The CTEK line of lead acid automotive battery chargers is built around a comparatively high quality interconnect between the charger cord and the battery.  Ctek uses a quality socket for this purpose with an o-ring gasket to prevent water infiltration and a rubber cover that keeps foreign matter out.  This is in stark contrast to other battery charger systems with rings or alligator clips which use just a 2 terminal plug, one being insulated and one being exposed metal.  After one season of weather these tend to corrode due to the 12V between them and the ability for water to get into the connector without any gaskets.Fortunately CTEK adopted their quality gasketed interface across all products, and included some unique features along the way.  All chargers use one gender and all batteries or things to be charged use a the opposite gender with a rubber cap to protect it when the charger is disconnected.  The interface features a snap to hold the two halves together .For connecting to the battery they make 6, 8 and 10 mm rings, alligator clips, and 2 very special connectors, this being one of them.  The second special connector is theCTEK 56-380 Comfort Indicator Panel.  Both this input which terminates in 8mm rings and the 56-380 which snaps into most import dash DIN outlets providing you with a convenient place to plug in your CTEK charger (any model) include a blinking (about 1 blink/second) red-yellow-green LED which indicates the present battery level.How do they do it?  For Lead Acid (and also other lead chemistry including SLA, lead - calcium, and optima batteries) the standardized test is to charge a battery and desulphate if needed (CTEK has a patented charging system which includes desulphating pulses).  Second, the battery is allowed to rest for 24 hours with no charging and no load on the cell.  After 24 hours resting a voltage measurement is made which determines how "full" the battery is.  The simple rule of thumb (and this is by no means perfect) is that 12.00VDC means the battery is dead while 12.80 means the battery is 100% charged.  The scale between 12.00 and 12.80 is roughly linear allowing the observer to determine the state of charge to a rough degree of certainty assuming a linear relationship.  12.40 volts after 24 hours of non-use would indicate the battery is 50% charged and would blink the yellow LED on the 3 LED display. Over 12.5V usually blinks the green LED and below 12.3V blinks the Red indicating you would be wise to charge now.The only difference in this part of the CTEK system and the one I purchased is the form factor.  This accessory does everything mine does but it is for battery systems without a din standard dash (where you would use the 56-380 panel mount charge jack and indicator) - here the indicator and charge jack are in a "blob" at the end of  55 cm of cable designed to be wired to the battery.  All CTEK features and quality are designed into this accessory.