I have a modest collection of solar panels. Four of them are currently installed in the garden, facing south, connected through a voltage regulator to an old car battery. The 4 panels are wired in parallel, providing 48 Watts in full sun.

During the summer this works well, the battery is kept fully charged by a high sun and long daylight hours. But now it's winter, the battery is struggling to stay above 12 volts and there's little power to be had during the evening before the low voltage alarm starts to sound.

It's time to get some more panels outside and increase the capacity of the system. But I'll also need another voltage regulator as the current one (rated at 4.5 amps) is near capacity. Rather than buy one though, I'm going to build one myself.

The point of a voltage regulator is to protect the battery from being overcharged. 12 volt lead-acid batteries can be charged safely to about 14.5 volts. If charging continues beyond this point, the battery is in danger of being damaged.

So, the regulator's job is to monitor battery voltage, and prevent further charging when the safe maximum has been reached.

This is a simple voltage regulator circuit. The solar panel is connected to the battery via the NC (normally closed) contacts of a 12v relay. A transistor energises the relay coil, pulling open its contacts when the battery voltage rises above a preset level. The point at which this happens is adjusted by the variable resistor.

It's a cheap and reasonably effective way to protect the battery against overcharging. It also has built-in hysteresis which ensures the relay contacts open reliably at the maximum battery voltage and close again at a lower voltage level. But dispite these advantages, there are a number of problems with this simple approach, and it's the relay that takes most of the blame.

• The relay is a mechanical device and therefore subject to mechanical failure, solid state switching would lead to a longer device life.
• The safest place for lead-acid batteries is outdoors, and the voltage regulator needs to be close by, so reliable protection against moisture is important - relays and variable resistors are often not well sealed.
• Several milliamps are consumed by the relay when energised, current that would be much better left in the battery.
• Hysteresis is a double-edged sword, valuable to prevent relay chatter at the trigger voltage, but obstructive to optimal charging practice.

Hysteresis is often desirable in voltage controlled switching applications to prevent instability at the trigger voltage. In the case of the simple regulator above, there are two trigger voltages, an upper one above which the solar panel is disconnected, and a lower one which reconnects the solar panel when the battery voltage has dropped back down again. But there's only one variable resistor, so these two points are not independently adjustable. It's actually the mechanics (or more correctly the magnetics) of the relay, not the electronic circuit that gives rise to hysteresis in this design.

In the case of a solar powered battery charger, hysteresis is undesirable. When the battery reaches its optimum (maximum) voltage, ideally it should be held there, indefinitely (or at least until the sun goes down). This requires a different charging technique, not realistically achievable using a relay as the switching element. This requires PWM (pulse width modulation) and that means looking at MOSFETs.