Eventually the battery will fail, and no amount of charging it will help.
I suggest you save your money, and put it towards a new battery which you will need before long.
A new battery is quite cheap compared with the cost and hassle when it dies on you at the worst moment (which it will)
If you make a few enquiries while the pressure is off about where to get one at the best price, and fit it yourself, it might surprise you how cheaply you can do it.
Consider doing it soon anyway, for peace of mind.