Can I use Li.ion 7.4v 5400mah in place of built in Li.ion 7.4v 1400mah battery of a portable DVD player and use same old charger?
I agree with @vini_i as far as the pitfalls go. At the core of the question though, if you are using a cc/cv charger with a larger battery than it was designed for you will be fine, even possibly with different chemistries, but not all, as long as the voltage is the same, it will just charge slower. I would not use it with a smaller battery however, as it could charge the battery faster than intended, causing undue strain on the cells and perhaps "rapid disassembly" of the battery. It all depends on the charge current that the cells can take and what ratio to the capacity it is for the specific chemistry. There are always outliers however I would conclude that in general what you are trying to do should work, aside from the possible smart components preventing it.
There is theoretically and practically.
Theoretically if the chemistry matches then it should be fine.
Practically there are more pitfalls than one could count. Is the battery smart? If it is smart then the internal communication to the battery may not match. If the charger is smart then it may know that it exceeded the capacity of the battery that it was designed for. This would cause the charger to shut off with the battery half charged. If the internal battery fuel gauge is smart it may be confused by the higher battery capacity. The connectors may not match. The new battery is going to be larger than the old causing space constraint problems.
Thank you very much vini_I ,Winny and Jared Good for sparing time to give valuable answer to my question. Now iam confident, I will connect two new batteries in series duly removing the protection board and taking centre tapping (for balance charging) and connect with the pcb as done with original battery. DVD's charger is 12v, 2A rated and player is having LED indication ON for charging OFF for fully charged. Thank you once again. Any suggestions/remarks thankfully welcomed.