I have two 12VSLA batteries which I want to keep trickle charged, but I don't want to have two chargers ($50-00 each) Does someone have a basic timer circuit which would work 24hrs on/24hrs off? I would join both -ves together, and have the two +ves toggles to a relay. Many thanks in advance. Rob. Melbourne.
The way for many people here would be to set up a simple PIC circuit, driving a relay to do the job, or use a 555 timer that outputs a pulse every 5 seconds into a 4020 binary counter. You could then use the /16.384 pin 3 to power the relay via a suitable transistor. This would give a changeover approximately every 24 hours. (The 4060 even has its own inbuilt oscillator, so it would be even simpler to get up and running :)
However If you want it ultra simple, cheap, and fast and 12 hours a battery is ok, go to Bunnings, buy one of those $5 mechanical timers intended for mains appliances, set it to 12 hours on/12 hours off, and use it to power a relay to switch between the 2 batteries, I think this would be the cheapest and most reliable (set and forget) system for you. Also if there is a blackout, the timer will remember its position, and the battery will still end up with the same number of hours of charge.
as for a relay, I would use one of those 12v high-current automotive relays, power it from a 12vdc plugpack that you plug into the timer directly.
If your like the rest of us, you would probably have a +12v dc plugpack lying around, or would be able to buy one for a few bucks from your local electrical store/pawnbroker etc.
If you want an indication of whether the relay is on or off, you could add a LED with a 1k resistor in series, or a small 12v bulb across the relay coil terminals.
---------------IMPORTANT (for car batteries) ------------------- Finally, keep the relay (and timer) away from the battery if possible, or put them in a sealed enclosure, since some batteries emit a flammable gas when charging, and a spark from relay/timer contacts opening and closing might be enough to ignite the gas and start a fire.