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A simple 'four-component' window detector, based on the ubiquitous 555 IC timer, to form a controller in conjunction with a low-cost charger for lead-acid batteries has been described by Phil Hine in Electronics Australia (September 1990). He uses it to switch the charger on when the load pulls the battery voltage down to about 12V, and off again when it rises to about 15V, although the limits are adjustable. In this case, the load comprises a couple of 60W automatically-controlled garden lights for which he uses a 4A car-battery charger with the controller, believing that it is better repeatedly to cycle a battery than to float it continuously.

Fig 1
Fig 1. Battery charger controller using an adjusteble 555 window-detector.

Fig 1 (a) explains the action of the 555 detector with the circuit arrangement of the unit shown in Fig 1 (b). He writes:

"A portion of the input voltage is compared to the reference voltage VZ across the zener diode. If V2, set by RV2, is less than half VZ the 555 flipflop is set - resulting in a Hi output capable of sourcing 200mA (for the relay). This will be maintained until V1, sot by RV1, exceeds VZ. VZ should be loss than the upper voltage to be detected. I used a 5V zener diode as that was what I had available. RV1 and RV2 can be any value from 10K to 100K with ten-turn trimpots best for fine adjustment."

"I used the window detector to drive a 12V relay which in turn switches 240V mains to a cheap commercial 4A automotive battery charger as in Fig 1 (b). If the battery is next to the charger then the output of the charger will be well filtered by the battery, but in my case the battery Is some distance away connected by about 0.25Ω of cable, hence the need for the 4700pF capacitor ricrose the output of the charger, Without this littering, the pulsing waveform out of the charger would set and reset the window detector at 100Hz. The 33V zener diode and the two fuses form a belt and braces approach tc 240V safety, and are only for my peace of mind."