In theory it is OK to connect them in parallel with two conditions: Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged.
Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged. The power supply is capable of maintaining the fixed float voltage.
There is series-parallel connected batteries. Series-parallel connection is when you connect a string of batteries to increase both the voltage and capacity of the battery system. For example you can connect six 6V 100Ah batteries together to give you a 24V 200Ah battery, this is achieved by configuring two strings of four batteries.
Connecting a battery in parallel is when you connect two or more batteries together to increase the amp-hour capacity, with a parallel battery connection the capacity will increase, however the battery voltage will remain the same. For example if you connect four 12V 100Ah batteries you would get a 12V 400Ah battery system.
There are three ways to connect your lead acid batteries—parallel, series, and a combination known as series/parallel. We cover each of these battery configurations in greater detail in our Battery Basics tutorial section of the site should you want to delve in a little deeper or reinforce what you already know.
Connecting 12V batteries in series will increase the voltage of the battery bank while keeping the amp-hour capacity the same. Connecting 12V batteries in parallel will increase the amp-hour capacity of the battery bank while keeping the voltage the same.