We would strongly discourage anyone from connecting batteries in series or parallel applications if the batteries are not identical in age size and type. For example the bmpro invicta lithium batteries are capable of being installed in parallel with up to 4 batteries.
Lead Acid Battery Production Model
Lead acid batteries in parallel. Each battery must be in a state where it can be voltage charged. As per good practice with lead acid setups all batteries should be of the same brand size and age. With no resistance to slow this charging process the charged units can overheat as they rapidly drain and the discharged battery can overheat as it attempts to charge at well above its design. Batteries linked in series cannot be charged in the same way as batteries linked in parallel and different numbers of batteries may require different types of charger. How battery charging works with a parallel battery bank. This is fine for lead acid batteries unless they are very run down.
Or connect 2 old batteries with 2 brand spanking new batteries. In order to balance a battery pack composed of multiple batteries in series you need to wire them all up in parallel like this. Check with your battery manufacturer first. Lets suppose you have 3 different 12v batteries wired in parallel to supply 12v power to your rv. In theory it is ok to connect them in parallel with two conditions. Or connect a group 24 with a group 27 and group 31 sized battery.
If you connect rechargeable batteries in parallel and one is discharged while the others are charged the charged batteries will attempt to charge the discharged battery. Not all lithium batteries are created equal especially cheaper batteries. 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. Balancing lead acid batteries i immediately disconnected the charger and disassembled the battery pack. For example you would not connect a deep cycle battery with a starting battery. Wiring batteries in parallel in order to balance them.
With a basic understanding of the difference between series and parallel connections anyone equipped with the right battery charger can safely charge multiple lead acid batteries. All flooded lead acid or all agm. Can i connect my new bluetop d31m in parallel with my one year old other brand marine 29 series. We assume when you plan to connect your batteries in parallel you are using the same type age and size of batteries. For secondary rechargeable batteries the stronger battery would charge the weaker one draining itself and wasting energy. They can have different capacities on account of size or age but the same chemistry eg.
It sounds like your batteries are different in each of those ways.