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.
In actual practice, people put lead acid batteries in parallel and cycle them that way frequently. Just look at RV's and boats and off-grid installations. A fuse for each battery would not be a bad idea. If you are charging them all anyway then what does it matter if one discharges into another?
Remember, electricity flows through parallel or series connections as if it were a single battery. It can't tell the difference. Therefore, you can parallel two sets of batteries that are in series to create a series-parallel setup. First, we recommend putting each set in series first.
Increased Capacity: If you find that a single battery doesn’t meet your power needs, adding another battery in parallel allows for more extended use without recharging. Cost-Effectiveness: Instead of purchasing a single larger battery, using two smaller batteries can be a more economical option.
There are two ways to wire batteries together, parallel and series. The illustration below show how these wiring variations can produce different voltage and amp hour outputs. In the graphics we’ve used sealed lead acid batteries but the concepts of how units are connected is true of all battery types.