Basic 24hour on/off timer

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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



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Why don't you put the batteries in parallel and trickle charge both at the
same time?



Re: Basic 24hour on/off timer


What happens when I put a used battery on the charger with a full one?  Will
it just trickle and not fully charge the empty one?

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Re: Basic 24hour on/off timer



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Just isolate the two batteries by joining the negatives together and
feed the charger through two diodes, one to each positive.
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Re: Basic 24hour on/off timer



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if it is an intelligent charger, and needs to measure terminal voltage
etc, this will create a major problem

Re: Basic 24hour on/off timer



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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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