Inexpensive solar panels.

Hero999 said:

That's a thought, I wonder if I could but a cheap batch of 20 solar lights and connect the panels in series.

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Of course you could, you will get 20 times the voltage, but still the same miniscule current.

G'day Hero999,
It's a well known fact pommyland hardly gets any real sun either by bad weather of the clouds of smog. Mutli-crystaline PV panels will only draw somewhere near the maximin output in full sun where amaphorous panels will work in overcast conditions. I'll give you an example on my house array I have about 900 watts of kaneka thin film amaphorus panels where in full sun I get around 30 amps in the batteries, when its overcast I still get 10-15 amps in too...

Now on my shed array I have 350 watts of multi-crystaline panels where in full sun they will output 12-14 amps, when a cloud comes over the output drops to under 3 amps.

So personally I would recommend you look at those amaphorous panels and forget about anything under 10 watts they are only toys suitble for charging AA nimh batteries or powering electronic projects.

Cheers Bryan

I don't see why I'd need a heat sink, average daytime summer temperatures here are only in the low 20s and it rarely gets above 30°C.

These lights are only used once every two weeks for a few minutes at the most so I think a 10W array would do. The 12V 14hA SLAs are low internal discharge types and will probably last me for a year given how rarely it's used even with no charging.

there are some cheap 20 W solar pnels on ebay £ 55 + 13 carriage, normally they go at 75+ +15 carriage (of course there are still idiots that expect to sell them for 150+)

I got two 20W panels from the tops of parking meters, before you say that's theft, my local Tesco had meters in the car park but noone used them and *if* you got a ticket you send you shopping receipt and the ticket and they dropped the charge. I think they got fed-up with the whole thing. After a year or so, they were gone and it turned out that they were thrown out back in the hedge - that's responsible! I managed to dismantle them to get the panels out.

Jules_Theone said:

I got two 20W panels from the tops of parking meters, before you say that's theft, my local Tesco had meters in the car park but noone used them and *if* you got a ticket you send you shopping receipt and the ticket and they dropped the charge. I think they got fed-up with the whole thing. After a year or so, they were gone and it turned out that they were thrown out back in the hedge - that's responsible! I managed to dismantle them to get the panels out.

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good for you mate !!!

Like most people, I don't like spending money on anything and until someone can produce solar panels cheaper and more efficiently I will continue to scavenge!

I went for the 2.4W panels from Maplin.

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I'm beginning to regret this decision, they're actually overcharging the batteries! I should have spent less money and bought the smaller solar panels.

I connected them up yesterday evening at 5pm, the weather was overcast and the current was 5mA. I measured the current at 11am this very sunny morning and it read 50mA, the terminal voltage was 32V. The batteries were making a weird creaking noise which I think is hydrogen bubbling off so I decided to disconnect the cells before the batteries start venting and drying up.

I need something to disconnect the batteries when the terminal voltage exceeds a certain level and reconnect them when it drops below a certain voltage.

We haven't even passed the equinox yet and it wasn't even mid day when I measured the current. If the batteries were left connected up they would fry in midsummer.

What voltage should I disconnect the batteries at?

The recommended float voltage is 2.3V per cell which gives 27.6V but I know SLA batteries can be to charged to a higher for short periods.

What voltage should I reconnect the panels at?

What should I use a a switch?

I'd rather not use a relay because of the current consumption but a MOSFET has the issue of a diode which will cause it to conduct in reverse.

I'm tempted to wait for them to discharge down to 24V but a higher voltage might be more reasonable.

What about using the LM317 regulator set to 27.6V?

I measured the open circuit voltage at 46V which gives plenty of headroom for an LM317 which will drop under 2V at 200mA and much less at lower currents. The only trouble is the leakage current, I want something that uses virtually no current.

Hero999 said:

I connected them up yesterday evening at 5pm, the weather was overcast and the current was 5mA. I measured the current at 11am this very sunny morning and it read 50mA, the terminal voltage was 32V. The batteries were making a weird creaking noise which I think is hydrogen bubbling off so I decided to disconnect the cells before the batteries start venting and drying up.

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50mA is a puny charging current (hardly charging at all), and should cause no trouble whatsoever for 14Ah batteries - and I fail to see how the voltage across the battery could possibly be 32V? - unless the batteries are duff?.

how about a regulator or switching regulator

Thunderchild said:

how about a regulator or switching regulator

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Why?, 50mA is so little as not to require regulating, and you certainly don't have anything to waste in a regulator.

I would agree with Nigel's comment [sorry Nigel] that 50mA at 30Vish suggests the SLA is defective.

I thought that too but it powers the two tubes without any trouble and the terminal voltage stayed up at around 25V even though it hadn't been charged since Christmas - I measured it yesterday before I connected the panels.

I've just tested it now. I measured the battery voltage without anything connected and it read 25.95V and I turned both tubes on and it dropped to 24.85V which I think is pretty normal given an ambient temperature of 8°C. The batteries also managed to light the tubes on the coldest night of the year, -10°C, I didn't measure the terminal voltage though, it probably would have been under 24V given the cold.

They're gel cells, I don't know if that makes any difference but I have a feeling that they can't stand a constant charging current and need to be float charged but I could be wrong.

Hero999 said:

I thought that too but it powers the two tubes without any trouble and the terminal voltage stayed up at around 25V even though it hadn't been charged since Christmas - I measured it yesterday before I connected the panels.

I've just tested it now. I measured the battery voltage without anything connected and it read 25.95V and I turned both tubes on and it dropped to 24.85V which I think is pretty normal given an ambient temperature of 8°C. The batteries also managed to light the tubes on the coldest night of the year, -10°C, I didn't measure the terminal voltage though, it probably would have been under 24V given the cold.

They're gel cells, I don't know if that makes any difference but I have a feeling that they can't stand a constant charging current and need to be float charged but I could be wrong.

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Check what the datasheets say, but I can't imagine any problems with such tiny charging currents?.

I think I'm going to try a simple LM217 first.
**broken link removed**

I'll use the LM217 and 2k2 for R1 and 43k for R2.

This will give me 27.8V. I might add a trimmer pot so I can tweak it a bit.

Yes I know that it won't regulate properly at low currents but I hope the battery can happily absorb the 5mA minimum load required by the LM217.

I'm also aware that there are no filter capacitors but I don't care about transient response - the battery is more than large enough to smooth and spikes out.

In the end I used 2k4 and 47k which gave an output voltage of 27.75V which is just a bit over the recommended 2.3V per cell recommended; it works out at 2.3125V per cell.

The battery still makes some weird sounds but it's nowhere as bad as before and the voltage regulation is perfect so I consider it to be a success.

EDIT:
When it's dark the regulator leaks 520µA from the battery which is not a problem because a 14Ah battery will take over three years to completely discharge.

The solar panels have built-in diodes which prevent the batteries from discharging back into them and the regulator from being damaged.

I like that

I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

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Think I'll add some thing like that. Any way Is there not some way you could use the same

solar panels have built-in diodes which prevent the batteries from discharging back

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seeing your getting to much volts any way. And regulating it.

The Japanese are so clever.

Why don't the American name one of their cities JAPAN, so they can say their cars are: "Made in JAPAN."

I have a Toyota Tarago. 250,000km. Only one serpentine belt and oil and filter. Not even a spark plug. Japan has perfected the car, just like they perfected the transistor.
Out of 35,000 TV repairs, I hardly replaced a Japanese transistor.

My Sony TV was 8 years old and stopped working.
I ordered the original parts from my local Sony repair depot and they sent the wrong ones. My Sony TV was made in Mexico.

What has your Sony to do with Japanese components?