how to charge a 12v 7ah battery?
A lead acid battery which also includes gel cell batteries is fully charged at 14.5 volts plus or minus .1 volt on most meters. A battery held at this voltage on charge for long periods of time build up heat, which is damaging to the battery, as well as evaporating the electrolyte, gel cells are sealed but with a vent that opens to let out the gases produced, also reducing the electrolyte and damaging the battery over time.
If you apply only 13.5 volts (which is not a full charge) the heat produced is much less and is dissipated without damage to the battery, thus you can leave it on this "float"
charge indefinitely without damage and only a small loss in charge capacity
There are chargers that will fully charge the battery to the full 14.5 volts then automatically switch to the float charge of 13.5 v. Then if you leave it a long time if the battery falls below 13.5 volts it will switch back to full charge until it reaches 14.5 v and the switches back. Thus you will get the benefit of having the battery fully charged most of the time you use it.