Lead-Acid Batteries

• According to Operating Technical Electronics, it's ordinarily safe to charge a 12-volt (12V) lead-acid battery at a maximum of 13.8V. It's also permissible to charge it at up to 14.4V for no more than eight hours.
  
  

Rechargeables

• The same recommendations typically apply to most types of 12V rechargeable batteries. For extra safety, check the battery label and instructions to see if they indicate a maximum safe voltage.
  
  

Consequences

• Charging beyond the maximum safe voltage, especially for many hours, harms the battery. According to the University of Hawaii ham radio club, excessive voltage can cause batteries to release hydrogen gas and pose a fire hazard. Bubbling is a sign of excessive voltage being applied.