Almost all batteries used to store large amounts of electricity for solar power are lead-acid based. There are other chemistries used for batteries, but lead-acid is the most common. True deep cycle batteries provide many years of service and are very reliable.

In off grid applications a key factor in solar energy cost is the type and construction quality of the batteries used. Total cost of a solar power battery system can be viewed as a function of the depth of discharge, the number of charge and discharge cycles, and the cost of the batteries. Monthly maintenance checks are critical for longest possible service.

In order to buy the right batteries it's important that you understand the terminologies that are used in marketing batteries and what you can expect in terms of service life under varying conditions. This will not be a complete treatise but it will be helpful.

The lead plates of a battery are immersed in a mild acid solution. The battery's ability to deliver large amounts of current to a load is related to the surface area immersed in the acid solution. Thinner and more numerous plates increase the current delivery to a load, but result in decreased life expectancy and an inability to cope with deep discharges. When using batteries for storing electricity for a home, the capacity of the batteries to survive when deeply discharged is important.

• Starting Batteries
  • These are like those used to start your car. They are designed to provide quick bursts of power, but are completely unusable for storing electricity for a home. One 80% discharge will usually kill these batteries. They just won't work.
• RV Batteries
  • RV or Marine batteries are a little bit beefier than a starting battery, but they're not much better. Save your money and your time, and skip these. They usually say they're deep cycle, but they won't hold up to everyday use. It cost me plenty to learn this lesson, so for your sake listen up. If they list CCA (cold cranking amps - a starting battery specification) as a feature that's a tip that it's not a true deep cycle battery.
• Gel Cell
  • These batteries are the industry standard for computer UPS systems. They are maintenance free and are better than RV batteries for solar power systems. They usually have half the life of a deep cycle battery. Battery life is expressed in terms of the charge/discharge cycles that a battery can withstand. Gel Cell's are designed for only occasional use, and while they are considered safer to handle and basically freeze proof, I think they are not worth it. My calculator says they are about 4 times more expensive than deep cycle, so the advantages just don't make up that kind of difference in cost. They also require different charging parameters so they can't be mixed with traditional deep cycle lead-acid batteries.
• Deep Cycle Solar Power Batteries
  • Surrette, Rolls Royce, Trojan, and others make true deep cycle batteries that hold up very well for this kind of duty. My personal favorite is the D-460 Surrette (or Rolls Royce) batteries. They are cost effective and reliable. They are the same shape and size as the Trojan L-16's, but have held up much better for me. There are better batteries out there but they cost more. Calculating the optimum value for your dollar is difficult with batteries because precisely predicting future power usage patterns, discharge/charge cycles, and depth of discharge is impossible. It's a very good idea to start with a less expensive battery set until you gain experience with batteries and solar/generator power systems. With real life experience you're more aware of what your actual usage rather than predicted usage will be.

Amp Hour Ratings

A deep cycle or solar power battery's ability to deliver power is directly related to the discharge rate. As I increase the amperage draw on a battery it's internal resistance rises. This increases the internal voltage drop and the available voltage to a load like an inverter drops. This characteristic is valid with all lead acid batteries.

When I purchased my batteries, I concluded that the best battery for solar power would be the Rolls Royce D-460 batteries. Their 100 hour discharge rating is 460 amp hours. This means that the battery can deliver 4.6 amps to a load for 100 hours before the battery is 'dead'. The same battery at a 20 hour rate can deliver 20 amps for 20 hours resulting in a 400 amp hour rating. The more current you draw on a battery the smaller it appears to be.

Large battery systems have a lower effective internal resistance and can deliver higher currents more efficiently. This results in more stable voltages, less stress on the batteries resulting in longer battery life, reduced power losses, and a happier owner. A beefier battery system makes it easier for an inverter to regulate AC output voltages.

Solar Power Battery Charger

The best way to charge a solar power battery set is to use one of the many good charge controllers from Xantrex, OutBack, or Magnum Solar. These units are incredibly efficient and are design specifically for solar power applications. Charge controllers are designed to take power from your solar pannels and convert it to the proper voltage to provide the maximum charge of your batteries. This results in 10-20% more power from your solar panels.
If poor weather requires you to recharge your battery from a generator you should be using the built in charging system common to high quality off grid inverters. (Once again I recommend Xantrex, OutBack, or Magnum Solar) These inverters have sophisticated charging systems that help to keep your battery system healthy, and help to extend the life of your battery investment. Even if you have the most efficient solar panels available, bad weather can limit your collection of power. A backup generator is important part of your solar power battery system.
Good local solar PV installers can be a great resource in choosing the right equipment for your area and weather conditions. Even if you go your own way and do it yourself get all the input you can. Don't rush into a system, choose wisely.

The Most Common Battery Mistakes

1.) Too Small A Bank

Most people buy high quality inverters, solar panels, and efficient appliances, but try to get by with fewer batteries than they really need for optimum service. This is a costly mistake. Even good batteries. if over worked, will fail sooner than they should requiring costly replacement. You don't want to have to replace your batteries until you've gotten years of good service.

Buy high quality batteries, and be sure you get enough storage capacity. You should be able to run 4-7 days without charging. This seems like a lot, but a properly sized battery bank will allow you to get long life from your batteries, increased charge efficiency, and allow your inverters to supply highly regulated voltage even under very high peak loading. If the battery bank is too small (due to internal resistance of the batteries) high current motor starts and the like can result in bumps in the voltage output of your inverter. This problem disappears with a properly sized battery bank.

2.) Not Properly Maintained

Batteries are very easy to live with, but every month the water level should be checked. When refilling batteries, use only distilled water! Not filtered. Distilled. You can buy low cost water distillers so you can easily make the distilled water you need for your batteries.

Frankly, you really can't trust some water sellers. They claim the water is distilled even when it's not. So be aware. The reason that this is important is that contaminants common to water can upset the battery's chemistry. This can easily shorten battery life.

Keep your batteries clean and dry. Use a baking soda and water mix to neutralize any acid on the exterior case of your batteries. A film of acid can provide a discharge path for your batteries, which wastes power. The acid eats clothes, and is harmful, so keeping things clean and acid neutralized is just good sense. Make sure that the caps are on the batteries when cleaning, and be sure that no soda solution gets into your battery.

3.) Wrong Battery Type

While buying gel cell batteries is not necessarily a mistake, it often is. These batteries cannot handle deep discharges very well and are rated for much fewer charge/discharge cycles. They shine in UPS systems, and eliminate watering, maintenance and freezing issues I'm told, but over all battery costs are nearly 4 times higher. You also need more since they have less capacity which makes wiring then in more of a pain. If this is for a summer home and you're worried about freezing in the winter and your use is going to be minimal then they might work out ok. I guess that's up to you...

4.) Charge Control

Battery charging systems common to large home size inverters are very sophisticated and will greatly extend the life of your batteries. If you are using solar panels, there are also excellent battery charge controllers that will maximize the power from your panels and treat you batteries with care.

Never attach solar panels directly to a battery bank, or otherwise charge your batteries without a high quality charge controller.

How Long Will These Batteries Last?

If you buy enough high quality battery capacity, you can easily get 20 years of trouble free service. Sometimes more. The batteries I'm using are good for 10 years if properly maintained, but they cost a lot less then top-of-the-line. I made this choice as a step towards the battery system I want, but could not afford right away.

One nice thing about batteries is that they usually fail gradually, giving you plenty of warning that they need replacement before replacement is required. This gives you time to allocate the money needed to replace your current batteries.