Lead-Acid battery discharged to 5V - how much damage?
When a battery sits for a length of time in a discharged state that's when the plates will suffer the most damage, the lower the voltage the worse it gets of course. if the discharge is very brief and is promptly recharged, then damage will be minimized or avoided. Keeping a battery on a float charger when it is very rarely used is a good way to keep it from deteriorating. I still try a high frequency pulse charger in the hope that it will bring back a weak battery, but I have not experienced any real positive results with this.