Connecting in parallel increases amp hour capacity only. Connecting batteries in parallel increases total current capacity by decreasing total resistance and it also increases overall amp hour capacity.

Why Does A Battery Have A C5 C20 And C100 Ah Rating Can T
Connecting different amp hour batteries in parallel. The basic concept is that when connecting in parallel you add the amp hour ratings of the batteries together but the voltage remains the same. Negative to negative and positive to positive. All batteries in a parallel bank must have the same voltage rating. It sounds like your batteries are different in each of those ways. In parallel you should use diodes to connect the batteries to the ups. In contrast batteries connected in a parallel configuration are able to increase the amp hour capacity of your batteries at the same voltage.
Following this example where there are two 12v 200ah batteries connected in parallel we will therefore have a voltage of 12v volts and a total capacity of 400ah ampere hour. Connecting four amp hour batteries in parallel 4 ampere hour batteries connected in parallel correctly. Connecting batteries in series increases voltage but does not increase overall amp hour capacity. Two 6 volt 45 ah batteries wired in parallel are capable of providing 6 volt 9 amp hours 45 ah 45 ah. 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. The voltage does not change.
You can connect your load to one of the batteries and it will drain both equally. Can i connect my new bluetop d31m in parallel with my one year old other brand marine 29 series. The diodes prevents one battery from chargingdischarging another battery. The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries keeping the same rated voltage. However you can connect batteries of different ah in parallel using diodes. We would strongly discourage anyone from connecting batteries in series or parallel applications if the batteries are not identical in age size and type.
As stated already you should only connect batteries of same typeagebrand in series. A parallel configuration is not meant to allow your batteries to power anything above its standard voltage output but rather it will increase the duration for which it could power an item at such a capacity. In this case 45 ah 45 ah 45 ah 45 ah 18 ah. To calculate the output when wiring in parallel add the ah ratings together.