take out battery to get battery internal Resistance (BIR):
clip one of its terminal/pole the easier one, together with an end of R ~2-3 Ohm and one of the MM probe correctly by use of an alligator clip
the other R end is gathered with the other one of the MM probe correctly (+ red ,- black), touch it to the other battery end and read V on MM and write down!
subtract this V from the MM-read battery's V
divide the result by this V / R whatever R is (well.: ~2-3 Ohm)
so:
(V batt. - V on R ) / (V on R / R)
if this BIR is greater ~ 0.5 - 0.7 Ohm it's time to replace
Read otherwise the charger output current (read it when a batt end on a charger pole the other to the MM probe the other probe into other charger pole) and V, it must get degraded, V =< batt. nominam V, I < .5A and needs caps or else comp. to replace