Batteries for Solar Systems

In off-the-grid systems batteries are an essential component used to store the energy collected by the photovoltaic solar panels so it can be reused later as needed.  Batteries can also be used as part of a grid-connected system to provide battery backup in the event the grid goes down for a period of time. Batteries used with solar photovoltaic systems do have some unique requirements and must be properly designed to fit the particular system you are planning to implement.  The more you understand about batteries the more success you will have in choosing the right battery to fit your needs and the easier it will be to add them to your PV system.

How Batteries Work

Let's start by going over some battery basics. Most of us have handled small batteries from time to time, such as a AA or AAA battery, and you will remember that batteries have one end or terminal marked (+), or positive, while the other is marked (-), or negative.  In a large car battery, there are two heavy lead posts (also positive and negative) that act as the terminals and which connect to lead plates. The electrons collect on the negative plate of the battery. It is the nature of electricity that the electrons want to flow from the negative to the positive terminal.  However this will not happen until some type of connection, or circuit, is created between the negative and positive terminals of the battery.  This is normally done by connecting the battery terminal to a load, which is any type of device that might want to make use of that flow of electrons.  The load might be something like a light bulb, an electric motor, a television, or an electric heater; really any electric appliance. This is shown below:

In a typical lead-acid battery that you would use with a home energy system the inside of the battery contains a sulfuric acid solution (also called an electrolytic solution). The lead plates sit inside that solution. When a load is applied to the battery a chemical reaction takes place which produces the electrons.  Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal in order for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts. The chemical reaction will keep on occurring as long as the load is there until finally all of the sulfur molecules in the battery have bonded to the positive lead plate.  When you recharge your batteries using the solar panels the process reverses itself.  Electrons flow  back into the sulfuric acid solution and re-bond with the sulfur molecules returning it to its charged state.  This process of discharging and recharging is called a charging cycle.

Deep Cycle versus Shallow Cycle Batteries

Lead-acid batteries are the most commonly used batteries in home energy systems because they are less expensive and readily available.  There are many different sizes and designs of lead-acid batteries.  In home energy applications nearly all systems use a type of lead-acid battery are what are called "deep cycle" batteries (as opposed to a "shallow cycle" battery like you have in your car). How much of its total energy capacity a battery is designed to lose is known as its "depth of charge".  A shallow-cycle battery is designed to apply a lot of current quickly so it is ideal for something like starting a car where you need a lot of current right away but once the car is started the car's generator takes over supplying the electricity. However, the drawback to this type of battery is that it is not designed to be deeply discharged. In other words it has a low depth of charge.  If they are repeatedly discharged by more than 20% they will have a very short lifecycle. A deep cycle battery on the other hand does not provide as much current quickly but is designed to be deeply discharge by as much as 80% of the battery's total capacity in each charging cycle.  This makes them an excellent choice for a home energy system because they can provide a lot of electricity by volume.

Deep cycle batteries, though, have their own limitations.  While they can be strongly discharged, it is not a good idea to let them stay discharged for days at a time.  You need your system to recharge them fairly quickly.  Another thing you have to be careful of with deep cycle batteries is overcharging them.  If you keep trying to push more electricity into them after they are already fully charged the battery can be seriously damaged. A properly designed solar electric system which uses batteries will always include a charge controller, a device that monitors the battery charge and prevents the batteries from becoming overcharged.  For more info on these systems look at our section on "Charge Controllers".

Batteries can be either sealed or open.  As a general rule we recommend that homeowners go with sealed solar batteries because they require little maintenance and are generally safer if their charge is properly managed.  The sulfuric acid in batteries is not something you want to mess with if you don't have to.  

Battery Sizes and Capacity

Most battery banks that are used with solar systems use either 6 volt or 12 volt batteries though bigger batteries are also available.  In a battery bank the individual batteries are interconnected into a string so that the voltage adds up to 12VDC, 14VDC or even 48VDC. Now you might be thinking in the back of your mind, isn't that a bit low.  My home electric system uses 120 or 240 volts. Don't worry.  The thing to keep in mind is that we are talking about Direct Current (DC) voltage when it comes to batteries. When you are ready to use the electricity for your home the inverter, a current conversion device that will be part of your solar system, will convert the DC voltage in your battery bank into the 120 or 240 volts of AC current that your home typically uses.

Voltage though is only part of it.  A battery's energy storage capacity is measured in amp-hours (Ah). To get the total capacity you multiply the amp-hour capacity for a battery times its voltage.  So if you had a battery which had an amp-hour capacity of 10 amp-hours and it was a 12 volt battery you just multiply the 10 x 12 to get a capacity of 120 watt hours. Lead-acid batteries come in different sizes.  Some batteries are the size of your car battery while others are about the size of a water cooler.  In general, the bigger a battery the more electrolyte it contains and the greater its capacity as measured in amp-hours.  However, voltage is not necessarily greater because a battery is bigger.  Two batteries might be different size but have the same voltage, usually 6 or 12 volts.  But the bigger of the batteries will usually have greater capacity in amp-hours. Because batteries contain lead and a solution they are extremely heavy. As a general rule most homeowners should not look to use batteries that are physically bigger than they can easily handle.

Sealed or Maintenance Free Batteries

Batteries contain fluid and as with all fluids they can, over time, evaporate. If you go shopping for batteries you will probably see some batteries that have little caps on them and some that don't. The ones with caps on them are designed so that you can add additional water to the battery as the battery dries out. The ones without fill caps are usually referred to as maintenance free or sealed batteries. This is a bit of a misnomer because even these batteries have small vents in them to allow gas to vent during charging. However, you cannot add water to these batteries. If overcharged too many times, some of these batteries can lose enough water that they will die before their time. A few industrial batteries have special caps that convert the Hydrogen and Oxygen that is given off during venting back into water, reducing water loss by up to 95%.  You can also buy a device called a "HydroCap" that will do the same job for conventional ("wet") lead-acid batteries.

As a general rule we have found that sealed or maintenance free deep cycle batteries work better for most homeowners.  Maintaining the water level in a battery bank is a hassle which is nice to avoid.  Moreover, it is just too easy for most people to forget to check them and then have a battery die as a result.  While maintenance free batteries cost a bit more we suspect they are less costly in the long run for this reason.

Gelled Batteries

A new type of lead-acid battery that is on the market is something called a "gelled" battery. In this type of battery the sulfuric acid electrolyte solution inside the battery has been turned into a type of jello-like gel by the addition of silica to the mixture.  The primary advantage of these batteries is that it is impossible to spill the sulfuric acid inside the battery even if it is broken. However, there are several disadvantages. One is that they must be charged at a slower rate to prevent excess gas from damaging the cells. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be adjusted to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries but it is still one more thing to worry about. Another disadvantage of gel type batteries is that they must be charged at about a 20% lower voltage then standard lead-acid batteries. If overcharged, voids can develop in the gel which then lead to a loss of battery capacity. This is again one more adjustment to make in your charging system.  Given these issues we think that the safety benefits of gel batteries do not outweigh these disadvantages.  Instead we suggest that homeowners use standard lead-acid batteries and take care to ensure that they are safely disposed of.

Absorbed Glass Matt Batteries (AGM)

AGM, or Absorbed Glass Mat Batteries are a newer type of sealed battery.  They use a very fine fiber boron-silicate glass mat to hold the sulfuric acid rather than a gel. Like gelled batteries the AGM batteries will not leak acid even if broken but they do not have the same charging issues that gelled batteries have. The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. Moreover these types of batteries offer several advantages over conventional wet batteries as well as gelled batteries.  A typical wet battery can freeze if stored outside and the temperature gets below freezing. In an AGM battery the sulfuric acid is contained in the glass matting and there is almost no liquid to freeze and expand. AGM batteries tend not to lose water as much as gel or wet batteries.  AGM batteries are "recombinant" - what that means is that the oxygen and hydrogen given off during the charging cycle recombine inside the battery. The recombining is typically 99+% efficient, so almost no water is lost.  Also, AGM's tend to lose energy very slowly when not in use.  They have a very low self-discharge - from 1% to 3% per month so they can sit in storage for much longer periods without charging as compared to standard batteries.

AGM batteries are considerably more expensive than other types of lead-acid batteries.  They will typically cost 2 to 3 times as much as a standard wet lead-acid battery of the same capacity.  While they offer some advantages they may not always be the most economic choice.  Prices on AGM batteries appear to be coming down so we recommend you price AGM batteries on the web or in your area and determine if their benefits outweigh their costs as compared to the costs of conventional batteries.  

The Lifespan of Batteries

The lifespan of a deep cycle battery will vary considerably depending upon how it is used, how it is maintained and charged, temperature, and how good the quality was in the first place. By far the biggest factor in batteries not lasting to their full potential is overcharging, something that is amazingly common despite the zillions of warnings that are provided against it.  Listed below are some rough estimates as to durability for different types of lead-acid batteries:

• Marine: 1-6 years
• Golf cart: 2-7 years
• AGM deep cycle: 4-7 years
• Gelled deep cycle: 2-5 years
• Deep cycle (L-16 type etc): 4-8 years
• Rolls-Surrette premium deep cycle: 7-15 years
• Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years

Battery Safety

Most liquid acid batteries do not vent gasses while discharging. However, near the end of a typical charging cycle, when the battery is almost “full,” the sulfuric acid and water electrolyte will begin to break down into hydrogen and oxygen—a very explosive combination. When ignited by a nearby spark or flame, an “explosion” can result. For this reason it is very important to store batteries in a well ventilated area. If you are putting your battery bank in a basement which has weak ventilation you should consider adding a fan and a vent to avoid any problems. The vent should be designed to direct all vented gasses to the outside. A 2-inch PVC pipe makes a good vent, but be sure it is located at the highest point in your battery enclosure where the lighter hydrogen gas will accumulate. Be sure it also includes a screened vent cap to keep out rain and insects.

A number of manufacturers make storage containers or boxes specifically designed for battery banks, many of which have built in venting systems. Most battery storage containers are also well insulated and so provide better temperature protection which can improve your systems performance.  Commercial battery containers are designed to be closed and locked and so provide a better safeguard against someone accidentally coming into contact with your system.

Whether or not you use a battery bank container, it is important to give some thought to the surface that your battery bank will rest upon. A single conventional lead-acid battery can, depending upon its capacity, weight as much as 65 pounds.  Multiply this by 6 to 12 batteries and you have a very heavy load to support.  For this reason most homeowners put their battery banks in a basement or garage having a concrete floor which can handle the weight.  We recommend, however, that the batteries themselves be placed on a well ventilated wood rack of some kind.  Concrete floors can get awfully cold and this cold will reduce your battery performance.

Another thing to consider is that batteries, particularly wet batteries, contain sulfuric acid. Always wear eye protection and acid proof gloves when working around batteries, and have lots of water and baking soda nearby. This will neutralize any acid spills from battery refilling and prevent corrosive damage.