What you are seeing is a natural variance between battery cells. It should not cause you a problem in most applications, but for very critical applications it is important to "balance" the batteries for voltage and discharge capacity. The only way to do this is to discharge the battery to measure it directly. For the critical applications you match the cells with similar performance.

You might see a problem ONLY if you run these slightly mismatched batteries completely flat. In that case, the battery with the lowest AH capacity will go flat first while the others still supply power and that can reverse charge and damage the flat (0.0 VDC)cell. This should not be a problem if the UPS stops working at something on the order of 40 VDC and that will make sure that none of the cells are reverse charged. But, if the UPS can run until the batteries quit, then battery damage is a possiblility.

BTW, a fully discharged cell does not mean 0 VDC, it means something on the order of 1.65 volts per cell. Each 12 V battery has 6 cells and should not go below about 9.8 VDC at full discharge. If you take a battery below this voltage it can be damaged. Using balanced batteries will assure that you don't damage one battery of the four you have in series if you do a full discharge on occasion.

Running batteries in parallel to get more AH capacity is generally not a problem. The parallel batteries will accept or discharge current based on their voltages and will tend to balance each other out. So, if you have two batteries in parallel and battery 1 has a higher voltage than battery 2, it will provide the most current when required and take the least current upon charging until the other cell (battery 2) catches up. This process will automatically go back and forth during charge and discharge and you don't have to monitor it. Any imbalance will also correct itself as the higher voltage battery (1) charges the lower voltage battery (#2) until they are the same voltage and current stops flowing between the two.

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