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Lead-acid deep cycle batteries - Flooded, AGM & Gel VRLA

Jason Svarc

Lead-acid deep cycle batteries previously (before lithium) were the most common battery used for solar off-grid and hybrid energy storage, as well as many other applications. They can be either a single cell battery (2V) or be made up of a number of cells linked together in series to operate at a required voltage. For example a 12V lead acid battery (such as the one in your car) is made 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. The three main varieties are listed below:

  •   Flooded lead-acid (vented)

  •   Valve regulated lead-acid (VRLA)- Absorbed glass mat (AGM)

  •   Valve regulated lead-acid (VRLA)- Gel electrolyte (Gel)

The most basic type is the flooded 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 widely used for 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 over time the electrolyte level is reduced and needs to be topped up, 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.

Valve regulated lead-acid - VRLA

VRLA 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 gelled 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.

* There are also more advanced gel lead-acid batteries being developed but we will discuss these in another article.

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!

Lead-acid Battery state of charge

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.


battery bank

Battery Banks

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

Charging a lead-acid deep cycle battery requires a dedicated multi-stage battery charger which is often integrated in the hybrid or multi-mode solar inverter (in older off-grid systems the DC solar regulator or controller is usually the main battery charger). 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 1-2 hours 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.

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.