This is a plug for a battery tender what is the top terminal use for.
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the tenders cant hook up backwards. on the batt/vehicle side the hot post is insulated,on the charger side its open. that way its isolated on the hot side for the battery.If it is on the 12V (or DC) side I am guessing that it is there as an alignment pin so that you cannot hook it up backwards.
I see what you mean. Kind of hard to tell from the view in the picture. I have an onboard charger on my boat and it is just wired directly to the battery with an AC 110V plug-in through the forward bulkhead. All I do is plug it in to and extension cord.the tenders cant hook up backwards. on the batt/vehicle side the hot post is insulated,on the charger side its open. that way its isolated on the hot side for the battery.
kinda like a trailer plug on your truck. all the hots are covered on the power side and open on the trailer side.
checked my supplier and it looks like there is a new model tender. its a 5amp model. works just fine with the 2.5 standard model. this is just another option and the third wire is used when "faster" charging is needed ,unhook it for standard storage..