use of batteries for lighting...controler question

[Ashnook]

I am looking into using a 12v leisure battery to supply my 12v lights in the kitchen. The battery would then be topped up using a PV panel during the daylight hours.

I want to use relays (?) to switch between the 12v transformer and the battery depending on the state of the battery.

I can design a setup where the mains to the transformer, from the light switch, enables the supply to a 12v relay from the battery to supply the lights. Using a zenner diode I think I can make the 12 volt relay disengage thereby reverting back to the transformer supply when the battery voltage starts to get low.

A problem I see is that as soon as the lights go back to mains the battery voltage will recover and switch back to battery, then voltage will fall, the relay will disengage,...... effectively setting up an oscillation.

What I think I need is either some form of delay circuit which will not allow the 12v relay to engage again for some time or a way where once the 12v has dis-engaged it cannot engage again until the lights have been turned off and the on again.

Thoughts from the group? Has anyone done this? Does something already exist to do this.

Obviously I want to keep costs low so no overcompliicated system.

Thanks, Brian

[Dave Plowman (News)]

Good luck - but I'd say the capital cost of the system and replacement batteries will negate any real money saving.

The 555 series timers are ideal for this. They cost pennies from the likes of Maplin and you can download a PDF of the spec with suggested circuits from here:-

Thoughts from the group? Has anyone done this? Does something already

A proper battery management system as used in a UPS etc would be better but will be more complicated.

[Stephen Dawson]

have a look for a firm called Sun Powered, they should be able to sell you a charging unit to do this

[garym999]

Dont know of a specific device but I'm sure they exsist. First off with a decent battery and PV panel you will probably have a shunt regulator to provide proper charging of the battery and prevent the battery discharging through the panel.

Then you need a (under voltage switch/voltage sensitive relay that switches when the battery voltage drops too low. When this happens it would switch two pairs of contacts. One would swap the lights from the battery to the mains supply and the other would enable the mains supply. The light switch would also have to be double pole to switch the lights on/off and the other in series with the relay contacts to the mains PSU. The mains PSU would only come on when the switch and the relay are on (flat battery).

[Ashnook]

yes your right, it's the under voltage system I need to build/buy so I need to get my thinking cap on.

If this system looks viable then I can use it on the many 12v lights in the house, bathrooms, hall etc. With a small 'bank' of batteries this could provide near zero on-going cost to run.

[Dave Plowman (News)]

The price of a leisure battery buys a deal of electricity. Also, low voltage lighting is very wasteful of the limited capacity. High efficiency units like fluorescents would be a better bet.

Have you worked any of this out? A battery will have a stated capacity in amp/hours. Very roughly, a 70 amp/hour leisure battery will cost about 100 quid, and run one 50 watt LV lamp for 16 hours if fully charged, 2 for 8 hours etc. Solar panels to charge just one in a reasonable time will be very expensive. Lead acid need to be recharged fully for longest life, but used every day in this way will be unlikely to last more than a couple of years.

[Ashnook]

I have sort of. I can get 100ah battery for about 40 pounds. The lights already exist so strip lights are out, also they are fly-over lights. Also they exist on bathrooms so again strip lights are out.

They are only used for about an hour or so each day, a 100 pound panel gets you about 30w of charge. say 10 hours a day(??) that's 300w of top-up. so....

I just want to try it out to see what result I get.

Thanks for you inputs, keep em coming :-)

[meow2222]

Any system run off solar power must start by being made efficient. Replacing your halogens with 12v fls will be cheaper than kitting up for the extra solar power for the halogens.

A 30w rated panel will give 30w under ideal conditions, and you certainly wont see those 10 hours a day year round! You might get that for a few hours a day in summer, the rest of the time will be lower output. What is the rating of your =A3100 panel?

Need to start by sizing your system. Working it out for both halo and fl will make things clearer.

So how many and what power halogens? ditto for fl replacements how many hours a day will they be used?

Only then can one start to work it all out and size the system. Dont assume 70Ah is enough.

Re undervoltage protection, a 12v relay is not ideal, and cant be used as is. Its not difficult to build a comparator with hysteresis to drive your relay - which should be coil unpowered in the battery supply mode.

If you definitely dont want to get into any electronics it fairly likely would be possible to make an undervoltage cutout using a 12v relay, a zener and some diodes. It wont be as accurate but should do the job. Relays close at around half their rated coil voltage, and will release at a bit less, so say we have a relay with apx 6v release voltage. We want cutout to occur at 10.8v, so the zeners/diodes must drop 10.8-6 =3D 4.8v. So with 4.8v of diode drop in series with your relay coil the relay would release when the battery fell to 10.8v. Dont use one zener, use some diodes and ideally a shottky diode too, so youve got fine-ish tunability. With some testing with a multimeter you'll soon get it to drop out at the right v or very close.

You'll want your battery placed centrally, unless wire runs are short, to minimise wire sizes and Vdrop.

NT

[garym999]

Yep a relay would do the final switching but for efficency, hysterisis, etc. some form of drive circuit would be required but you would only need the one.

On the lamp front Halogens are going to be the nicest, most natural but also consume the jucie. I would use IRC lamps, these use the heat from the lamp to burn brighter for less energy.

There are various flouresents availaible, many use small 8W straight tubes and older drive electronics, the more efficent seem to be CFL,

2D, and I think that these are going to be the most efficent and brightest overall.

One final option is to look at Luxeon LED's. There are loads of colours with cool white being the brightest white and warm white being more natural (but less efficent). These will need drivers to run them correctly and in theory once fitted have emmense lifetimes, 30000 hours plus. You can even get retrofit modules that will go into MR16 halogen fittings. (Steer clear of the ones with 5mm LEDs in though). Some drivers even have dimmers in so you neednt end up with an eco looking solution. Go for RGB and you can colour wash walls with any colour your mood takes you.

[Ashnook]

On a new installation I think you are right with the choice of LED's for fancy-ness and flors. for efficiency where the are hidden, under/over cupboards etc.

I want to try the experiment on the currently installed 20w 12v halogens in my kitchen. The hysterisis is the main problem, I would like to hold off the battery for say 1 hour after it has dropped below a pre-defined lever, say 10 volts. I think I will have to design my own circuit for this.

[John White]

Details snipped

I can't help feeling that you are introducing more complexity than is really needed here.

Perhaps you might take a look at the off-the-shelf systems that are used in both caravans and boats.

In such systems the 12V lighting is permanently wired to the leisure batteries (through appropriate switching and fuses). These batteries are in turn kept topped up by a combination of mains chargers, PV panels, windmills and engine alternators.

Whatever route you decide on, make sure that you fit the appropriate fuses and isolators to the leisure battery circuits. These things can generate serious currents when things go wrong.

You should also make sure that there is plenty of access and ventilation where the leisure battery is located. Unlike car or UPS batteries they need regular topping up and generate hydrogen when being charged.

As other posters have pointed out you will also need to pay special attention to cable sizes as the voltage drop can be very significant.

John

[Andy Wade]

ISTM you just need a schmitt trigger or comparator circuit for this, with appropriate hysteresis. (If in doubt consult Horowitz & Hill's "the art of electronics.")

Lead-acid batteries shouldn't be discharged below 1.8 V per cell, so switch to mains at 10.8 V min., say 11 V to allow some margin, and switch back at say 14 V which would indicate that the battery has been significantly recharged.

A more sophisticated approach would be to use one of the 'fuel gauge' chips that are used in mobile phones and laptops and integrate actual amp-hours. But then you'd need a microcontroller and firmware and ...

[Andrew Gabriel]

Find a relay with 3V hysteresis (probably going to be something like a 5V one) and use a zener diode to drop the excess (9V in that case, or possibly a bit less and the rest done by a resistor to anable fine tuning). Extra contacts on the relay could be used to drop the coil out of the circuit when it opens, and have it so it can only close again when charging voltage is present. Not tried drawing out a circuit, but it might not need much extra around the periphery.

[Andy Wade]

Trouble is that relay pull-in and release voltages aren't very stable with either temperature or time. Much less hassle in the long run I'd have thought to use the zener as a reference and add a couple of transistors or a '311 so that the electronics sets the thresholds. You could also add pre-set pots to make the thresholds easily adjustable.

Hmm, that's a good idea.

[Ashnook]

... along with a 555 timer for a delay to stop oscillation...?

[meow2222]

re solar pv setup:

Relays have hysteresis, but certainly an electronic solution with the relay unpowered when on batteries would be better. Relays arent very accurate as threshold detectors.

LEDs are not a good choice for general lighting, though theyre great for nightlighting. They have their uses but are much overhyped.

Lead acid battery cut off voltage should not be below 10.8v.

An electronic control for the relay is simple enough, and could be discreet or comparator based.

NT

[Ashnook]

The problem I see is that the voltage falls below, say, 10v and the relay disconnects. Once the load is disconnected the battery voltage will recover, I don't want to re-connect the 12v until the battery has re-charged somewhat otherwise oscillation occurs. I possibly need to detect a 'charged' voltage, say 13.5v?

[meow2222]

Lead acids dont do that, theyre not like dry cell batteries. Fully charged is around 13.2, so a 13.5v threshold would leave you without power. 11v should be fine, higher would keep the battery at a higher charge level some of the time, thus might extend life a bit..

NT

[Dave Plowman (News)]

As others have said you *really* don't want to let the voltage drop below

10.8v as you'll seriously shorten the life of the battery. And the fully charged voltage is more like 13.2v.

[Ashnook]

Thanks.