated Lead-Acid batteries".
|Reduces explosive gas production, water loss and leakage.||Does not allow maintenance or control.|
|Requires less maintenance.||Imposes a perfectly regulated load according to the temperature to avoid gas losses by excessive pressure.|
AGM batteries are a type of sealed/VRLA battery, in which the electrolyte is a liquid but it is held in place in a fiberglass blotter, and hence its name: Absorbed Glass Material.
View the inside of an AGM battery
|Maintenance-free with minimal release of gas||They do not perform well in hot conditions (loss of electrolyte in the form of gas at higher temperatures) A temperature above 49°C (120°F) is very dangerous for the battery life.|
|They maintain the electrolyte homogeneity well.||They are sensitive to overcharging and high voltages (loss of electrolyte in the form of gas)|
|Withstand colder temperatures well because of their homogeneous electrolyte (Since the electrolyte is held in the glass mat separators, it won't expand when frozen like it will in a flooded battery)||They have limited shelf-life (as the acid concentration inside is higher than in others, which leads to faster battery degradation).|
|Allows high peak currents (CCA) to pass|
|Shock-resistant (Vibration-resistant) (Because of the fibre glass mats are woven tightly and the plates are packed tightly, making them immune to vibrations)|
|Low self-discharge (1-3% per month)|
Gel batteries are a type of sealed battery / VRLA. In a gel battery, the electrolyte is gelled by adding silicate.
View the inside of a gel battery
|Maintains the homogeneity of the electrolyte perfectly.||Limited peak current.|
|Low self-discharge (1-3% per month).||Slow charging and discharging. (charge current limited to 5-10% of capacity).|
|Cannot withstand high temperatures (loss of electrolyte in the form of gas - permanent effect).|
|Longer life span/shelf life.||Sensitive to overload (loss of electrolyte in the form of gas).|
Freezing of Electrolyte: When a battery is discharged, the electrolyte is mostly water. At low temperatures, it can freeze and irreparably damage the battery.
Corrosion of the battery terminals : Following acid splashes, acid vapours, or simply galvanic corrosion (two metals brought into contact), lead oxide deposits may form on the battery terminals. This can cause electrical conductivity problems.
Equipment: Choose your battery carefully according to the intended use.
Never mix new and used batteries.
Never mix batteries of different technologies.
Correctly and solidly install the wiring of your battery bank to avoid fires.
Regularly check the connectors if they are subject to vibrations.
Battery not in use – Self-discharge: When a battery is not in use, it slowly discharges. This phenomenon depends on the type of battery and the temperature. o An unused open battery must be recharged every four months at room temperature (between 10-25°C). o An unused open battery must be kept permanently charged in temperatures below 0°C. o Sealed batteries can be left for up to 6 to 8 months without recharging at ambient temperature. o When a system containing batteries (RV, car, etc.) is not used for a long period, disconnect the batteries to avoid leakage currents.
During discharge, lead sulphate (PbSO4) forms on the positive and negative electrodes. If the battery remains discharged, this lead sulphate crystallizes and hardens. Once crystallized, it can no longer turn into sulfuric acid when charging the battery. This causes the battery capacity to drop: "it no longer holds a charge" it is a weak battery/it is a dead battery.
Battery regeneration is a process of sending high-intensity electrical pulses (300-400A) at a given frequency, based on the battery's resonant frequency. This is calculated automatically by the machine and evolves over time. These impulses break down the crystalline layer formed by the amorphous lead sulphate deposited on the electrodes and convert it back to sulphuric acid. The plates are reconstituted and the battery returns to its original condition.
Success rate: Since sulphation is not the only phenomenon underlying battery degradation, not all of them can be regenerated by desulphation.
Duration of the process: This process can last from a few hours for a starter battery to several days for traction batteries.
Research to follow (For Further Reading)
Document by Guénolé Conrad with the help of Loup Girier, Wiam Razi, Elliot Harant and Pascal Criquioche within the framework of the Scholar Grid project. A project initiated by the Schneider Electric Foundation with the technical support of Energie Sans Frontières, Atelier 21 and the Low-tech Lab