My opinion is if the batteries use the same chemistry, are of the same type of manufacture type (flooded, VRLA, AGM, gel) and are hooked in parallel, it doesn't matter if the size, manufacturer or age matches. It does matter when you hook batteries in series. You will hear overheating, mismatched voltage, parasitic draw stories which are universal but only apply to specific cases not represented here. Hooking flooded or AGM batteries in parallel just adds the amount of lead available for the chemical traction. Only the internal resistances of the different batteries will regulate how fast the batteries charge and discharge. I will admit if you mate a half discharged battery with a fully charged, you will loose more water from the fully charged battery, but you in no way will hurt either battery. As soon as both reach the same charge (internal resistance), they will loose water at the same rate (flooded batteries). Of course AGM batteries do not loose water because of the ability of the hydrogen irons to recombine oxygen ions liberated from the plates when the batteries are being charged. This recombination does occur in flooded batteries but at a much slower rate, so some water is lost. The slower you charge the batteries the less water lost. That is why the better chargers monitor battery temperature rather than the cheaper one assume the batteries are at the same temperature as the charger. It is just simple chemistry, not three dimensional fluid flow.
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Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow
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