>However, I don't think it's true that the charge controller is just a cheap way to improve efficiency; it's actually necessary to keep the battery from exploding like a firebomb when you try to charge it.
You only need to worry about it when you're riding near the edge of what the batteries can handle. If your solar array outputs 4 volts and needs a minimum of 6 hours to mostly-charge the batteries, and your batteries are in a nice cool basement, I'm pretty sure you can just hook them all together. It wouldn't be ideal but I don't think you'd have any real fire hazard.
You only need to worry about it when you're riding near the edge of what the batteries can handle. If your solar array outputs 4 volts and needs a minimum of 6 hours to mostly-charge the batteries, and your batteries are in a nice cool basement, I'm pretty sure you can just hook them all together. It wouldn't be ideal but I don't think you'd have any real fire hazard.