The battery is "sulfated"- - - - -there is a thin layer of crystallized lead sulfate on the surface of the plates that must be broken down before the current will flow to rejuvenate the acid. Hit the battery with the "12 volt boost" setting on a commercial charger for a few minutes to force-charge the battery for about 15 minutes, then drop the charge rate down to about 10-15 amps overnight. Put a load on the battery such as a sealed beam headlight bulb and totally discharge it. Repeat that charge/discharge cycle several times. Eventually, the sulfate crystals will bvreak down and the battery will take and keep a charge. It usually takes a week or more of this process to reclaim a sulfated battery. A way to fool a "smart" battery charger into working with a totally dead battery is to connect two batteries in parallel- - - -one with a little charge in it. The charger will top off both of them.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!