Is there any way to renew a battery cell that is weak?

Nothing that really works every time. The usual reason a cell goes bad is sulfation. That is a buildup of sulfates generated by the acid's interaction with the lead plates in the battery. There are chargers that "fix" this but all they actually do is dump a high currant into the cell and in effect that burns the sulfate connections apart. It might extend the life of a battery some but how long is a guess. If the sulphates vibrate around and short the cell again, you lose that cell. I have heard of people dumping out a cell and putting in new electrolyte, however that isn't a good idea as most newer batteries don't use a simple water and acid mix any longer. They use a special blend that helps the battery perform better. That falls into the same category as putting an aspirin in the battery or adding acetone to the fuel. Has it actually worked? Maybe, is there real proof that it does, nope. Overall, the best solution when you have a failing cell is to replace the battery. If you don't want a new one, find a salvage yard. When cars come in one of the first items removed is the battery, they then put a charger on it and test it. If it passes the testing it goes on the sale rack, commonly for much less than a new one.

Nothing I have tried off the list of tricks has actually worked for more than a couple days. Usually once a single cell starts to fail they tend to cause problems because the alternator is still trying to charge that cell and it starts to overcharge the other cells. The big thing to ask is, why is only one cell low? They all get charged at the same time, so it's more likely to be a mechanical issue in the cell, nothing chemical will change that.

If it is starting the vehicle take it to a place that does free battery testing, they can put a machine on it that will tell you if the battery is good or not. No guessing that way.