I have salvaged a lithium-ion battery pack from a laptop.On the case,it is written each battery has 3.7v(they are 18650s),but when I measured using my multimeter I found each battery to be just 2.8v(some were 3.1v and 3.0v). My question is,that is it safe to charge these batteries using normal lithium ion battery chargers(TP4056)? Any answer will be of nice help!
3.7 V is purely the nominal voltage. The working voltage is typically around 3.2 to 4.2 V. 2.8 V is lower than you would like but not throw the cell away low.
They should be trickle charged until the voltage recovers back to the normal range but after that they should be fine. Which a decent charger should do automatically.
Obviously all the the normal caveats about begin careful with unprotected Li-ion cells and charging / discharging them using the correct limits and protections still apply.