Making use of that old garden light solar cell

[nking]

I was thinking of using an old solar cell to trickle charge my WH1081 transmitter batteries (they are rechargeable). I am NOT an electronics engineer so I thought I would ask for some advice from those on the forum rather than damage my transmitter.
I thought I would be able to simply connect the + and – connectors from the cell to the battery connection (no I haven’t done this) but then, what happens when there is no power from the cell will the power then flow from the battery to the cell? I have measured the cell and it outputs approx. 2 volts in full sunlight (it was previously powering 1 x AAA 1.2 volts battery).
So there, you can appreciate my ignorance and hence advice needed for this little project. Many thanks

[geoffw]

Now that sounds a brilliant idea ............ to a layman like me.

Too good to me true I fear!!

[steve]

This is going to be a completely useless answer, as I don't know for certain (I know enough electronics to be dangerous), but I think you probably need a diode in there.

[Gina]

Yes, I think Steve is probably right though I've never actually checked. You will need 2 of those solar cells to charge the 2 rechargeables (maybe more if the light is poor). I don't think you need to worry about overcharging - the garden solar cells don't give much current.

[nking]

Thanks Steve/Gina, OK I need a diode (non return valve to me). Next question, what do I need to get from Maplin? Do I need two, one for the + and - flow or just one on either the + or – flow? I probably need to know a part number or a link to what I should get. I am just going to try with one cell first as it's really just to get the batteries to last longer, however, I guess it depends on what milliamps the cell produces (notice the word GUESS!).

[tjaliwalpa]

I would have thought that the solar garden lights would have a diode in the circuit. I've got some lying around but have never pulled them apart.

Bob

[Gina]

You only need one diode (and non-return valve is an excellent analogy). It's usual to put it in the positive line though it doesn't matter in this case, as long as it's connected the right way round.

From my experience, these stations need a minimum of about 2v to work. Two NiCD batteries will give 2.4v when charged and should power the station transmitter unit as long as they stay reasonable well charged. Daily top-up from solar cells should do this. BUT you do need enough voltage. Taking the nominal 2.4v for the two batteries in series then add 0.6 to 0.7 volts for the diode drop gives a good 3v. To get some charging current, the voltage from the source will need to be higher. Now you say your cell gives 2v - this is not enough. The ones I've got will give up to 3v but only in bright sunshine - 2v would be a better value to work on (like yours). For the diode, almost any diode will do, though some signal diodes might not take the current. I suggest one of the 1N4000 series such as Maplin QL73Q - 1N4001. They cost 16p (but if you haven't got a Maplin shop nearby, P&P is £2.99 unless you spend £35 or over).

[nking]

Gina - You have been most kind, many thanks. I'll order up the part (I might splash out and get two or three ) when I next put in an order for some other things that I need. I'll recheck the voltage of the cell as It will be easier to construct if I only need one.

[nking]

I decided to go through my “box of rubbish” as my wife affectionately calls it and I’ve found a larger solar cell which originally charged 2 x AA batteries. I’ve just tested it and it outputs approx. 3.8 - 4 volts and when the sun came out up to 70 milliamps. The circuitry seems intact with diode so it looks very promising but before I go ahead I thought I would just check to see whether any alarm bells start to ring for those people in the know (Gina). If the amperage is likely to be an issue with overcharging I could always angle the cell accordingly I guess?

[sanramonrover]

I guess lots of people think about this one.

I found this link from a guy who has done it and posted a "how-to". He says you don't need the diode as there is not sufficient reverse current to worry about.

http://www.kwarc.org/Tedds%20Tech%20Corner/thermor.pdf

As soon as I find my solar garden lights, I'm going to rip into them as well!@

[nking]

Many thanks for the link, I'll post how it goes once I've completed the mod

[Gina]

I was going to say that the maximum current from the solar cells shouldn't overcharge the batteries but now you have the full gen Incidentally, the solar lights shown in his article look exactly the same as mine, which I originally bought as a box of 10 (from a garden centre) and didn't use them all. I might try that little project some time but ATM I'm quite happy with Energizer lithium cells and changing them once a year (or whatever). BTW Our transmitter units are not so easy to take apart - they're glued - but can be got apart with a knife worked around the join in the case. There's a thread about it somewhere on here.

[Charlie]

There's a long discussion on this subject part way through this thread: https://cumulus.hosiene.co.uk/viewtopic.php?f=13&t=2378 including some design details and pitfalls to avoid.

[ian e]

Before I found this thread, I was going to replace my Fine set transmitter Batteries, with a remote DD size Battery holder and Batteries from Maplins. Working on the theory that once the system is on the Roof I'd not want to climb up there to often to change the Batteries.

Now I think I will remote the Batteries in a seperate Battery holder, and try this too !!

Thank You Ian

[Gina]

Better if you could move the transmitter/temperature unit away from the house and at around 5ft above ground. Heat rising from the house will give you an elevated temperature reading.