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September 20, 2006, 6:33 am

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.

Re: Basic 24hour on/off timer

Just isolate the two batteries by joining the negatives together and
feed the charger through two diodes, one to each positive.
--
Sell your surplus electronic components at
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Sell your surplus electronic components at
http://ozcomponents.com
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Re: Basic 24hour on/off timer

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 :)
http://www.kpsec.freeuk.com/components/cmos.htm#4020
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.
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