Solar Battery Storage Systems: If You Can't Tell Your AGM From Your Gel

Most homeowners planning on installing a solar panel system on their roof never have to worry about the many choices in solar batteries. But if you are a homeowner or other solar energy user who is planning a solar energy system that will be off grid or have a emergency backup power, then you will definitely need to know something about solar batteries. If properly conceived a solar electricity storage system can be a reliable and affordable method of powering your cabin, RV or home. In this article, we will help you navigate the choices you will face when designing an off-grid or gridtie battery bank and understand the world of electrolytes and amp-hours.solarbatteries.jpg

Grid-Tie and Off-Grid Battery Systems

There are two main kinds of solar battery storage system: grid-tie with battery backup and off-grid.

Off-grid systems are exactly what they sound like: independent of the grid. During nights and cloudy days, an off-grid solar home relies on batteries. There is an unfortunate disparity between when we can produce solar energy and when we want to use it. Peak solar electricity generation happens around midday, while peak consumption occurs in the evenings when lights and appliances are turned on. To reliably power a home, a battery bank must charge in the few hours of intense sunlight and discharge throughout the evening and night. Batteries designed for this kind of environment are called deep-cycle batteries. The most common solar deep-cycle batteries are flooded, AGM and gelled lead-acid batteries. These three batteries are meant to repeatedly discharge a lot of their capacity, but have low discharge rates.

Grid-tied systems with battery backup use batteries only when there is a power outage. During regular operation, DC power from the solar panels is passed through a grid-tie inverter that converts it to AC. This allows the solar panels to run appliances in your home. At the same time some of this electricity charges the battery bank. If there were a power outage the inverter would disconnect from the grid and your home would run as an off-grid system. In general, battery banks can be smaller in grid-tie with backup systems since they are only used when the grid is down. You can view a grid-tie battery system as a hybrid between pure grid-tie and off-grid systems.

Before starting to design your off-grid system, please make sure you have read this article on the components in an off-grid system:

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Choosing Your Solar Batteries

If you have decided to use either an off-grid or grid-tie with battery backup system to power your home, the next step is to choose your solar batteries. While many different kinds of batteries are used in solar electricity storage, lead-acid batteries are most common. Competitors such as nickel-cadmium and nickel-iron can be expensive, inefficient or hard to dispose of. The three main lead-acid batteries are:

  • Flooded (FLA) → unsealed with liquid electrolyte
  • Absorbed Glass Matt (VRLA) → sealed with electrolyte held captive by glass mat
  • Gelled (VRLA) → sealed with gel electrolyte

Each type has its own benefits, drawbacks and requirements. Some batteries are well suited for individuals who enjoy monitoring and maintaining their system while others are designed for those who have an 'out of sight, out of mind' attitude. Regardless of the relationship you want to pursue with your solar electricity system, there is a battery for you.

Flooded Batteries (FLA). Flooded batteries are the least expensive and most common of the lead-acid batteries. For this reason we will treat them as a kind of reference point for the VRLA batteries. Unlike AGM and gel batteries, they are unsealed and therefore have to be well vented. This means they must be stored in a battery box that has a vent leading to the outdoors. The main thing you have to know about unsealed batteries is that they require periodic care. But don't let that scare you! We have outlined how to keep your battery bank running in our Battery Care and Safety article. Flooded batteries also need to be properly oriented. VRLA batteries are more forgiving in this respect and can be stored on their side. While sealed and unsealed cells have similar performance, they each produce a distinctly different solar storage experience. For a list of SolarTown's flooded batteries please follow the hyperlink.

Learning Point: Special applications for flooded batteries:

  • Telecommunications
  • Emergency Back-up systems
  • Golf carts/electric vehicles

Gelled Batteries. Gel lead-acid batteries are specialized in deep-cycle applications. They use fumed silica as a thickening agent for the electrolyte, which makes the cells more sturdy. The viscous electrolyte prevents the cell from leaking when damaged. Gel batteries also vent less and discharge at a slightly higher rate than flooded cells (low Peukert exponent).

While gel batteries are sturdy and low maintenance, they have a few serious downsides that make them a sub-par option in solar storage. First, they are not compatible with flooded and AGM cells due to much lower charging voltages. The low charging voltage also makes it very easy to accidentally overcharge the battery and destroy it. This is particularly true in solar electricity systems, where electricity production is variable. If you choose to use a gel battery bank, the charging voltage must be limited to a very precise range. In addition, gel cells are more expensive than flooded cells, do not provide much better amp-hour capacity and will generally not last as long as FLA. When you buy a gel cell, you are paying for extra convenience, not extra capacity. You can find gel batteries for sale on SolarTown's Store.

Learning Point: Applications for gelled batteries:

  • Hot weather environments
  • Very-deep discharge situations

AGM Batteries. Absorbed glass mat batteries are the latest development in valve-regulated lead-acid batteries. They are designed with the electrolyte held captive by sheets of fiber glass. In some respects AGM batteries are similar to gel batteries. AGM cells are leak-proof due to the captive electrolyte and do not need much ventilation under normal conditions. However, unlike gel batteries, AGM cells use the same charging voltage as flooded batteries and are more compatible with solar electricity systems. This means they can be used in combination with flooded cells, though this setup is not recommended as it may degrade battery life.

Other advantages of AGM cells are that they are non-hazardouslightweightresistant to cold temperaturenot inclined to heat up (low internal resistance) and able to hold a static charge for a long time. In addition, they have the highest discharge rate of the three lead-acid batteries. The most notable downsides to AGM batteries are there price and their life span (potentially). Lead-acid battery life is a topic of heated debate. However, overall it seems as if AGM batteries have a slightly shorter life-cycle in exchange for their discharge rate. This said battery life fluctuates a lot based on temperature, depth of discharge and maintenance. SolarTown offers many models of AGM batteries.

Learning Point: Special applications for AGM batteries: 

  • All flooded battery applications
  • Stand-by applications (street lights, computer backup)
  • Cold weather environments
  • Wet environments
  • High end cars, motorcycles and recreational vehicles

Below is a table comparing some of the characteristics of the three lead acid batteries. In order to look at battery capacities and other technical data, please see the batteries section of the SolarTown store. 

 Lifespan*Minimal GassingSpill Proof RatingFlexibility in MountingCharging Voltage SensitivityMaintenancePrice
Flooded 3 1 1 1 3 1 3
Gelled 2 3 3 2 1 3 2
AGM 2 3 3 2 2 3 1


*if well cared for


1 - poor

2 - good

3 - excellent

Your choice of battery will dictate your solar experience. For the average homeowner, we do not recommend gel batteries because of their high sensitivity to charging voltage. If you make a mistake, you could quite easily destroy your battery bank. If you don't mind monitoring and maintaining your system, and want the lowest installation cost possible, you should be looking at flooded batteries. On the other hand, if you would rather pay extra and never have to touch the batteries after installation, AGM is the way to go.