Charging a 6V Pos ground while in machine.

I disconnected the battery + to ground cable. Do I need to disconnect both battery cables in order to charge the battery normally ?

I've got a small 6/12 V charger but when I hook it up, the "Polarity Warning" light on the charger comes on.

Thanks in advance.

As a precaution, I always set the charger on a wooden or plastic table or chair or similar stand, or even a plastic bucket with a lid,sitting next to the tractor, rather than on the tractor itself, I have run across a charger or two over the years, usually older units, which have the (-) in common with the charger case. With one of these chargers, there's a potential for a short between the case of the charger, and metal parts of the tractor. While it's unlikely that there's many of these old units around anymore, keeping the charger from sitting directly on the metal of a POSITIVE GROUND tractor is a simple safety precaution.

I usually disconnect the ground cable anyway (safety for kids and prevents slow drain on battery).

Thanks.