The most important thing here, in my experience, is understanding the batteries.
The panels and inverters are cheap - and very difficult to damage. Batteries on the other hand are fragile and can easily eat up all your money if you make a mistake.
Lead Acid batteries will only last for a few weeks before they are damaged. If you understand, protect and care for them properly, they can last many years, even up to 10 years if you do everything right.
The key is to understand that a battery is just one of many chemical reactions - and that depends on a very specific voltage amperage. If the voltage or amperage is too high, or too low, then a different chemical reaction happens, which usually destroys the battery. This is why we talk about battery chemistry. This is “Lead Acid” or “Lead Zinc Acid” or “Lithium Ion” or “Lithium Iron Phosphate” or “Lithium Cobalt Oxide” or “Glass Sulfide” or many more. Each is a different chemical reaction. If you are interested in inventing new battery types, read about electronegativity.
The two most common battery types we commonly have access to, are “Lead Acid” variants, and “Lithium Ion” variants. The “Lead Acid” chemistry is almost 200 years old, and require the most care. The much newer “Lithium Ion” variants too, but you usually don’t have to worry about this because a “BMS” or “battery management system” is built into the battery, to protect it.
So I will focus on the Lead Acid variants here. A 12V battery has 6 cells internally, connected in series, each of about 2.12 Volts. Because they are connected in series, the voltages adds up to usually around 12.7 Volts. There are many variants and each reads a little bit more or less, but always under 1V difference. You can usually find out the chemistry’s values by consulting the specification sheet, which you can request from the manufacturer (or find on their homepage) or if you are lucky it will be printed on a label on the battery itself. If a good battery that measures 12.7 Volts is left unconnected on a shelf, it will slowly decrease in voltage over 2-3 years.
If the voltage of a Lead Acid battery never drops below 11.7 volts, it can get cycled over 1000 times. That means that in a solar set-up where it is cycled every night, it could last over 3 years, and you can extend the lifetime up to 10 years, maybe even more if it never gets to this point. By contrast, many charge controllers and inverters will continue draining the battery until 10.5V. Cycled to this depth, it will not even last 2 years and you will have to replace them much sooner.
This is the first problem with Lead Acid batteries. The second problem is that the rating of the battery is based on only cycling them down to 11.7V. This means that if the battery says 100Ah, and if you want it to last, you should only think of it as a 20Ah battery. To get a comparative rating in Watt-Hours (Wh) you can multiply the Ah rating by the Voltage. So instead of a 100Ah 12V battery being 1200Wh, you should think of it as closer to 240Wh.
Because of the built in BMS, Lithium Ion batteries don’t have these problems. Even the cheapest Lithium Ion batteries last about double as long - and good ones easily last more than 4 times as long. If you work out the long term cost of batteries, it should always be more effective to use Lithium Ion batteries.
As an example: To run a repeater tower you usually need 400Ah x 12V ~ 4800Wh of Lead Acid batteries. This would cost you around US $400. And you will have to replace these batteries after 4 years under ideal conditions. So over the course of 10 years, the cost of batteries for this site will average to $1000, excluding the costs of actually going out and replacing them. By contrast, you would only need 960Wh of Lithium Ion batteries, and this would cost $600.
The problem with comparing costs, is that if you looked 4800Wh of Lithium Ion batteries, it would cost $3000. Which is a lot more and makes it look much more expensive. But in reality you only need to spend $600. Which is more than the up front price of $400 for the Lead Acid batteries - but over the course of time, the Lead Acid batteries would cost $1000 whereas the Lithium Ion batteries would in reality cost almost half that at $600.
The only legitimate excuse you would have for using Lead Acid batteries, is if you got them donated, or you got them at 50% discount ie. around $50 per 1000Wh or if you made them yourself.
The main reason for this is because the label rating on Lead Acid batteries is misleading. I would go so far as to call it a lie. Of course you can also buy Lithium Ion batteries that are not what it says on the label. A good way to know is to check the weight of the batteries, and to actually test them, or buy the same batteries that someone else has tested.
Hope this helps. I have found some good resources on batteries on the internet, and have learned much of the above the hard way, by spending too much money on lead acid batteries. I will share the links when I manage.
So, if I had to prioritize the important points here:
- Use Lithium Ion batteries because they are cheaper in the long run - and you don’t need as many Watt-Hours as with the label rating on Lead Acid batteries.
- If you can’t, the most important thing is battery protection. - and to set the cut off voltage above 11.5 V, ideally 11.7 V. And rather than be tempted to set it lower, just start with more batteries.
- If you use Lead Acid batteries, prefer to connect them in parallel, because it is more difficult to damage them like that. (If you put them in series, and one of them gets damaged, or is weaker, then it will get damaged when you charge it, because it will have a higher internal resistance, so it will have a higher voltage across it, which will cause the wrong type of chemical reaction to take place.)