Features and Benefits: Hydrometer immediately and accurately tests battery life whether the battery is hot or cold Help extend battery life and detects overcharge Assists in preventing dead batteries Works on all 6-12-24 volt systems Designed to help to get your job done right Hydrometer immediately and accurately tests battery life whether battery is hot or cold. Helps extend battery life and detects overcharge. Assists in preventing dead batteries. Works on all 6- 12- and 24-volt systems.