Batteries store electricity for night time in off grid solar systems. An off grid home will have 4 to 6 days of storage in the batteries, while grid tie systems may have no batteries at all. Batteries also help loads to run at a constant voltage, while allowing the solar modules (or other power sources) to still charge as much as it can. In On Grid homes, batteries are only used to power loads if there is a blackout.
Batteries are the "weakest link" in a modern solar system. They're heavy, full of lead and sulfuric acid, require regular maintenance, and will wear out sooner than any other component. Most complaints on older systems are due to old batteries.
So how do we deal with this integral, but unpleasant side of solar power systems?

Other Battery Types and energy storage machines :

First, what about Lithium Ion (Li+), Nickel Metal Hydride NiMh, NiCad, fuel cells, fly wheel storage, perpetual motion machines, etc.?
We've been waiting (100 years now) for a better energy storage method. Li+ looks like the next step followed by fuel cells and flywheel storage. Charging large banks of Li+ batteries is proving troublesome compared to traditional chargers for Lead Acid batteries. Nickel based batteries are just not enough of an improvement to justify their extra cost. Fuel Cells are available now, but reasonably priced Hydrogen to run them is not. Flywheel storage is still too heavy, bulky, and expensive. Perpetual motion machines? Windmills on the roof of a car to power the car's electric motor? Soon we'll have a section on energy and efficiency that will explain why those don't work.
For the time being, we'll just say that every time energy changes form, some of that energy is lost to friction, heat, etc. How much energy is left after each conversion process shows the efficiency. Extra energy always comes from somewhere. Solar panels and wind mills are the closest thing to free energy machines you will ever find. Only the machine costs money. The energy is free for the taking after that!

How a Lead Acid battery works:

When a battery is connected (you turn on a light for instance), this starts a chemical reaction between its sulfuric acid and the lead in the plates submerged in the acid. This reaction produces electricity (a flow of electrons from the chemicals) to run the light. The reaction also depletes the sulfuric acid and builds up a crust of sulfate crystal on the battery plates. When the battery is recharged (by PV modules, or other source), it reverses the reaction, electrons are introduced back at the plates, the sulfate is dissolved into acid again and the battery water becomes more acidic. At full charge, the acid quits increasing in concentration, and further charging releases hydrogen gas. This battery gassing, in small amounts, stirs the electrolyte, knocks sulfate loose from the lead plates, and brings the battery to fuller charge. Too much gassing can boil the battery water dry, or cause overheating both of which shorten a battery's life.

It's a Chemical Reaction:

Chemical reactions usually slowly when colder, so this means a colder battery can't produce as much energy as a hot one. However, colder batteries will last much longer than ones that overheat (over 115°F) Also, we know that the greater the surface area of the lead plates, the greater the number of electrons that will be allowed to react at once, creating more current. But if the plates are thin, they can't react for very long before being drained or even damaged. This is how a relatively small starting battery can start a car. By increasing the surface area of its lead plates, a starter battery allows for a rapid flow of high amperage -- but they can't last as long because of their thinner plates. A deep cycle battery, the type used for energy storage in renewable energy systems, is the opposite of the starter battery -- it has much thicker plates with less surface area, and thus will produce power much more steadily over a longer period of time.

Battery Voltage:
A battery cell that completes its own chemical reaction, in its own acid bath only produces 2 volts. So smaller batteries are usually sets of cells wired together internally. Notice a 12 volt car battery will have 6 separate cells. Many different voltage batteries are available (2, 4, 6, 8, 12) and can easily be wired for whatever the nominal voltage of your system. Go to our Battery Wiring Diagrams for more details.

Amp Hours (AH):
A battery's energy storage capacity -- literally, how many amps can be taken from a battery in how many hours (amps x hours). A 100 Ah rating could mean different things -- 1 amp for 100 hours or 100 amps for 1 hour -- the difference lies in the rate of discharge (see side bar).

Only true deep cycle batteries are rated in Amp-hours. (Cold Cranking Amps is a rating for starter batteries -- this is the measurement of how many amps can be produced by a new, fully charged battery at 0°F for 30 seconds, while maintaining at least 60% of its nominal voltage.)

Comparing Cycle Life:
Batteries with thicker lead plates can withstand more cycling. A battery can only store, then release,so much energy before it needs to be replaced. By multiplying a battery's cycles x its energy storage rating, gives the total energy capacity of the battery for its entire life. Cycle life depends on depth of discharge(DOD). For example, a Deka Golf Cart Battery can be discharged and recharged many more times at 20% DOD(3500 cycles) than at 80% DOD (900 cycles).

Steps to Success:

1) Start with Deep Cycle batteries, and compare cycle life. For small to medium systems, we generally recommend deep cycle Golf Cart batteries like the Deka Golf Cart Battery. They can be drained completely (to 0%) and recharged an amazing 750 times. A car battery can only do this maybe 5 times before it's dead. A marine RV battery might say "deep cycle", but it can only be fully cycled 100 times. So a marine battery could last a year or so, while Deka Golf Cart Batterys regularly last 5 to 10 years. Compare this to the HUP Solar One that can be cycled 2100 times and can last over 20 years!

Comparing Brands of Golf Cart Batteries

2) Get as large a battery bank as is reasonable. You need enough batteries to store electricity for at least 3 days of usage for cloudy weather. We recommend 4 to 5 days of storage, and some systems have 10 to 14 days of capacity. Spending more money now, means less trouble, and longer battery life. Deep cycle batteries are rated in Amp Hours AH, the amount of energy that can be stored in the battery. 100 Amp- Hrs means that the battery will deliver 1 amp for a 100 hrs or 100 amps for 1 hr. (see side bar) Get a battery that lasts longer (has more cycles), in the long run you'll be happier.

4) Don't mix new and old batteries, or different types of batteries. Just as too many batteries causes problems, mixing new batteries with ones that are older than 6 months will cause problems. Basically, the newer cells are cycled more than the other cells, so the whole set only lasts as long as its weakest cell.

5) Keep the batteries at 40 to 80°F, with 55°F being optimal. A Basement is perfect! Batteries last longer if kept cool, but conversely they have more capacity if kept warm. They don't need to be inside, but in cooler climates outside they do need an insulated box with ventilation. A fully charged battery is filled with strong sulfuric acid and can't freeze till 30° below zero Fahrenheit. A completely discharged battery, however, is filled with very weak acid (almost water) and can freeze at 25° above zero.

6) Take Care of them! Check the water level regularly -- we recommend doing so every 3 months. Only use distilled water when adding water to the batteries, and don't overfill them. Clean the terminals with baking soda water, and recoat all connections with petroleum jelly. The HUP One Industrial Batteries have much more water over the plates, so they don't need to be maintained as often. We now even carry an automatic battery watering system, that keeps all the cells filled from a central 5 gal. tank of distilled water.

Comparing Battery Types

The Table Explained:

Cycles (to 80% Depth of Discharge) is how many times you can drain most (80%) of the energy from the battery and recharge it completely. Eventually the battery will lose its storage capacity and need to be replaced. In a normal solar system, the battery is only discharged about 10 to 20% a day, so it can last many more cycles than if it is drained totally every night (as in an Electric Vehicle).

Lifespan is the real world life expectancy of the batteries based on the experience of ourselves, our customers, and other installers. We have overlayed all this experience with the laboratory test results for cycle life. Actual lifespan depends on proper maintenance. If you don't check the water in your batteries, they could die in less than a year!

Price per kWh : This is the bottom line -- how much you are paying over the years for each type of battery. While initially more expensive, the Industrial Batteries are a good buy in the long run . When you figure the cost of extra maintenance, power outages, swapping out sets of heavy and corroded batteries, they become an even better choice.

Total cost is calculated by figuring total kWh that can be cycled from the batteries over their lifespan. Maintenance cost assumes a half hour (@ $50/hr) to check water and terminals every 2 months for golf cart batteries, every 4 months for L16s and every 8 months for the Rolls. Installation cost are included. Since the Industrial cells don't need to be replaced as often, their life cost is better.