CTEK D250S Dual DC to DC Battery Charger with Variable Charge Relay
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CTEK D250S Dual DC to DC Battery Charger with Variable Charge Relay
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2 Year Warranty

This charger is identical to the D250S Dual 12V DC-DC 20Amp Battery Charger, with the optional Variable Charger Relay included.  See D250S Dual 12V DC-DC Battery Charger for details on the charger itself.

Variable Charge Relay
If your vehicle is a four wheel drive and was manufactured from mid 2011 on, there is a good chance it is installed with a variable charge alternator also called a "eco charge" or "smart alternator".  The problem with this type of alternator when used with the D250SDual is when you first start your vehicle, the alternator would charge your start battery at normal rates approx 13.6v but once the start battery is fully charged the alternator then backs off the voltage down as low as 12.2v. The problem is dc chargers generally needed 12.7v to turn the charger on. So once the start battery was fully charged the dc charger would then turn off.

With the addition of a 30amp auto relay (as shown in the diagram) you can now keep on using the Ctek D250S dual with your variable charge alternator vehicle. The relay when voltage is above 12.7v allows charging via the alternator input but once voltage drops below 12.7v it closes the relay sending the alternator current into the solar input of the Ctek D250S which will operate at lower voltages then the alternator input. The price includes both the Ctek D250S Dual as well as the 30amp relay. Simply wire the relay in line as shown in the diagram and let the D250S Dual do the rest.