Until recently lead-acid deep cycle batteries were the most common battery used for solar off-grid and hybrid energy storage, as well as many other applications. Lead-acid batteries are available in a huge variety of different types and sizes and can be anything from a single cell (2V) battery or be made up of a number of cells linked together in series to operate at a required voltage. For example a common 12V battery (such as the one in your car) is actuallymade up of 6 x 2V cells in series. The most common battery voltages available are 2V, 6V and 12V.
There are several different types of lead-acid deep cycle batteries available with the four main varieties listed below:
The 4 main types of deep cycle lead-acid batteries
Flooded lead-acid (vented)
Valve regulated lead-acid (VRLA)
Absorbed glass mat (AGM)
Gel electrolyte (Gel)
Lead-carbon (VRLA) - Gel electrotyte
Deep cycle solar tubular gel (VRLA)
The oldest and most basic type is the flooded lead-acid battery where the electrolyte (acid) is in liquid form. Until 10-12 years ago flooded batteries were the most common deep cycle battery available and are still used for some large off-grid systems. During the charging and discharging of a flooded battery volatile gases are produced and are vented out of the battery to prevent the build-up of pressure. The disadvantage of this is that the electrolyte level is reduced and needs to be topped up regulary, however this also enables the battery to be maintained and thus flooded batteries can last a long time (up to 20 years or more) if regular maintenance is performed.
2. Valve regulated lead-acid - VRLA
VRLA or valve regulated lead-acid batteries (AGM and Gel) work in the same way as a flooded battery but sealed in a leak proof enclosure with the electrolyte in a non-liquid form. VRLA batteries use a gas recombination system which recombines the gases created during the charging/ discharging process back into the battery. This prevents almost all losses (close to 99%) of electrolyte through gassing. These batteries are thus non-serviceable or maintenance free as they are sealed and there is no chance of acid spillage. This means they are much safer, easier to handle and transport than flooded batteries.
AGM Sealed lead-acid
AGM batteries use the VRLA system but store the electrolyte in an absorptive glass mat between lead calcium plates. This is the most cost effective type of VRLA battery and has become very popular over recent years. However cycle life (lifespan) of this type of battery can be quite low compared to flooded and gel batteries, usually 6-10 years.
Gel Sealed lead-acid
Gel batteries use the VRLA system but have a gel electrolyte which is created using a special gelling agent to create a thick immobilised electrolyte. The gel batteries have been known to perform very well under high discharge rates and generally last longer than AGM batteries, however they are typically more expensive.
3. lead-Carbon VRLA batteries
Lead carbon batteries are an advanced lead acid battery which uses lead for the positive plate (anode) but has carbon added to the negative plate (cathode). The carbon acts as a sort of 'supercapacitor' which allows faster charging and discharging, plus prolonged life at partial state of charge.
Like more common Gel sealed batteries lead-carbon batteries are typcially sealed with a gel electrolyte for improved safety and less maintenance.
The REXC series Lead-Carbon batteries from Narada uses a nano carbon material cathode which the company claims also provides a much longer cycle life than traditional lead-acid batteries.
4. Solar Tubular gel batteries
There are several high quality battery manufacturers which have engineered very high performance, high cycle life batteries specifically designed for solar installations. These are tubular gel VRLA single cell (2V) batteries are linked together in series to form 24V or 48V deep cycle battery banks. Often referred to as OPzV batteries, the manufacturers below use advanced tubular-plate electrodes with woven polyester gauntlet and solid grids in a corrosion-resistant lead-calcium alloy.
These batteries can offer very high cycle life up to 5000 cycles (15 years) at 20% DOD and 3000 cycles at 40% DOD the specific charging parameters are met and the battery is maintained within the correct temperature range (usually 15-30 degC).
Some of the leading OPzV solar battery manufacturers include:
Advanced tubular gel deep cycle batteries are a proven technology with 15+ year design life. There are many well established trusted companies manufacturing high quality tubular gel lead-acid batteries for solar applications.
Battery Capacity - Depth of discharge (DOD)
The available energy stored in a battery is known as the battery capacity which is measured in amp hours (Ah). All deep cycle battery manufactures will provide the total capacity of a battery in Ah. When a battery is being discharged the amount of battery power used is known as the depth of discharge (DOD) and the amount of energy still available is known as state of charge (SOC). The level of DOD and SOC are directly proportional to the battery or cell voltage under no load. A typical fully charged 12V battery will have a voltage reading of 12.7 to 13.0 volts, where as a fully discharged 12V battery will read approximately 11.8 to 12.2 volts depending on temperature and the age of the battery. Refer to the diagram shown below for a typical 12V lead-acid battery:
Note: SOC can only be accurately measured with no load on the battery!
Battery temperature has a significant effect on SOC – lower battery temperatures will reduce the SOC and amount of energy available. However, lower battery temperature will not reduce battery life unless the electrolyte freezes, in which case it can destroy the battery. On the other hand high battery temperature can drastically reduce the life of a battery. The ideal operating temperature range is 15 to 25°C. Generally once the battery temperature (not air temperature) is 35°C or higher then the life of the battery is reduced. The damage done is proportional to the temperature. For example higher temperatures above 40°C for a prolonged period of time can destroy a battery with an 10 year design life in as little as 1-2 years.
It is important to note that over time as the battery ages the cell voltage will slowly reduce. This is normal and should be taken into account when reading battery SOC. In addition batteries will slowly discharge over time if no charge voltage is applied. This is known as self-discharge and is usually around 3-6% per month depending on the type of battery. Thus a deep cycle battery can be left in a fully charged state for several months but should never be left in a fully discharged state as this can cause rapid sulphation and permanently damage the battery.
A deep cycle battery bank is the combination of a number of batteries linked together in either series or parallel. When batteries are connected in series then the output voltage increases, whereas if they are connected in parallel the overall output current increases. By doing this the battery bank voltage (or system voltage) can be matched to the inverter and other components. The most common system voltages used for domestic off-grid and hybrid power systems are 12V, 24V and 48V. As a general rule the larger the system the higher the voltage used. For example a large off-grid system will use 24 high capacity 2V batteries linked in series with interconnecting cables to form a 48V battery bank. A smaller battery bank often used in hybrid systems will usually be made up of a small number of 6V or 12V batteries to form a 24V battery bank.
Battery Charging - The charge cycle
Charging a lead-acid deep cycle battery system requires a dedicated multi-stage battery charger. Most modern hybrid or multi-mode battery inverters have multi-stage charging functions while off-grid DC-coupled systems the DC solar regulator or solar controller charges the battery. The typcial charging cycle consists of 3 main stages with optional 4th equilisation stage for flooded batteries. There are also up to 7 stage chargers which incorporate various other battery recovery functions.
The charger must match the battery bank voltage - for example a 24V charger can only used on 24V battery systems. Most modern off-grid inverter/chargers or multi-mode inverters have dedicated multi-stage chargers built in together with temperature sensors and precise voltage terminal sensors.
The four main battery charging stages
Bulk charging stage - typically 15-40 minutes*
Absorption charging stage - typically 1-3 hours*
Float stage - indefinite
Equilise stage (only for flooded batteries)
* Actual charging time depends on battery DOD, battery temperature, rate of charge, load and battery age. Always refer to battery manufacturers specifications (datasheet).
The various charging stages of a multi-stage charger ensure the battery is fully charged back to 100% without over-heating or over-charging the battery. Multi-stage chargers do this by first charging the battery at a higher voltage for approximately 1-3 hours (bulk and absorption stage) then reduce the charge current to lower the voltage to what is known as the float voltage, which can be maintained indefinitely. However it is worth noting that many cheaper chargers are essentially just float chargers and do not charge the battery at the correct charging voltage. They are generally fine for maintaining a battery as long as the float voltage is close to the float voltage specified by the battery manufacture. All new hybrid and interactive inverters (off-grid) incorporate a multi-stage charging system so this is not a concern.
Fully charge a battery every 2 weeks for longer life
To increase the life of a deep cycle lead-acid battery they need to be charged to 100% state of charge at least every 2 weeks. This ensures the battery life is maintained by completing the chemical reaction and breaking down any sulfation which occurs when a battery is not charged for a prolonged period of time. During winter a battery may not obtain full charge for many days so an external backup power source such as a generator should be used.
Since temperature can have a huge effect on the battery state of charge, a high-end battery charging system (multi-mode inverter, regulator or controller) will have a battery temperature sensor input and has the ability to automatically adjust the charging voltage to accommodate for cell temperature. This is very important in hot climates as the battery can over-heat while it is being charged, which will cause permanent damage.
Note: Never use cheap automotive battery chargers. These do not have any feedback control about the battery voltage and should never be used for charging a deep cycle battery.
*IMPORTANT: Always refer to the battery manufactures specifications and recommendations. Each battery will have specific charging voltages which will ensure the battery operates in a safe and reliable manner. Always use a high quality battery charger or regulator with automatic cut-off and overload protection to prevent damage and over-charging.