Battery defective?

Hello, especially in the cold my small compact trecker is hard to start. It has a 1l 3 cylinder diesel and glows about 15 seconds ahead. Then the battery has a voltage of 10.5V and can of course not start the engine any more. Sounds like defective battery. However, this is a dry charged John Deere (47Ah, cold start current (CCA) 500), which was produced only 4 years ago. Level of the cells is OK. Before winter I had no problems, but have the trecker the battery is charged with an Aldi charger, if the tractor is rarely and briefly used. Is it due to the battery, the Aldi charger or the Trecker?