So, will a battery tender completely restore a dead battery on a motorcycle, ATV or a lawn mower? What about a Car? It seems from the manual that it would with some exceptions. This one has not been started in about three weeks.....
Completely? No. Likely to make it usable? Also no.
But if giving it a try anyway, you'll need an actual battery charger that can push say 10A max. Be sure that if the battery needs water, to fill it with distilled water first. Partially dry cells won't take or deliver charge very well.
Completely discharged/dead battery could* be restored again if it is properly/slowly recharged with low Amps, takes time.
AMG/Gell type, will take less time than wet type
*most likely discharged but not defective, i.e. bad cell.
More factors to the ‘what if’? One could be age of the battery itself, previous care, last used, etc..
It would never hurt to hook to a tender for awhile, but can’t expect miracles with a 5+ year old neglected battery.
I use a tender periodically on an assortment of batteries, not normally plugged in all the time. I’ve had many last a good long time.