I believe the Leece-Neville units are insulated so they can be used on either polarity, so the negative terminal will need a ground wire the same size as the one you're using on the + terminal. You should definitely be at 13 volts or more to recharge the batery, going up to about 14.5 as the full-charge capacity of the battery is reached. A fully-charged battery will read about 12.6 volts at rest, with all current draw removed. Parasitic drain, with all circuits turned off, should be 1/10 amp or less.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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