What can be the consequences of charging a lead-acid battery with an excessive voltage?
This information can be found at various lead acid battery manufacturers, such as Power Sonic. Here is an excerpt from their FAQ:

Q: Does overcharging damage batteries?
A: As a result of too high a charge voltage excessive current will flow into the battery, after the battery has reached full charge. This will cause decomposition of the water in the electrolyte and premature aging. At high rates of overcharge a battery will progressively heat up. As it gets hotter it will accept more current, heating up even further. This is called thermal runaway and it can destroy a battery in as little as a few hours. For further information about charging please refer to our Technical Manual pages 12-19. To access our Technical Manual please go to the Literature - SLA Batteries section of our website