That is a question I get asked all the time. Most people don't know how to tell a good brand of battery from a cheapie, so what is the difference? why is one selling on ebay for $25 while others sell for $50.
Let me explain. Lead acid batteries vary in price because the weight varies. That is the amount of lead in the battery. Less lead, the cheaper the battery. Good quality 12v 9Ah batteries should weigh around 2.6kgs, a cheap one would weigh 2.1kgs so it will have a shorter life..If your system has 12v 7Ah batteries, replace them with the 9Ah model, they are the same size and store more power.. When you are looking at new batteries and your system has two batteries wired in series to make 24v, make sure you replace them as a pair. If one battery is not holding charge so you replace that battery only, then the batteries are different in performance and quality and the batteries will not last that long because one battery will try and control the other battery, so they both fail early.. Always replace as a pair and some kits have the batteries laying down not standing up with the terminals at the top, so just make sure you buy the batteries that are OK laying down as not all are, but if standing up all is good for any of them. Check your spade connections for clean fittings and good wires.
If your system has a separate solar charge controller, then you can look at the read out to see the solar panel output, (usually the default screen,) then you can hit the program mode and it will tell you the battery voltage so you actually have your own built in voltage tester. If you have a system that has a built in charge controller on the control board like the Ahouse system, , you can't tell anything like that. so you would need to use a volt meter.
A new product to use is our new 24v 10Ah lithium batteries. They are expensive at $250 but you only need one because they are 24v and they have a much longer life, usually around 6-8 years, where acid batteries you need two at $50 each, ($100) and they usually last around 2 years, so our lithium batteries are a good long tern fix, and they are smaller that the two acid batteries. so plenty of extra room for phone apps etc.
These batteries are suitable for swing, slide or articulated style systems that are 24volt. We also have a 12v 12Ah lithium battery for your 12v system $149
So choose your batteries well, or you will have to keep replacing them and throwing your money away.