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Re: Solar charge controller question
If the existing controller is a simple charge controller, there will always be a voltage drop in the regulator, and if that is 0.6 volts, I'd suspect that is perfectly normal and acceptable.
You have 130W of panels and some 2000W of battery, so you certainly go hook up the panels directly for a couple of hours, or a day, and see how that helps.
Diodes can be of different kinds for different purposes. Somewhere, in the panels or in the wiring or in the controller (that's 3 places) there should be one diode which prevents the batteries from discharging back into the panels during the night. If a system was just cobbled together, and there are two diodes (i.e. one in the charge controller plus a second in the wiring into it) then you've got two diodes serving the same purpose--but with two voltage drops, so the extra one in the wiring can and should be removed. There may also be "bypass diodes", one on each panel, so that if one panel is shadowed or fails, the other panel still puts out full power. Those would stay.
A lot of stuff can be "wrong" but still work well enough not to be noticed. Of course once it is 20 years old and the system isn't as robust...could be time for some housekeeping. A sketch of the whole system, showing what is where and accompanied by a description of the bits, could shed some light on all of this too.
Some peeking and poking, i.e. battery condition and capacity testing, separately from the other questions, probably also would help. The whole system has to be questioned, really, in order to figure out which pieces might be misbehaving.