As a first course of action you should consult your local Polaris dealer regarding this situation; however the boiling, over bubbling of batteries or the pooling of electrolyte is something we have heard about from a few Polaris users and does not necessarily mean the charger is not functioning correctly.
The Rangers large parallel string of batteries is designed to allow longer run time. The default charging algorithm of the Ranger charges is #73, chosen in part due to a higher finishing current which keeps charge time down to approximately 8-10 hours for a fully discharged pack( actual charge time varies on a variety of unique factors to each battery set).
The by-product of using the algorithm #73 is the increase in battery maintenance required to monitor water levels and clean of the pooled electrolyte that can collect on top of the batteries. If you wish to reduce the maintenance required we would suggest changing to algorithm #72 as a compromise between charging performance and reducing maintenance frequency this will increase overall charge time slightly. The steps to do so are demonstrated in the YouTube link below.