[philb]

The hydrometer is about $10 at the auto store. That's the first thing I'd check too.

It seems to me that you could hook the panels directly into the batteries if you have an open circuit voltage that's less than 30 volts. Good batteries will clamp the voltage down to 12.5 to 14 volts. Remove the connection occasionally, let the batteries settle down for a few minutes and check volts. Maybe you could save your batteries with a bulk charge if the controller is at fault.

Check the batteries case temperature to find out if one is warmer than the rest. The warm one is the one that could be bad. Take it out of the circuit and see if the other ones will charge properly.

If the batteries are 5 years old, they may have bit the dust and need replacing.