Important Concepts
The batteries are the most expensive part of the system, often costing more than buying generator fuel would cost. Choosing well and taking care of them will save you a lot of money.
The cheapest batteries you can find will not be the least expensive in the long run since they will need replacement more often.
A few large batteries will last longer and have fewer problems than a lot of smaller batteries.
Battery types
Lithium Iron Phosphate batteries (abbreviated LFP or LiFePO4) are the new technology in the solar market. They are the most cost-effective batteries for Solar Power storage, and many solar equipment manufacturers now support and recommend LFP batteries. Their up-front cost is higher, but they more than make up for it with a much longer lifetime, especially in hot climates. However, shipping LFP batteries by air can be expensive or even impossible. If going by air, they must ship on cargo-only flights and you’ll have to pay an extra fee for hazardous goods. On the other hand, they are much lighter than lead-acid batteries. The various brands have different restrictions on putting their batteries in series or parallel, and varying charge/discharge current limits. If you want to consider these, we can help you “do the math” and find the right batteries for you system.
Until recently, Lead-Acid batteries were the only choice for solar energy storage; the same basic chemistry used in your car battery. However, there are a variety of design differences and terms to understand. The rest of this page focuses on these types of batteries.
The form of acid containment:
Flooded – The acid is in liquid form. Making them “dangerous goods” when shipping because the acid might leak out.
Sealed – There are no caps for adding water. Could be Flooded or Gel or AGM, starter or deep-cycle it’s totally ambiguous. If the acid is liquid (flooded), these are still Dangerous Goods.
Gel – The acid is in a jellied form and won’t run out if the plastic case is cracked. Therefore NOT “Dangerous goods.” Usually these are deep cycle, but often made for “standby use” (see below).
AGM – Like Gel batteries, they are not Dangerous goods because the acid is contained inside fiberglass matting and can’t leak out. Usually, but not always, these are good for deep cycle use. Some are designed as starter batteries.
VRLA – (Valve Regulated Lead Acid) A more general category encompassing both AGM and Gel types. This means they are sealed and spillproof. Usually these are deep-cycle, but not always.
The Intended Use the Battery is designed for:
Starter – These have thin lead plates inside and can provide very high amperage for short bursts to startup an engine and then be recharged immediately by the alternator. These have a very short lifetime if used for energy storage since the plates get destroyed if you run them way down repeatedly.
Marine – A compromise design that can be run down more and still have good amperage for short bursts to start a motor. Marine batteries are often labeled “deep-cycle” and can be used for solar energy systems, but are not the best choice.
Deep-Cycle – This is the type you need. They’re made to supply less current for longer periods of time and be cycled repeatedly. However, not all are created equal, and there are some variations to watch for:
Standby – These deep-cycle batteries are designed for UPS systems which keep them fully charged most of the time and only deep-cycle them a few times per year. Not the best choice for solar energy systems.
Renewable-energy – These are deep-cycle batteries designed for the heavy daily charge-discharge cycles seen in wind or solar power systems. These are the best; they will last the longest in solar power systems, more than offsetting their somewhat higher price.
Sizing the battery bank
See the Sizing and Building a Battery Bank page for the calculations and details on doing series and parallel combinations to make up your battery bank.
Shopping for batteries locally
People often have no choice but to look for batteries in the local marketplace. If you’re doing this look for the words “Deep-Cycle” on the label or literature. If they talk about “Cranking Amps” they’re really not a true deep-cycle battery, avoid them. Even better if you can find batteries labeled “renewable-energy” or something equivalent. Secondarily, see if they’re AGM or Gel or perhaps VRLA, these are all good terms. If they are “sealed” but don’t say deep-cycle avoid them. If you can hear the acid slosh when you shake or tilt the batteries, or they have caps on them, they are flooded batteries and they won’t last as long unless they are maintained perfectly for years on end, which almost never happens in remote locations.
When shopping, take your multi-meter with you. All lead-acid types need periodic recharging every couple months and will ruin themselves if left alone on a shelf too long. Fully charged voltage is 12.7 V or 12.8 V. Before purchasing batteries, check that the voltage is 12.6 V or greater. Lower than this means it’s been sitting too long, and the lifetime will be shorter than a new battery. If they’re down to 12.4 V or lower, their service lifetime has been seriously reduced.
Mounting and maintenance
All Lead-acid battery types can give off acidic vapors and hydrogen gas. Flooded batteries vent gas all the time. Sealed AGM and Gel batteries don’t usually vent any gases, but they can vent occasionally if they are overcharged, which will probably happen as they get old. You need to mount them in a location with some ventilation.
To protect your battery investment, keep them cool. Batteries are designed to run at about 22° C (72° F). Running them at ~32° C (90° F) cuts their lifetime in half! In the tropics you can put them on the ground, or better, below ground and protect them from heating by the sun. Keep a little space between the batteries so air can flow between them and cool them since they generate their own heat when in use.
Unbalanced Batteries lead to premature failure
Fewer batteries of a larger size are best to minimize risk of imbalance
Manufacturers recommend no more than 3 strings of batteries in parallel, with a single string being preferred. This is because “identical” batteries from the same manufacturer will have variations in capacity and internal resistance. The weakest batteries get overworked and fail very quickly, and the problem gets worse with the more strings you have in parallel. Making a large battery bank out of dozens of 12-V 100-Ah batteries will work OK at first, but fail much sooner than it should due to imbalances. If you need more than 400 Ah out of your battery bank, consider getting individual 2-V cells in larger sizes like 800 Ah to 1200 Ah.
Proper cabling practice is important to keep your batteries in balance
See the Sizing and Building a Battery Bank page for details on how to avoid imbalance caused by cabling. 
Check the connectors every few months and clean any corrosion as necessary. After connecting or cleaning, covering the terminals with battery grease sold in automotive stores can help reduce corrosion.