Some useful information and links regarding charging lead acid batteries.

Minimum voltages and preventing ‘gassing’.

• 6V batteries need to stay below 7.1V to avoid gassing, and typical charge voltages are 6.9V (float) to 7.5V (bulk charge).
• As you increase the voltage to get faster charging, the voltage to avoid is the gassing voltage, which limits how high the voltage can go before undesirable chemical reactions take place.
• Charging voltages range between 2.15V per cell (12.9V for a “12V” 6 cell battery) and 2.35V per cell (14.1V for a “12V” 6 cell battery).
• If the battery is not yet fully charged you can use much higher voltages without damage because the charging reaction takes precedence over any over-charge chemical reactions until the battery is fully charged. This is why a battery charger can operate at 14.4 to 15 volts during the bulk-charge phase of the charge cycle.

What the voltage reading means:

A voltage of 13V after charging a 12V lead-acid battery is generally good, but it’s best to let it rest for a few hours or overnight to see the resting voltage. A fully charged battery’s resting voltage should ideally be between 12.6V and 13.0V; if the voltage drops below 12.6V after a rest, the battery may not be fully charged or could be degrading.

• < 12.6V: The battery is not fully charged, or it may have sulfation. 
• 12.6V to 13.0V: A healthy, fully charged battery. 
• Above 13.0V: This can be normal right after charging or if the battery is still warm, but the voltage should decrease over time. 

To get a more accurate reading:

1. Disconnect the charger: Remove the charger from the battery. 
2. Let it rest: Allow the battery to sit for several hours, or preferably overnight. 
3. Measure the resting voltage: After the rest period, measure the voltage with a multimeter. 

If the resting voltage is high (e.g., 13V+): 

• Should be ok if the battery is new and/or was charged aggressively
• The voltage should naturally decrease to 13.0V or less within a day or so.

If the resting voltage is low (e.g., <12.6V):

If the battery is old, this could indicate degradation and a shortened lifespan. Refilling fluids and giving it a little shake to mix and recharging it may be a temporary solution.

Links & References

Reviving a lead–acid battery | SAVAS.CO.UK
https://savas.co.uk/2020/reviving-a-lead-acid-battery/

How to charge all lead acid batteries; how to charge SLA lead acid batteries, a tutorial for engineers about lead acid chargers and charging.
https://www.powerstream.com/SLA.htm

12v lead acid resting voltage question forum discussion/thread
https://www.helifreak.com/showthread.php?t=679157