BU-201b: Gel Lead Acid Battery

Learn the unique advantages of lead acid batteries

The early gelled lead acid battery developed in the 1950s by Sonnenschein (Germany) became popular in the 1970s. Mixing sulfuric acid with a silica-gelling agent converts liquid electrolyte into a semi-stiff paste to make the gel maintenance free. The AGM that arrived in the early 1980s offers similar performance to gel but each system offers slightly different characteristics to fill unique market needs. Gel batteries are commonly used in UPS, big and small, while AGM has carved out a market with starter and deep-cycle applications. Gel and AGM batteries are part of the valve-regulated lead acid (VRLA) family to make the traditional flooded lead acid maintenance free.

Energy storage systems (ESS) deployed for frequency regulation and energy buffering use lithium-ion batteries. Unlike lead acid, Li-ion can be rapid charged when excess energy is available. While UPS normally dwells at full-charge and is only discharged occasionally, Li-ion in an ESS can operate at mid-state-of-charge of 40 to 60 percent without inducing sulfation. UPS for standby applications continue to be served by lead acid batteries because of economical cost, ruggedness and superior safety, Li-ion is making inroads into applications that need cycling by delivering the best price per cycle.

A gel battery generally lasts longer than AGM; improved heat transfer to the outside is one reason. (The gel separator moves heat whereas the absorbent glass mat of the AGM acts as insulator.) A further advantage of gel is the dome shaped performance curve that allows the battery to stay in the high performance range during most of its service life before dropping rapidly towards the end of life; AGM, in comparison, fades gradually.

Gel is known for good performance at high ambient temperatures, is less prone to sulfation than other systems, but it needs the correct charge and float voltages. In comparison, AGM is superior at low temperatures with better current delivery because of low internal resistance. The cycle count on gel is said to be larger than AGM and the secret lies in holding more acid due to its design. Because of higher internals resistance, gel batteries are not used for high current applications.

One of the secrets of building a good gel battery lies in the valve construction. Small and economical gel batteries use a valve consisting of EPDM-rubber (EPDM stands for ethylene propylene diene monomer). High quality large gel batteries for use in high and low temperatures use a more elaborate valve design to improve moisture retention.

In terms of suitability and cost, the flooded lead acid is most durable when used in standby operation, but it is also the most expensive and requires maintenance by replenishing water. Gel is cheaper than flooded and is the preferred battery for the UPS installations in communications. AGM comes at a lower cost and is also superior in load capabilities to gel. Both systems have a promising future and will continue to serve for standby applications that require limited deep cycling. Table 1 illustrates the advantages and disadvantages of the gel battery over other lead acid systems.

Advantages Maintenance free; can be mounted sideways; low self-discharge
  Long lasting due to its ability to transfer heat to the outside
  Performance stays high until the end of life, then drops rapidly
  Produces water by combining oxygen and hydrogen
  Safe operation and forgiving if abused; less dry-out than AGM
  High cycle count, tolerance to abuse and heat
  Large variety of battery sizes available
Limitations Higher manufacturing cost than AGM 
  Sensitive to overcharging (gel is more tolerant than AGM)
  Moderate specific energy and load current
  Subject to release gases. Ventilation needed
  Must be stored in charged condition (less critical than flooded)

Table 1: Advantages and limitations of the gel battery.

Last updated 2017-10-11


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Comments (5)

On June 28, 2017 at 7:04am
Remco Schrijver wrote:

This is a very usefull website. Thank your for making this possible. In reading this piece however there are some things that are not correct if I may be so bold.

Gel was invented in the mid 1950’s by the Sonnenschein factory, AGM is invented later on. AGM batteries are commonly used in UPS systems, big and small. Not GEL.

Gel is used in deepcycle applications, however AGM is gaining marketshare in that same area due to lower costs and higher specific energy. Please note that there is a difference between UPS and Deep cycle AGM batteries. Not all AGM is desgined for cyclic application.

AGM is more sensitive to overcharging in comparance to GEL. Gel is more ;;robust’’ and can take abuse better. AGM performes better in cold conditions and GEL performces better in higher temperature surroundings. These days there are GEL batteries specially designed for high temperature surroundings (DEKA Fahrenheit for instance).

One of the reasons GEL can do more cycles lies in the additive of fosforic acid and the ability to hold more acid due to its design.

On July 16, 2017 at 9:39am
Joe Chau wrote:

Thanks for this good info article. Is “but cheaper than flooded” mentioned in the Limitation section might a typo? It is often said that Flooded Lead Acid is the cheapest of AGM and Gel. Are we talking about some special type of flooded that is also VRLA? Also “Vale-regulated Lead Acid” near the end of the first paragraph is a typo.

On December 21, 2017 at 5:55am
Al Mamun wrote:

Dear, as i read somewhere that, Sealed Maintenance free batteries are like dead when their charge level drops to less than 40%. Means these batteries cannot give full power by recharge, if they are reached to 40% of their capacity. Is it true?

On May 10, 2018 at 8:22pm
hung wrote:

Tôi rât thic trang we nay

On January 16, 2019 at 12:09am
spencer wrote:

I have a problem.. with EVERYONE who claims that these SLA batteries.. needs to be charged at 13.5 - 13.75 volts.. (for sake of reference).
A LEAD-ACID BATTERY ... MUST BE FLOATED AT 13.2V… ONLY..!!!!!!!!!!!!!!!”!

This is EXTREMELY important…!!! A LEAD-ACID battery chemistry… is designed to be a common terminal voltage of 2,2V per cell.. or 13.2V per battery of 6 cells… (common voltage for 12V SLA battery.

Charging - FOR ANY TIME LONGER than the “so called charge time”... destroys the battery, and heats up the inside.!!! causing very slow dehydration of the cells.!!!!

I have designed an intelligent SLA battery charger… by which I keep the SLA battery perfectly operating…

This charger - automatically monitors the terminal voltage of the 6 cell .. 12V battery.. and will ONLY switch to the max 14.4 volts level.. when the battery is being discharged at a higher rate, than deemed suitable…
Whilst in the 14.4 V level of charge… there is a timer in the software… which lasts for a maximum of 1 hour..!!! If the SLA battery is not charged within this 1 hour period.. the charger shuts down… THE BATTERY has reached END OF LIFE..!!! PLEASE REPLACE…!!!

I have spent over 6 months in developing this very simple intelligent battery charger.. using a very simple 8 pin PIC processor… I have recovered.. so called.. US… batteries, which have been abused in usage time.

My general notice of problems.. is that the 12V batteries are FLOAT CHARGED at.. 13.5V or higher…WHICH IS TOTALLY INCORRECT.!!! this voltage destroys the chemistry.. and dehydrates the battery cells…

I have made up what I call DCW…. which is ULTRA PURIFIED DISTILLED water… which has a TDS… of 0.00001 uS/cm or better..!!

I refill the… dried out batteries.. gently with this water… and then recharge with my charger… then add additional water until the AGM.. is saturated once again…

PLEASE DO NOT OPERATE a SLA battery on it’s end or side… This allows the electrolytes to seep out the AGM.. and so dry parts of the plates and AGM… WHICH DESTROYS THE CELL GENERALLY…. The SLA has to be placed down flat on it’s base.. on a highly insulated surface… else electrolysis coupling occurs.. through the base of the thin battery casing… destroying the battery before it’s time…

The surface tension between the electrolyte and the AGM is broken down.. by charging currents… and so the AGM releases the electrolyte which runs out .. and accumulated outside the AGM… in so doing drying out the AGM and the lead plates.!!!

SO OPERATE a SLA in the FLAT HORIZONTAL position ONLY…as it stands on it’s base..!!!

I have SLA batteries - that have now been in operation for over 6 years..!!!
and they do not seem to be any worse for wear.!!

All my “recovered” SLA batteries seem to be working very well now…

You may contact me.. with any queries.. if I can help ... I will…!!!

Spencer - in South Africa… you may whatsapp me.. on +27 71 743 4842… respect my time zone please…!! DO NOT CALL ME after 6 PM… my time..