Batteries are, by far, the most expensive part of any solar system. I can say this because I have bought MANY batteries since going off grid, and of all the equipment I have purchased, these things are the big, heavy, ugly, dangerous, expensive downside of alternative energy! The first thing you will need to know is the different types of batteries for solar.
AGM batteries, or Absorbed Glass Mat, are the non spillable type that you find (normally) as small 10 amp hour alarm system back-up batteries.
This type of battery is also available in larger capacities for solar applications, and have advantages, such as a bit more available energy per pound of battery, and are safer because there is no electrolyte (battery acid) to deal with, there are also disadvantages such as cost, and, for some, the fact that they are ‘maintenance free’ means that once they are done, there is not much you can do.
For me, the biggest disadvantage to these is that they are hyper sensitive to over charging and over drain. Stories of battery banks costing thousands of dollars going bad because of a simple mistake in charging or discharging them are all over the internet.
Gel type batteries are another option, and unlike AGM, are a bit more forgiving as far as ruggedness goes, and are a great option for solar, or other alternative energy setup. The issue with gel cells is that they are SO over the top expensive, for most applications, they are just to expensive. The cost is coming down, but as with all things that are new technology, you pay a premium for it. For me, the extra cost at this point does not justify the advantages. I would rather get MORE of a less expensive battery for capacity, but, you’re mileage may very due to size restraints, budget, etc.
Then, there is the lead acid battery. While this is what I use, and they are the most popular as far as solar systems go, they are not created equal. The WORST type of battery you can use for alternative energy applications is the standard car battery. They will ‘work’, and some will argue that given the cost and availability, they are the best for someone just starting out, and in some limited way, I agree.
For someone in a garage that just wants to tinker with a 100 watt inverter to run a couple lights and that’s all they will ever want out of their system, I would say yes, grab any battery (A deep cycle if you can get you’re hands on one), a cheap inverter and panel hooked into a controller (usually available as a set) will do just fine, and for a couple hundred bucks, you have a working system.
However, if you think that you may ever want to expand you’re system, I would recommend cutting costs someplace else. My advise would be to go for the cheap panel/controller/inverter set on you’re favorite auction site (you can find them for $100), and save up for 2 6 volt golf cart batteries!
A typical golf cart battery costs about the same as a 12 volt car or deep cycle battery. The problem is, that to get to the 12 volts to run you’re inverter, you need 2 of them. This doubles the cost right up front, and if you’re anything like me, this seems like a waste of money at first. Even after reading posts and reviews from experts on solar systems, I fell into this trap myself! After I bought my first set of golf cart batteries though, I could run for SO much longer and these batteries are designed for this type of use, so, at double the cost, I got 4 times the capacity. However, like I said, I will never be one to say a setup is ‘wrong’ just because of a personal preference or budget limitation.
However, another trap that I (and a lot of newbies) fall into is to MIX batteries. So, the thinking is/was that since I have 2 or 4 golf cart batteries, then why not hook the 2 deep cycle batteries into the system as well and get the capacity boost from them? If you read any post by someone that uses solar, they will tell you not to do this, and why. I found out (the hard way) that this is TRUE! While it may seem perfectly logical to try to squeeze out an extra 100 amp hours, the fact is that you do not get any extra capacity, and at the same time, you lower the life of all of the batteries.
The reason is that if you have 10 awesome, expensive Trojan 6 volt batteries and hook them to 2 12 volt car batteries, the entire bank will only run at the capacity of the 2 car batteries AT BEST, and this effect will worsen over time. The 2 car batteries will drag down the 10 good batteries to the point where you will have 1000 amp hours according to the labels but only get 300-600 actual amp hours. This truly IS a thing and one of the most frustrating things you can do.
In my research, even 2 battery banks of great quality but different types, sizes, and even (to a lesser degree) brands, will have this effect. I have 4 Rolls Surrette 6 volt S-480s as a bank. I also have 6 Trojan J185H-ACs. These are top end batteries, and even hooking these together, you would think they would be fine. They are not! If I tie them together, I actually get LESS capacity than ONE of the banks by its self!
I know this is a long section, but I really want you to get the most from you’re system, and for me, the hardest part is storing the energy.