Originally Posted by Herk7769
Even with current you won't see a difference in voltage since the voltage delta would be too small for most home voltmeters. Where you will notice a difference is in battery longevity as one of the 6 volt batteries will be more heavily tasked when providing current and will be charged "ahead" of the other during charging. This unequal treatment will effect the depth of discharge between the two batteries and the number of potential charge/discharge cycles.
Thus the "unbalanced" loading I mentioned previously.
Bank batteries (6 or 12 volt systems) work best when matched by manufacturer, age, cycles, depth of discharge, and "inter-battery/cell resistance".
Actually, that is not really true. The two 6 volt batteries are in series, you cannot charge or discharge one without the same current flowing from/to the other. When you charge them, the same voltage will be applied (and dropped by each battery.) Herk is correct in that if they are different in any way, one battery(or cells within a battery) will potentially drop a shade more or less voltage than the others, resulting in the other series battery (or cells) seeing a more or less voltage across it in order to recharge and hence a slightly lower final SOC and different specific gravity. In actual fact, they will never be exactly the same anyway, even if they are the same manufacturer and exactly the same age and this is equally true of the cells within each battery as well. That is why vendors recommend occasional "equalization" (only when needed) where you up the voltage way beyond the normal recharge voltage (like above 15 volts and carefully monitor the current) in order to equalize the charge state of all the cells and bring the specific gravity of all of the cells to exactly the same level.
Parallel banks are way different. The current will choose to flow through the path of least resistance. Make sure that each parallel battery has EXACTLY the same cables and cable length to the converter and the RV. If they are different you will eventually find one battery being overused and the other "loafing." After a significant time this will result in one battery ageing faster than another and a loss of total bank capacity.
In series installations...the current has no choice! But when comparing #4 to say #2, or #0, at some reasonable but relatively high charge or discharge current (like 100 amps) we aren't talking volts, we are talking millivolts and only a few at that! Even in a parallel bank installation, a short inter battery cable makes little practical difference, as long as it can handle the current that you want from the bank when it is in use. The total resistance and resulting voltage drop is proportional to the length of the cable, and these are only 6" to 1 foot.
Spend more time worrying about how corroded the lugs are on the batteries...now there is a place to drop some voltage and generate significant heat at the same time...and make sure they all have sufficient water to totally cover the plates at all times.