I have a 12v / 5A battery nobreak gel that I already left connected with a 1N5402 + diode one bulb in series to the battery terminals for a few hours but the battery is not holding the charge. Is there any method to save or recover the battery?

 

Not really. Contrary to what so many people think, batteries are a consumable item that wear out or become damaged and need replacing from time to time.

 

Throw it.

 

Deep cycles that are filled with liquid sulfuric acid can sometimes be emptied out and rinsed to remove sediments and be placed back in service. These are heavy 100 plus pound industrial use batteries. Glass mat and gel type sealed batteries are not designed for such maintenance. They are use, abuse, then lose. Buy another.

 

The closest I have ever come to getting any reasonable recovery from a LA battery has been with larger automotive and deep cycle type batteries by letting them deep bulk charge soak in a slightly overcharging rate (~ 15 - 15.5 volts @ 1 - 2 amps) for several days to a week to regenerate some of the sulfation back into workable lead on the plates.

It's not a permanent fix and most(if they recover at all) still go back to self discharging or having very low capacity in a few weeks to a few months but for non critical applications (seasonal or low use equipment batteries), before becoming core exchange units for new batteries, they are good enough.

For small batteries like in a riding lawn mower or UPS unit I just buy new ones being having a questionable battery in a more critical application is just not worth the hassle of them going bad at the least convenient time they can (which they always do) for the their overall cost.