There's plenty of fake batteries out there. Batteries of course are the weakest component of a solar system. I've bought quite a few solar kits from Lazada and the batteries are always the first to go. Sometimes very quickly, sometimes they last a year or so.
Being annoyed with this l a) taught myself how to replace a battery in these kits and b) bought a battery capacitance tester. One of the best investments l've made; only about 200baht for a basic tester. This is all you need. My first battery test showed a new 18650 battery rated at 9800mah had an actual capacitance of 980mah