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Batteries 101: Solar Battery Basics

Because we get a lot of questions about which batteries are right for a project, we have put together this guide in consultation with MK Battery so that you can learn the fundamentals of lead-acid batteries and their advantages in a solar context.


There are several components that compose a lead-acid battery. Each battery is comprised of one or more cells, the electrolyte itself, and in the case of VRLA batteries, at least one pressure relief valve.

MK Deep Cycle Lead Acid Solar Battery

A cell contains the lead plates that hold the battery's charge. There are two sides to the cell, one positive and one negative, divided by a porous separator. You can tell how many cells a battery has by looking at the voltage listed on its spec sheet or by counting the terminals on top of the battery. Every battery cell adds two volts and one terminal to the final product, so a 12 volt battery would have 6 cells.

Valves release excess gas when the internal battery pressure passes a certain threshold, which can happen when the battery is overcharged. Overcharging should be avoided whenever possible because your battery will lose hydrogen and oxygen which will eventually dry it out and destroy it. In a wet battery, the valve is also where you will add water when necessary. There is one valve for each cell.

The electrolyte is the sulfur mixture that fills the battery. When a battery is completely discharged, the electrolyte becomes water soluble and will no longer be able to fully charge. On the other side of the spectrum, when a battery is overcharged, the electrolyte releases hydrogen and oxygen gas causing pressure and heat within the battery.

Type Cost Weight Life Maintenance Heat Tolerance Cold Tolerance
Wet Low Light High* High Low Low
AGM Middle Middle Low Low Low High
Gel High Heavy Middle Low High Low

*if maintained properly


Flooded batteries (also known as WET) are more cost-effective and lightweight. They can handle deeper discharge rates more often than AGM or Gel batteries and you can add water if they start to get dry. If they’re properly maintained, they can have the longest life cycles of the three types of lead-acid batteries. With that said, there are a host of disadvantages that make VRLA batteries a better choice for many newcomers to off-grid.

If you tip a flooded lead-acid battery over, it will spill and become an environmental hazard. They are not recommended for extremely cold climates. The acid inside can also stratify when the batteries have not been cycled for a period of 30 days or more which would cause the sulfuric acid and water inside to separate.

Since sulfuric acid is more dense than water, the electrolyte at the bottom of the battery becomes more corrosive and damages the plates inside. If you do not maintain a flooded battery properly, it will fail very quickly. Please see the manufacturer spec sheet of your flooded batteries to see how often they need to be checked. Usually, they need to be watered every 30 to 45 days depending on the location.

Although flooded batteries are lighter than the alternatives these batteries must ship freight because they are considered hazardous materials.


Valve-regulated lead acid batteries, or VRLA, feature a valve for each battery cell. Unlike Wet batteries, these valves open automatically when necessary and are not meant to be opened by hand for any reason. They tend to be heavier than their flooded counterparts and are not considered hazardous materials. Some of these batteries can ship ground in small quantities.


Because they are low cost and low maintenance, AGM batteries are a good choice for someone new to off grid. AGM batteries require no maintenance, can be placed in any position except upside down, will not spill like flooded batteries, and have superior tolerance for low temperatures. If you have an application where the batteries will be vibrating (such as an RV), you may want to use AGM batteries, as constant vibration can cause bubbles in Gel batteries.

Unfortunately, they do tend to gas out in high heat, so they are not well-suited for hot climates. Of the three types of lead-acid batteries available, AGMs have the shortest life span, though they still last much longer than unmaintained flooded batteries.


Like AGM, Gel batteries require no maintenance and are not hazardous. Gel batteries handle the heat very well and can last nearly as long as properly maintained wet batteries. While they are more expensive than AGM or Wet batteries and are extremely dense, they do a great job of balancing the benefits and disadvantages of each. If you can afford them, they make a great choice for a low-maintenance off-grid system. Due to their weight, Gel batteries usually will ship freight.

Protecting your Batteries

Batteries are the most expensive component of an off-grid system so it makes a lot of financial sense to invest in some optional components to protect your batteries:

Charge Controller with Low Voltage Disconnect

As a battery charges its voltage shifts slightly. An excellent metaphor: imagine you are filling a bucket with water, but you are doing it through a hole in the bottom of the bucket, rather than from the top. At first, this will be easy, but as the bucket grows more full and the pressure of the water builds, you will need more pressure to continue filling it.

Similarly, the voltage to charge a battery shifts as it is charged. A higher voltage is required as more power fills the battery. A charge controller adjusts the incoming voltage to match what the battery currently requires. This protects the battery and increases its life.

If you discharge any type of lead-acid battery completely, it will not last long. A low voltage cutoff switch prevents this by looking at the battery’s current voltage constantly. If it drops below a set point, it completely cuts off any loads drawing power from the battery, preventing deep discharge.

Temperature Sensor

A battery is rated at 77° F. If the ambient temperature is below this mark, you will need to put in a higher voltage than you would normally; if it is warmer, the voltage needs to be lower. A temperature sensor will account for this factor and improve your battery life. Even if your area is usually around this 77° mark, a temperature sensor is a smart investment because temperatures do vary.

Battery Monitoring

Battery monitoring allows you to see how much charge is actually making it into your battery. If something in your system goes wrong, your batteries are the first thing to go. Investing in a battery monitoring solution can show you something is not right before the most expensive component of your system is destroyed.

Temperature-Controlled Environment

If you are in an extreme environment, investing in an enclosure or putting your batteries in a temperature-controlled room may be a good choice. Even though the right batteries can survive extreme temperatures, they will perform much better in moderate conditions. Please note that batteries should never be placed in a fully sealed container, because it could cause an explosion.