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Battery powered on/off timer

Postby kel123 » Mon Jun 16, 2008 6:10 pm

Hi all

First time poster needs help and advice?
Central heating is the closest area that seems to fit my requirements/needs, so here goes hope you can help.

I am after a battery powered on/off daily timer to switch a 12v heater on and off.

I will explain

It is costing me £250+ per year to heat my 15' X 39" above ground swimming pool. So I have converted a 600x600 rad into a heat exchanger and by using a 12v pump and sensor controler I am hoping to utilise the sun (when it isn't raining). The pump is run from a 12v car battery trickle charged by 2x4.8w solar panels. I have stripped down an old cold/hot 12v (cool) box and the heat sink gets quite hot, so my intention is to put this in the air gap against the rad to increase the efficiency by direct contact with the rad and by heating the air gap. I don't think the battery/solar panels will be capeable of running the heat sink 24/7 and the pump will activate (drain the rad) when the temp differencial is above 4 degrees C. So I want to put a timer inline so as the heater only come on say between 11am and 5pm. The nearest I can find is the Danfoss TP500 at about £42. Have you any other ideas?
I have put up a drawing to help ... iagram.jpg
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Postby plumbbob » Tue Jun 17, 2008 6:55 pm

You haven't mentioned the power loading the timer will be required to switch. Note these timers such as the Danfoss TP5000 is only designed to switch central heating boilers / pumps up to 3 amps at 230 volts. Low voltages will generally require much higher currents.

As far as I am aware, battery operated timeswitches can only operate low current relays because of the drain on the battery. Higher current capacities require a more specialist timer or be mains operated.

Google RS Components for a range of different timers.
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Postby kel123 » Wed Jun 18, 2008 8:03 am

Yes this is my problem - I only want to opperate a 12vdc system. There doesn't seem to be anything that meets my needs?

I have a very basic knowledge of electrics/electronics but my thoughts are a battery (AA or AAA) powered clock with a daily on/off function (like an old fashioned central heating timer) that activates a normally open relay to carry 12vdc.

I have tried to make sense, probably haven't used the correct words
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Postby ericmark » Thu Jun 19, 2008 5:30 am

9.8W will hardly make any difference to the heat in any swimming pool and not really worth bothering with. But principle is good. I would not use timers but would use voltage sensor. Switch on at 13.4 volts and off at 12 volts but battery of wrong type needs to be traction battery not a car battery the latter will not stand cycling. The problem you will find you use more power running the electronics and compensating for the internal loses of the battery than ever goes to your pool. If you consider a trickle charger designed to just keep a battery topped up uses about 1.5 amp or 18 watt then you can consider the first 20 watt of what you produce is total waste and only power over and above that is of any real use. 1 watt = 1 joule per sec and there are 4.2 joules in a calorie which will heat 1 gram of water 1 degree C. Once you work out how many Kilograms of water is in your swimming pool you will quickly see you have to be talking of kilowatts not watts to do any good. Painting the radiator black and putting it behind a double glazed window would do a lot more good. Connect the solar panels to a caravan water pump to circulate the water still a drop in the ocean but a little more effective.
All best Eric

Postby kel123 » Thu Jun 19, 2008 9:36 am

This heat sink was an adddition to the system. The system works fine.

Durring 30 min tests under sunny/cloudy sky the water in the rad rises on average 10 degrees c. It raised to a max of 14 dergees c and a min of 6.

It is now a matter of placing the censors in the correct position to gain the best over all (positioning of the pool censor is the key because the pool can have a 8 degrees c differential from bottom to top)

What I have done is mounted a 600x600 single rad in a frame, bottom feed top outlet. 4" of expanding insulating foam was pumped into the back to prevent heat loss. At the front is a 2" air gap with a sealed perspex front. The face of the rad is sprayed black and has a 1" foam suround to prevent heat loss from the air gap. Now my thoughts were if I can maintain or increase the temp in the air gap the efficiency of the heat exchanger will increase hence the heat sink on a timer to only opperate during peak perriods of the day, I would agree that it would be useless at any other time and that any direct heat to the rad from the heat sink would only be minamal and just a bonus.

I envisage that there will be no pull of the battery after 5pm and before 11am because the exchanger will be in shade. Given this the battery will recharge by the next day. Although as the days get shorter this may not be the case. the solar panels are 4.8w (9.6 total) and 17.5v at optimum so given that at present there is a charge period of 8/10 hours (daylight with no pull of the battery) this should be enough.

I have a very good heat blanket on the pool (big bobble rap, Blue top silver bottom) which was well worth the money I paid. and paid for it's self the first year (cut the heating bill down by £150 per year and increased swimming perriod) I am hoping the exhanger will do the same. I will still have to put innitail heat into the pool using my 6kw heater but I am hoping that this will be the only time I have to.
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