I have 2 DeWalt Lithium-ion batteries Apprx. 4 years old, charged 3 times...won't charge now

Its also important if is the batterie 14,4 or 18 V there is an other safety circuit on the batterie that must have a little lower basic dc on it that will referenced on the loader , i have do a short loading 4 amp for about 20 seconds 12 volt DC battery charcher from a car. and then direct disconnect from the connecting wires and place direct in the normal Dewalt li-ion loader then you see its functional only Dewald batteries where made functionally, panasonic ,hitachi bosch or others you can better send the batteries to a service where arre special cadex charchers to get back the deep discharge protection , makita has modificated machines from 2 to 3 connector contacts.also to have a prior deep discharge protection the machine will stops earlier .

Either bad batteries or a bad charger. You can use a multimeter to test for DC voltage output in the charger.