Agree with BamaRambler...but suggest you inspect water level closely before adding distilled...IF you see the metal plates don't have water over the top of them...chances are you damaged or killed the batts.
If the plates are covered...add the distilled water and charge by plugging in somewhere for at least 12 hours. THEN remove the negative cable from the batt for 24 hours.
Then take a voltmeter measurement at the battery terminals...12.6V would be great...12.4 not so much (loss of capacity) ...anything under 12.2 is a candidate for immediate replacement.
NOTE...you MUST disconnect for 24 hours to get an accurate reading.
Alteratively...you can get a hydrometer (turkey baster) from the auto parts store pretty cheaply and test each cell in each battery after filling & fully charging which is the preferred way.
3rd Alternative...buy a $20 load tester at Harbor Freight an test under load after fully charging.
All three methods work just fine.
If you find a problem with the battery..then you have to determine if it was murdered by neglect or a bad charger. If the latter...you'll soon have the same issue with a new battery. Easiest way with the new battery in place...is to plug in somewhere and measure voltage AT THE BATTERY TEMINALS. It it is between 13.2V and 14.5 volts...all is well INITIALLY. Charge for 4-6 hours and you should then have a reading not in excess of 13.6V.
If not...you might wanna have someone look at your converter/charger.