Tweaking a configurable shunt controller

[ 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 (single stage) controller can be a very good deal, sometimes less than $10 shipped. While they share the same lower-yield issues with PWM and for the same reason¹⁾ they can be tweaked for better performance.

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 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 much or even at all.

So if you are deep cycling lead batteries daily one can use the highest charging voltage (Absorption voltage, “boost” voltage) listed by the battery manufacturer.²⁾ It might be ~14.4v for AGM, ~14.2v for GEL and 14.7v for FLA. This approach works because lead batteries are often underabsorped and sunlight has a limited duration. Those with flooded lead-acid remember to check your water levels!

LiFePO4 is the opposite: unaffected by partial states of charge and does not like to be held at higher voltages. Something like 13.6v would charge somewhere in the upper half of SoC without overcharging. 13.8v would yield gentle overcharge if desired. You will have to observe-and-correct to see how your particular setup behaves.

Low Voltage Disconnect, called LOAD OFF on the screen, should be set 12.1v-12.2v for lead deep cycle batteries. If you are using “marine” or wally world 12v “deep cycle” batteries 12.4v will keep the battery in a healthier 25% depth of discharge zone. Lithium works well at these cutoff voltages, but you can run down to 12.0v without much cause for concern.

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, no lower than 13v.