For example if a 3v and a 9v battery were wired in parallel the output voltage would be 6v 93 divided by 2 however the current will be total amperage of all. Take another set of batteries and wire them together in parallel as well.
Different Batteries In Parallel
Mixing batteries in parallel. Take two batteries and connect them in parallel. Unlike wiring batteries in series when batteries are wired in parallel the voltage does not increase the output voltage is the average voltage of all batteries in the circuit. 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. In a pair of 6v batteries in series the voltages of each are not guaranteed to be the same as they are when wired in parallel. According to safety materials from the oak ridge national laboratory mixing old and new batteries or different brands of batteries that have different performance levels causes the stronger. Mixing batteries in series.
You can connect your load to one of the batteries and it will drain both equally. Negative to negative and positive to positive. 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 capabilities. To join batteries in parallel use a jumper wire to connect both the positive terminals and another jumper wire to connect both the negative terminals of both batteries to each other. Neither is a scenario you want to have happen to your batteries as it will probably shorten the lifespan of both and could create a potentially create a dangerous situation if one battery gets severely overcharged. Its common in many rvs to make use of pairs of 6v deep cycle batteries wired in series.
Finally take a jumper wire and bridge the positive terminal of one parallel connection to the negative terminal of the second parallel battery pair. When you connect two or more batteries that dont charge and discharge at the same rate one battery will probably end up overcharged andor one battery will end up undercharged.