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There is more to comparing batteries than just cost or amp-hour ratings. For example, based only on cost, the Concorde AGM's do not look so good. On the other hand, you probably would not want to store a flooded battery in your computer room. There is no one best battery for all applications. If the batteries are in a remote communications site, low maintenance might be the most important feature. For the off-grid full time home, capacity. life, and long term cost would probably be the most important.
Universal - Sealed AGM
Concorde - Sealed AGM - Sun Xtender
Crown - Flooded, industrial (fork lift type) and standard deep cycle.
Surrette Canada (Rolls) - All solar batteries.
Trojan deep cycle solar batteries (RE series)
The "Best" battery for a particular system is not always the most expensive, but it is seldom the cheapest either. There are many things to consider:
How much storage, in amp-hours, do you need? This will vary with the application and what part of the country you are in. As a rough rule for home solar systems, the total battery capacity (in amp-hours) should be three to five times your daily usage. 3 days is usually sufficient in most of the sunbelt states, 4 in most of the Midwest, in the East and Northwest, 4 to 5 days is better. If you are in a good wind area and have a wind generator, you can probably cut this down a day, but wind turbine output can be highly variable in many areas.
For backup power systems (UPS), the total capacity should be enough to cover about twice the longest anticipated outage. To figure how many amp-hours storage you need, take your average daily usage in watts and divide by the battery voltage. For example, if you use 5 kWh (kilowatt-hours) per day, and have a 48 volt system, then dividing 5000 by 48 gives you 105 AH. Since you do not want to discharge the battery more than 50% in most cases, you would need 210 AH. If you want to keep running for 4 days of bad weather with no sun, multiply that times 4, which gives you about 850 AH total capacity. UPS and backup power systems are the same, except that the times are often in hours instead of days.
How many Watt-Hours in a battery?: Watts are pretty simple - it is just battery voltage times amp-hours. A 12 volt 105 AH battery can supply (under perfect conditions and to 100% discharge) 12 x 105, or 1260 Watt-hours (1.26 kWh).
This is what most people look at. Batteries are the component that are most often skimped on, but can make or break a system. There is a lot of hype about batteries, especially those being sold outside the solar industry - that "deep cycle" marine battery on sale for $39 might seem like a good deal - until you find out that it will probably only last about 2 years. In general battery types run (from cheapest to most expensive): generic golf car (such as those sold at various discount stores), "regular" deep cycle, such as the Trojan's, heavy duty or premium deep cycle, such as the Rolls-Surrette, and then the sealed AGM by Concorde & Deka, and the industrial batteries, such as the Crown forklift batteries.
Price is a fair indicator of the overall expected life when used in a particular application, with the exception of the Concorde AGM's, which are a special purpose sealed battery. Weight of the battery is also something to look at - if one battery rated at 220 amp-hours weighs 60 pounds, and another weighs 68 pounds, that is a pretty good indication that something was skimped on in the lighter battery.
A note here on "free" batteries - they are usually worth about what you pay for them. There are exceptions, such as occasionally stumbling over a pile of 4-year old telephone batteries, but they are few and far between. In fact, we usually do not recommend ANY used batteries - regardless of the price. While they may be "new", as in never used, that does not mean they are any good. If they have been sitting around for months or years at less than full charge, you may be getting nothing but scrap Lead.
This is what really determines the long term cost, but sometimes the up-front costs can be higher than what was expected. There is more to the lifetime cost of a battery system than just the battery costs up front - you have the interconnects, shipping costs, maybe Hydrocaps or Water Miser caps, maintenance, and eventually disposal. Maintenance may not cost money, but it can certainly cost time - if you have an underachieving set of batteries that you have to keep watering constantly, that can get old real fast. As a general rule, the more expensive the battery, the longer it will last. Generic golf car batteries can be expected to last 3 to 5 years, Trojan and similar types around 4 to 6 years, Surrette (depending on type) 8 to 20 years, and industrial 15 to 25 years. AGM's will generally go for 5 to 8 years. There are pro's and con's to both sides - if you are only going to use the system for 5 years, there is not much point to buying 20 year batteries.
The best and/or cheapest battery in the world is not much good if it so heavy that you cannot move it. While the industrial batteries, such as the Crown are the heaviest duty batteries, they are also the heaviest - some can weigh as much as 2,000 pounds for a 12-volt pack. On the other hand you don't want to end up with 50 or 60 small batteries that are a nightmare to wire up and keep maintained. A good rule is to try and keep the total number of batteries between no more than 8 to 12. If you need more capacity, get bigger batteries, not more small ones. If you use Hydrocaps, they cost around $9 each - whether you need 48 for a pile of small batteries, or 12 for a few larger batteries.
Sealed batteries, such as the Concorde, make sense in some applications - but not all. Sealed (AGM) batteries are generally about 2 times as expensive as flooded, and the cycle lifetime is usually similar or a little longer. Nearly all sealed batteries are AGM (Absorbed Glass Mat) or gelled - we are not big fans of gelled.
In areas with no or poor ventilation:
In areas or applications where maintenance is a problem, or is poorly done
Any place that a flooded battery might spill or break and cause damage
Where the batteries are in a spot where it is difficult or time consuming to check water levels
In situations where the batteries must be mounted on their sides to fit
Flooded batteries are fine for most applications. They are nearly always cheaper, and there is better availability of types and sizes. They are the best choice where:
You have adequate ventilation for the batteries.
You do not have to worry about tipping, spillage, or breakage.
The batteries are in an area where maintenance can be performed without too much hassle.
You need a very large battery bank - AGM batteries have a pretty sparse range of choices for large capacity sizes, such as over 250 amp-hours.
This is usually only an issue where some of the "exotic" batteries, such as NiCad, are concerned. But it may also be a concern where large heavy batteries are used that will need a crane or forklift to move.