Batería de plomo-ácido

Una batería de plomo-ácido sin su carcasa. El interior de una batería de plomo-ácido está compuesto por placas de plomo sumergidas en una solución de electrolito.

Las baterías de plomo-ácido usan reacciones químicas reversibles entre el plomo y el ácido para proporcionar energía cuando se necesita y para almacenar energía cuando se produce. Han existido durante más de 150 años y han demostrado su durabilidad, bajo costo, reciclabilidad y rendimiento en condiciones variables hasta el punto de que casi todos los automóviles del planeta dependen de una batería de plomo-ácido para arrancar y funcionar. Las baterías de plomo-ácido han sido la forma preferida de almacenamiento de energía para los sistemas FV fuera autónomos desde que comenzaron a construirse por las mismas razones mencionadas, por lo que la mayoría de los componentes FV autónomos están diseñados para su uso con configuraciones de baterías de plomo ácido en 12 V, 24 V o 48 V. Las baterías de plomo-ácido tienen la ventaja adicional de estar disponibles en una variedad de voltajes (2 V, 6 V, 8 V, 12 V) y clasificaciones de amperios-hora (5 Ah a 5000+ Ah). Pero debido a su alta densidad de plomo, estas baterías son extremadamente pesadas. Una batería de plomo-ácido inundada de 12 V, 225 Ah pesa alrededor de 60 kg, lo que se acerca al límite superior de lo que se puede mover fácilmente sin equipo.

Al igual que un módulo FV se compone de varias celdas FV diferentes conectadas en serie que producen un cierto voltaje, baterías de plomo-ácido se componen de celdas conectadas en serie y cada una de las cuales produce alrededor tiene un voltaje nominal de aproximadamente 2 V. Esto significa que una batería de 12 V consta de 6 celdas.

Hay muchos tipos diferentes de baterías de plomo-ácido, pero inicialmente se pueden dividir en dos categorías: baterías de arranque y baterías de ciclo profundo.

• Las baterías de arranque se utilizan en automóviles y están diseñadas para proporcionar grandes cantidades de energía durante cortos períodos de tiempo con una profundidad de descarga superficial. Funcionan bien para este propósito, pero no pueden suministrar energía continuamente más allá de esta profundidad de descarga superficial sin acortar severamente su ciclos de vida. Estas baterías fallarán prematuramente en un sistema FV y no vale la pena invertir en ellas.
• Las baterías de ciclo profundo tienen un diseño más robusto que les permite suministrar continuamente grandes cantidades de energía a una profundidad de descarga más profunda, generalmente considerada alrededor del 80%. Estas baterías son más pesadas y cuestan más que las baterías de arranque, pero son la batería adecuada para su uso con un sistema FV.

Hay varios tipos diferentes de baterías de plomo-ácido, pero esta página se centrará en las dos categorías principales: Baterías de ciclo profundo de plomo-ácido inundadas (FLA) y de plomo-ácido reguladas por válvula (VRLA).

Baterías de plomo-ácido inundadas (FLA)

Diagramas que comparan diferentes tipos de baterías de plomo-ácido. El anillo más externo es la calificación o rendimiento más alto. El anillo más interno es la calificación o rendimiento más bajo.
(1) Densidad de energía (2) Densidad de energía (3) Costo (4) Seguridad (5) Vida útil (6) Rango de temperatura (7) Mantenimiento (8) Eficiencia

El diseño original de la batería de plomo-ácido. Son las baterías de plomo-ácido más simples, duraderas y baratas. Son un poco más duraderas ya que tienden a ser más tolerantes con las descargas profundas que las baterías VRLA que no requieren mantenimiento.

Caracteristicas:

• Requieren un mantenimiento mensual ya que las baterías pierden agua a medida que se cargan y descargan. Durable, de larga duración y económico solo si se mantiene adecuadamente. Si el mantenimiento no se realiza con regularidad, las baterías fallarán rápidamente y el reemplazo costará significativamente si inicialmente se han usado baterías libres de mantenimiento.
• Libera cantidades significativas de gas hidrógeno.
• Tener una solución de electrolito líquido en el interior que requiere que permanezcan en posición vertical.
• Puede someterse a un carga de igualación que puede ayudar a prolongar su ciclos de vida ya que se reduce la sulfatación.

Consideraciones de uso:

• Usuarios finales que sean capaces de mantener una batería (con capacitación y equipo de protección protección adecuados) o en un lugar donde hayan técnicos pueden reparar el sistema.
• Los usuarios deben tener acceso confiable al agua destilada ya que cualquier otra forma de agua tiene impurezas y dañará la batería a lo largo del tiempo.
• Requiere un espacio seguro con ventilación adecuada debido al peligro combinado de que las baterías derramen ácido y su tendencia a liberar cantidades significativas de gas hidrógeno. También crean un olor nocivo a azufre. La combinación de estos riesgos significa que, idealmente, las baterías inundadas siempre deben almacenarse en una habitación que sea segura y no habitualmente habitada como un almacén, un cobertizo o un garaje.
• Puede que no sea el mejor tipo de batería con temperaturas extremadamente altas o bajas.
• Bajo presupuesto y altas necesidades energéticas.

Baterías de plomo-ácido regulado por válvula (VRLA)

En la década de 1970, las baterías de plomo-ácido reguladas por válvulas comenzaron a ingresar al mercado. Estas baterías resolvieron muchas de las principales fallas de las baterías de plomo-ácido inundadas: no requieren mantenimiento, no tienen un electrolito ácido líquido que pueda derramarse y no expulsan cantidades significativas de hidrógeno. Sin embargo, existen compensaciones: son menos duraderas porque no son tan tolerantes a las descargas profundas, tienen menos ciclos de vida y cuestan significativamente más que las baterías de plomo-ácido inundadas. Hay dos subcategorías principales de baterías VRLA: AGM y Gel.

Caracteristicas:

• No requieren mantenimiento.
• Están sellados (aunque no completamente) por lo que hay poca o ninguna emisión de gases.
• No tienen una solución de electrolito líquido en su interior y además están sellados para evitar fugas, por lo que no es necesario mantenerlos en posición vertical en todo momento.

Consideraciones de uso:

• Usuarios finales que no pueden realizar mantenimiento.
• Ubicaciones en las que no existe un espacio separado que pueda dedicarse al almacenamiento de energía. Estas baterías no pueden crear derrames peligrosos, emitir cantidades significativas de gases peligrosos o emitir un olor nocivo a azufre. Aún así, deben almacenarse de forma segura en una caja de batería, pero pueden ubicarse en un espacio de usos múltiples si es necesario.
• Requieren un presupuesto mayor.

Baterías de plomo-ácido de matriz de fibra de vidrio (AGM)

Una batería VRLA en la que la solución de electrolito está contenida dentro de una matriz de finas fibras de vidrio. Estas baterías cuestan en promedio 1,5-2 veces más que FLA y tienen un ciclo de vida más corto en comparación con una batería FLA debidamente mantenida.

Consideraciones específicas de uso:

• Permiten tasas de carga y descarga más altas que las baterías FLA y de celda de gel.
• Funciona mejor que las baterías FLA y de celda de gel en ambientes fríos. Las baterías FLA pueden congelarse y dañarse durante la carga y descarga.

Celda de gel

Una batería VRLA en la que la solución de electrolito se convierte en una pasta de gel. Las baterías de gel son la opción de ácido de plomo más costosa. Prefieren la carga y descarga lentas, lo que no es ideal para los sistemas de energía renovable que a menudo hacen ambas cosas todos los días.

Consideraciones específicas de uso:

• Funciona mejor que las baterías AGM y FLA en ambientes calurosos.

Capacidad de almacentamiento

El capacidad de almacenamiento de una batería de plomo-ácido se mide en amperios-hora (Ah). La cantidad de esta energía que es en realidad energía utilizable depende de:

1. La velocidad a la que se extrae energía. Si una batería de plomo-ácido se descarga rápidamente, la cantidad de energía utilizable disminuye. Por el contrario, si se descarga lentamente, la energía utilizable aumenta. Esto se mide en términos de tasa C. Una tasa C de 1 significa que toda la capacidad de la batería se descarga en 1 hora. Una tasa C de 20 o C/20 significa que toda la capacidad de la batería se descarga en el transcurso de 20 horas. Las baterías de plomo ácido se clasifican típicamente por su tasa C/20. A continuación se muestra un ejemplo de una batería AGM Trojan de 12 V 205 Ah:

C-rate	Amp-hours
10 hour	174Ah
20 hour	205Ah
48 hour	210Ah
72 hours	213Ah
100 hours	216Ah

2. The chosen depth of discharge limit for a battery. The cycle life of a battery depends greatly upon how deeply it is discharged and how frequently. The numbers of cycles vary depending upon the type of battery as well.
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  Depth of discharge vs. cycle life for Trojan FLA, AGM and Gel batteries.

Temperature

The temperature of a lead acid battery or battery cell directly influences its resting voltage and the voltages at which it should be charged. It is worth noting that batteries have significant thermal mass, so brief periods of high or low ambient temperatures do not tend to quickly change battery temperature on their own. Batteries do internally generate heat as they charge and discharge.

Higher temperatures give batteries an increased capacity, but also greatly shortens their life. For lead acid batteries it is estimated that for every 10°C increase in average temperature above 25°C shortens the batteries life by half. This means that operating a lead acid battery for one month at 35°C is equivalent in terms of battery life to operating the battery for two months at 25°C.

Low temperatures decrease the capacity of all lead acid batteries can leave them prone to damage during charing and discharging. Extremely low temperatures can cause flooded lead acid batteries to freeze, which can crack the case of the battery and cause electrolyte to spill. The freezing point of flooded lead acid batteries varies with their state of charge. For all lead acid batteries it is important to use them lightly if they are at a very low temperature (below -10°C).

Locating the batteries in an appropriate location and properly storing them is essentially to ensure that a system performs properly. A charge controller that is able to take into account battery temperature through a remote sensor can greatly prolong battery life and avoid accidental overcharging that can severely damage batteries.

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  Capacity vs. temperature for Trojan FLA, AGM and Gel batteries.

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  Cell voltage vs. temperature for Trojan FLA, AGM, and Gel batteries.

Efficiency

There is no type of storage that is perfectly efficient. Lead acid batteries inherently lose some energy as it is put into the battery and some as it is withdrawn, generally as heat. A FLA battery is typically only 80-85% efficient, whereas a VRLA battery is has a slightly higher efficiency of 85-90% efficiency. This is round -trip efficiency, meaning that if 100Wh of energy arrives from a PV module arrives that the terminals of a FLA battery and a VRLA battery:

• 80-85 Wh will be able to be withdrawn from the FLA battery.
• 85-90 Wh will be able to be withdrawn from the VRLA battery.

Charging

Lead acid batteries have specific charging requirements that must be followed to ensure that they continue functioning for a their rated cycle life. For more information on the the different charging phases - see Charging phases.

Sulfation

It is important that batteries reach a full state of charge (90-100%) on a regular basis. If batteries do not reach a full state of charge every few days, then the plates within the battery will begin a process of sulfation which is the buildup of sulfate crystals on the plates of the battery that will start to impede performance and can ultimately lead the battery to cease functioning. Although a 30% depth of discharge is rather shallow for off-grid systems, if a battery regularly sits at 70% state of charge (the inverse of a 30% DoD) the batteries will begin to sulfate, lose performance, and fail prematurely.

It is necessary to build a system with a PV charging source that can supply sufficient current to bring the batteries up to charge regularly while loads continue to operate. With FLA batteries it is possible to do a monthly equalization charge to reverse some of the sulfation, but this is not possible with VRLA batteries.

Charge rate

If an off-grid PV system will rely solely upon PV as a charging source (no generator, no other renewables) then it is necessary to ensure that a PV source is properly sized for this role. The PV source size should be checked to ensure that it can supply sufficient current to properly charge the energy storage system. If lead acid batteries do not regularly receive an adequate charging current - typically because the PV source is undersized relative to the energy storage system - they will begin to experience sulfation and they will last for fewer cycles.

Lead acid batteries last longer and perform better when they are regularly recharged with a current in a certain range - typically between 5-13% of their C/20 rating. It is best practice to consult the manual or manufacturer for recommended maximum and minimum charging currents. If a system uses many loads during the day, this will limit the available charging current for the energy storage system and should be taken into account.

• A minimum of 5% of the C/20 Ah rating is recommended for a system that is used infrequently or is primarily used at night.
• A 10% of the C/20 Ah rating is recommended for a system that is used regularly with significant load usage during the day.
• A maximum of 13% of the C/20 Ah rating is recommended for gel and flooded lead acid batteries. A maximum of 20% of the C/20 rate Ah rating is recommended for AGM batteries..

Discharge rate

Lead atteries have a recommended maximum continuous discharge current which is typically the same as the maximum charge current of the battery. This rate can be exceeded for brief periods of time when surge loads are operating or a heavy load, like a well pump, is operating for a few minutes, but the size of the energy storage system should be increased if the amount of current that the inverter will require to operate at its continuous duty rating exceeds the recommended value for the battery. Commonly used values for the maximum continous discharge current for different lead acid batteries are as follows:

• A maximum of 13% of the C/20 Ah rating is recommended for flooded and gel batteries.
• A maximum of 20% of the C/20 Ah rating is recommended for AGM batteries.

It is important to consult the manufacturer of a particular battery as there are batteries that permit higher charge/discharge currents.

Battery storage

A battery box with a hinged lid, an entrance for cables and added ventilation.

Due to safety issues posed by batteries (high available current, hazardous gases, dangerous chemicals) it is always necessary to put batteries in a secure room or enclosure to avoid unauthorized access to them. For small-scale off-grid systems they are typically housed in a battery box that is constructed to the required size of the battery bank. The most common and easy to work with material in most areas is wood, but it is also possible to build a suitable enclosure out of metal. Batteries should be checked regularly and serviced as needed, therefore it is important that any type of enclosure that they are placed into provides sufficient working space.

Location and security

Batteries must be placed in a location that will provide adequate cooling, a dry environment, and protection from unauthorized access. It is best practice to avoid installing batteries or other electrical equipment in living quarters meaning bedrooms, kitchens, or living rooms. This is especially true with flooded lead acid batteries as they generate a noxious sulfur smell during charging. This is often not possible in many locations due the lack an additional appropriate space or concerns over theft. In this case, using VRLA (sealed) batteries and installing them in the living room would be preferrable to a bedroom or bathroom.

If a battery box is to be placed in an accessible location, then it is necessary that it has a means to avoid unauthorized access, like a lock. If the batteries are in a room without a battery box, it is necessary that the room itself be locked to avoid unauthorized access. A battery box should not be become means for storage - adequate space should always be left so that the lid can easily be opened without the need to move other items. Many batteries come with a rubber or plastic cover for the terminals, this can be a useful safety measure to prevent accidental short-circuits from accidents like tools being dropped across the terminals.

Ventilation

FLA batteries produce potentially explosive hydrogen during charging, it is recommended that they are stored in a location with adequate free airflow. This is additionally important for cooling. If batteries are to be placed in a battery box, then holes should be made near the top and bottom of the box to enable rising hydrogen - which rises because it is lighter than most other molecules - to escape and other air to enter. A sloped lid on the battery box can help to improve ventilation. It is also a best practice to locate the point where any cables exist near the bottom of the battery box to avoid any hydrogen entering the conduit and making its way to the other electrical equipment.

AGM batteries do not release hydrogen during normal charging, but it would nonetheless be a good practice to follow the same practices recommended for batteries. At the minimum they should be provided good ventilation to provide appropriate cooling to avoid overheating during charging.

Spacing between batteries

Adequate spacing is important to permit adequate cooling of batteries and to permit safe working conditions. In cool climates - maximum temperature indoors under 30°C - a battery bank should allow for:

• At least 2.5 cm should be left between batteries.
• At least 15 cm of perimeter around the batteries. This may be 15cm between the batteries and the sides of the battery box or 15cm between the batteries and the nearest object if they are not in a box.

In warmer climates, more spacing can be provided and additional considerations should be taken for battery cooling. In cooler climates, insulating the battery box may be an appropriate choice to improve performance and prevent freezing.

Spill containment with FLA batteries

As FLA batteries contain an acidic electrolyte in liquid form, it is common for them to overflow during maintenance or if they are over-charged. It is important to build a spill containment system that can handle the amount of electrolyte that a battery bank could potentially spill. Placing the batteries in a plastic or rubber tray with a tip of at least 2" is recommended.

Dating batteries

It is important to understand the age of batteries when purchasing them and when servicing them. Batteries use a unique dating system. The month of production corresponds to a letter - beginning with "A" for December. The letter "I" is skipped due to its similarity to the number "1." The year of production corresponds to a number with "0" being the first year of a decade. For example a batter produced in March of 2018 would have a date code of "C8."

Top row: letter system for months. Bottom row: number system for years.

Safety

Basic PPE for working with lead acid batteries.

Lead acid batteries contain lead and acid, both of which are hazardous if they come into contact with the skin or are ingested. Installing, maintaining, or troubleshooting lead acid battery banks requires that appropriate personal protective equipment is worn at all times with the bare minimum being eye protection and gloves. Baking soda and water should always be stored in case of contact with the acidic electrolyte solution or if a spill were to occur. Baking soda is a basic substance, which is the opposite of an acid, and will work to neutralize the hazardous effects of the acid. Water works to dilute the acid and wash it away to minimize damage. If acid comes into contact with eyes or skin, baking soda can be applied and then area should be washed thoroughly and continuously for at least 15 minutes to dilute and remove the acid. If electrolyte solution comes into contact with clothing, it should also be neutralized with baking soda and diluted or else it will eat away the clothing. It is a good practice to store the appropriate personal protective equipment, baking soda, and water in the same location where with the batteries. After working with lead acid batteries, even if gloves were used, hands should be washed thoroughly to remove lead with which they may have accidentally come into contact.

For more information on electrical safety when working with batteries see: Electrical safety with batteries.

Recyclability

Lead acid batteries contain lead and acid, both of which are hazardous materials that must be disposed of properly. Lead acid batteries are often pointed to as a success story for recycling as the majority of lead is used for batteries and an estimated 95-96% is ultimately recycled. This largely has to do with a well-developed market, supply chain and abundant processing facilities for lead acid batteries as lead is a valuable material and is readily recyclable. This knowledge has spread and has resulted in batteries being returned for cash in even the most remote places.

Notes/references

Hydrowires - Energy Storage Technology and Cost Characterization Report
Isidor Buchman - Batteries in a Portable World
Thomas Reddy - Linden's Handbook of Batteries, 4th Edition