VSR (Voltage Sensitive Relay) allows two batteries to be charged at same timeEliminates possibility of draining the start battery and protects sensitive electronic equipment from start-up spikesRed LED indicates combined charge mode12V DCEngages: 13.7V DCDisengages: 12.8V DC2.75" x 2.75" x 2"D (69 x 69 x 50 mm)