Battery-based solar power systems in the 10W–100W range often use a switching regulator to control battery charge. These have the advantage of high efficiency and facilitate peak power point tracking, but only at the cost of an inductor, circuit complexity and noise. As a simpler alternative to a switching regulator, linear control is feasible in applications up to about 20W. While simple and quiet, linear charge controllers generate heat, which must be shed by means of a heat sink. The bulk, cost and assembly complexity of a heat sink somewhat nullify a linear charge controller’s perceived advantages over a switching regulator approach.
A hysteretic controller that simply connects or disconnects the solar panel as needed to limit the battery’s state of charge provides an excellent anodyne, one devoid of inductors, complexity, noise and heat sinking.
Both series and shunt hysteretic switch topologies are possible. A series configuration opens the connection to the solar panel when the battery has reached its maximum charging voltage, then reconnects when the battery voltage falls to a lower threshold. The chief difficulty with a series configuration is driving the high side switch, which requires either a charge pump for an n-channel implementation or a high voltage, high side gate drive circuit for a p-channel MOSFET.
The preferable shunt arrangement is shown in Figure 1.
In this case the switch (S1) turns off when the battery voltage falls below a certain threshold, allowing the solar panel current to charge the battery. When the battery voltage exceeds a second, higher threshold, the switch turns on to divert solar panel current to ground. Diode D1 isolates the battery when S1 shorts the solar panel. The switch is easily implemented with an n-channel MOSFET, directly driven by the output of a ground-referred comparator.