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[What is] the make and type of battery charger used in 2005 Guardian 25?

Are you certain the charger was delivered with the boat from Boston Whaler?

What is the HIN on the boat? If it starts with BWC, call Boston Whaler customer service and inquire with them about the charger.

If the HIN starts WCG, call Brunswick Commercial and Government Products customer service.

Usually any electrical device has external markings that give information. A close visual inspection of the device may reveal a model designator and a manufacturer's name.

All Guardians were custom built to each customer's specifications. This included electrical components.

Rather than try to replicate a part installed 14 years ago, purchase a readily available part via a chandlery that serves the commercial marine operators.