Negative to negative and positive to positive. As stated already you should only connect batteries of same typeagebrand in series.
Battery Wiring
Can you connect different amp hour batteries in parallel. The diodes prevents one battery from chargingdischarging another battery. They can have different capacities on account of size or age but the same chemistry eg. However you can connect batteries of different ah in parallel using diodes. You can connect two different ones with no damage to either. That can be true even if the batteries are the same size. The amp hour ratings tell you the capacity of the battery.
How battery charging works with a parallel battery bank. 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. In parallel you should use diodes to connect the batteries to the ups. 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. 27 year in the auto business. Connecting batteries of different amp hour capacities in parallel this is possible and wont cause any major issues but it is important to note some potential issues.
Connecting two battery banks of different amp hour capacity together in series is a bad idea. All flooded lead acid or all agm. Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Different types of batteries flooded or agm also can have different chargedischarge characteristics. You can connect your load to one of the batteries and it will drain both equally. Lets suppose you have 3 different 12v batteries wired in parallel to supply 12v power to your rv.
Check your battery chemistries sealed lead acid batteries for example have different charge points than flooded lead acid units.
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