Gel and AGM Battery

In 1839 a French scientist Edmond Becquerel has discovered, described and demonstrated the photovoltaic effect. 20 years later, a fellow Frenchman Gaston Planté has invented another vital element of photovoltaic system: a lead-acid based energy storage technology. Inexpensive manufacturing, thanks to good availability of the resources needed for fabrication, made the lead acid-battery popular and cost-effective option for cars, small electric vehicles (golf carts, forklifts) as well as solar installations for self-consumption of energy.

An electrochemical battery consists of a cathode, an anode and electrolyte that act as a catalyst. When the battery is charging the electrons (negatively charged particles) are being drawn to cathode by positively charged ions, which results in creation of electrical potential between anode (always negatively charged) and cathode (always positively charger). In case of lead acid batteries, the cathode is usually built using lead dioxide alloy, while the anode is made of pure lead alloy, the electrolyte which allows the ions and electrons to flow between the electrodes is the sulfuric acid. The use of lead alloy (combination of lead with for example tin or calcium) is necessary since lead itself is very soft and deforms easily, mixing it with other material is therefore done to improve the mechanical properties of the electrodes.

Several types of lead acid batteries have been widely available on the market, we mainly distinguish:

  • OPzS – flooded type of tubular-plated with liquid electrolyte
  • GEL – valve regulated with gel form of electrolyte,
  • AGM – valve regulated with fiberglass mesh electrolyte.


While the flooded lead acid batteries are extremally cost effective it has several drawbacks, notably requires feable currents to charge (charging takes more than 10h), requires to be fully charged during storage (to prevent sulfation) and have a very moderate lifespan. The electrolyte needs to be refilled as charging and discharging the battery consumes water.

The gel and AGM batteries are maintenance free, so called sealed lead acid batteries. Thanks to the ability to combine hydrogen and oxygen to create water as well as low content of electrolyte, the batteries are mostly maintenance free. Since the operation of the batteries causes gasses to build up, the integrated valve is used to release the pressure if it gets too high. Hence the alternative name for those batteries: VRLA which stands for Valve Regulated Lead Acid.

The VRLA batteries:

  • Have shorter recharge time than flooded lead–acid batteries.
  • Cannot tolerate overcharging: overcharging leads to premature failure.
  • Have shorter useful life, compared to properly maintained wet-cell battery.
  • Discharge significantly less hydrogen gas.
  • AGM batteries are by nature, safer for the environment, and safer to use.
  • Can be used or positioned in any orientation.

It needs to be noted that there is quite a number of factors which can impact the lead-acid battery’s lifespan. Among most important ones there is the operating temperature. An average temperature which is 20°C higher than the recommended one for operation, can result in reduction of projected service life by a factor of 4 which means a difference between for instance 8 and 2 years of service. Interestingly low temperatures have a negative impact on the usable capacity of the battery, while the capacity is more less the same in temperatures around 20°C and higher, it falls rather sharply below 0°C. The lead acid batteries are best cycled using low currents (not to increase the operating temperature of the electrolyte) in relatively cool temperatures.

Another important factor is the Depth of Discharge (DOD). In fact, in most cases the lifespan of the battery is expressed as a number of the charge-discharge cycles the battery can support in function of a parameter called depth of discharge (DOD). It’s easy to understand: if the DOD is 20% it means that the battery discharges 20% of its capacity before it can be charged again. The volume of energy which passes through the battery during charge and discharge (so called energy throughput) is insignificant and therefore does not cause a lot of wear and tear. This would change if the DOD is set to 80%, the volume of energy passing through the battery during 1 cycle is 4 times higher and therefore wears out the battery quicker.

The last important factor is the charging / discharging current. The battery manufacturers, instead of speaking of currents, refer to C-rate, which is an indicative parameter linking current applied to the battery to its capacity since speaking of one and while omitting the other may not be representative. It is easy to understand that a current of 10A would be significant for a battery having 1Ah of capacity while negligible for a battery having the capacity of 100Ah. The charging rate of 1C denotes a current which would fully charge the battery in one hour – in case of the 100Ah battery this would mean a current of a 100A. The lead acid batteries, due to their chemical characteristics prefer to be charged and discharged gently, with lower currents relative to their capacity.

Imeon solar inverters are capable of supporting many different storage technologies, including supercapacitors and state of the art lithium-ion technologies. Proven and optimized over the years lead acid management strategy allows Imeon inverters to optimize the service time of the battery in multiple various ways, attempting to limit the number of daily cycles the battery is subjected to, by reducing charge-discharge currents and by intelligently adapting the DOD.