After doing a little research, it looks like the big thing that I have to watch for is Voc - Volts Open Circuit - how many volts the panel can make in full sun on a cold day.
For my panels, it can be quite high. I already measured 97 volts in full sun in 15 degree F weather.
The trouble is that MOST of the more simple solar charge controllers for 48V battery packs CAN'T handle that high of a voltage.
The MPPT Chargers CAN handle that - usually up to 150V.
There are a few NON-MPPT that can also handle a higher Voc from the panels.
The
Xantrex C40 can handle up to 125 Voc, which would be enough for single panels run in parallel.
It is my understanding that plain charge controllers basically "throw-away" any extra voltage that you have. So, that extra "headroom" of voltage I have with the 67V panels isn't going anywhere. The panels would put out 67V at one amp, but only something like 58V at one amp would be going to charge the batteries.
A MPPT charger would take advantage of that extra voltage to convert to a lower voltage but at higher amperage - thus a higher amp charger for the same amount of PV panels and sun! Yea!
The only products I have seen so far that handle really high voltage (5 or 600V) are grid tie inverters. I would LOVE to just plug one panel to the next to the next in series. It would make the wiring SO much easier.
But I don't want an inverter really - I just want a way to charge batteries. So, it looks like I need to run each panel individually to a combiner box, and then THAT into a solar charge controller.
You know what would be really cool? A 600V to 48V DC to DC converter. That would solve all my problems. Any got one of those kicking around?
Another thing that I did notice about the MPPT chargers is, although they are more expensive, they usually have some sort of built-in monitoring, and aux 12V (to activate a fan, alarm, etc.) and some other built-in features.
The less-expensive NON-MPPT chargers tend to be very basic, adding a display, etc, is optional and costs extra.