[ Note: this article was written by secessus about the type of controller seen on the right. They are called CMTD among other names. ]

A configurable shunt controller can be a very good deal, sometimes less than $10 shipped.

There are three setpoints that interest us: charging setpoint, low voltage disconnect, and low voltage reconnect.

The charging setpoint, called PV OFF on the screen, is the highest voltage you want your battery to see. There are no separate Absorption voltage (Vabs) or Float voltage (Vfloat) settings, just the one PV OFF setpoint. So you have to pick a compromise. There are a couple reasons to use a higher-than-usual voltage: the battery will charge faster and the controller will be able to run the panels at a higher voltage where they make more power (closer to Vmp). This means you can also run bigger loads without affecting the battery in addition to just charging faster.

So if you are deep cyling the batteries daily and can check water levels I would use something quite high like 14.7v or as high as your battery manufacturer allows for Absorption. Check your water levels! If you are deep cycling but can't check water levels (maintenance free or AGM batts) I would use something like 14.2v. If you are using the battery each night but not deeply, you might set it to a high float-like voltage like 13.8v. If the batteries are just being maintained and not used I would drop to a normal 13.2v float voltage.

Low Voltage Disconnect, called LOAD OFF on the screen, should be set no lower than 12.2v for actual deep cycle batteries. If you are using “marine” or wally world 12v “deep cycle” batteries 14.4v will keep the battery in a healthier 25% depth of discharge zone.

Low Voltage reconnect, called LOAD ON on the screen, should be set high enough that the panel is making meaningful power before the load is applied again. Maybe 13.4v, I'd say no lower than 13v.