A battery that is closed but has no valve.
The partly used term "open" battery is misleading, because also a so called "open" battery is closed by plugs or similar. However, it can be opened for maintenance (low-maintenance battery). Maintenance-free closed batteries are closed at the factory and cannot be opened. They usually have a central degassing system or special lids through which only very little gas escapes under normal circumstances. In contrast, the sealed battery (VRLA) is valve-controlled.