I’m planning a new project — a small garden light based on a 3V LED. I’d like to design a solution that uses two NiMH batteries (1.5V each) to power the LED during the dark hours. I also want to charge the batteries with a small solar panel (or several), like this one:
I’ve tested the solar panels, and they seem to provide around 30–60 mA and about 1.5 V according to my multimeter.
(The battery instructions say it should be charged for 16 hours at 245 mA.)
Could you please advise me on a few points:
Do I need a controller to charge my batteries, or since the solar panel output is so low, can I just connect it directly without worrying about overcharging?
If I do need a controller, what kind should I use?
How can I control when the LED switches on? I’ve experimented a bit with simple transistor-based circuits using photoresistors, but maybe there’s a better or more efficient way to do it. Ideally I wanted to try using the esp32c3 microcontroller, for a bit more complex setup, however I don't have a good idea how to power it.
Those batteries are actually 1.2v not 1.5v . Is that enough to power you light ?
Before they are completely full i guess.
I would say that if they provide 1.5v all you would need would be to limit the current for the 1.41v float level of the battery to reduce current to almost nothing.
The obvious solution is to use the solar panel as a switch. If there is voltage then the light should be off. You can use a simple PNP transistor.
Several issues here. NiMh batteries have nominal voltage 1.2V. Two of them in series is not enough for 3V LED.
Solar charging is next issue, the panels you linked are 5V. And you can't draw much current from them without collapsing the voltage. Also, charging batteries in series should be done with proper circuit.
Here's the interior of $1 garden light.
Even if it's the cheapest possible one, there is specific ic ( YX8018) for boosting the battery voltage /switching charging.
Yeah, I have disassembled a similar one as well and it looks the same. However this type of lights switch of only during the dark time, and I don't quite understand how to tweak it's turn on/off time (esp32 for example has this deep sleep feature, I am using in my home projects to keep it running for quite some time). Also I wanted to understand it a bit more as a topic itself, e.g. if I would need to power two lights at the same time, or if I would want to use a bit different load (similar power consumption), how would I solve it.
In that case the simplest solution would be to just charge them that way as well. NiMh batteries are not hugely complex and can be stacked to charge as well (a 9v block typically consists of 7 layers making up 8.4v )
The tricky bit will be to protect against overcharging. Once the batteries reach 1.41v each, adding up to 2.82v, charging should stop. Again a very basic schematic will most likely suffice and overcharging will only really reduce the lifetime of the batteries. Make sure to never exceed the 245mA charge current limit and rather stay well below that.
Solar charging is quite complex job, especially since you need regulated supply for esp.
Solar lights are typically done the way that there's no simultaneously charging and light.
There are readymade esp boards with battery and (solar) charge circuit onboard.
I did something like this just recently to light a terra cotta pumpkin. I used an ATtiny85, LDR, and a NPN transistor. It turns on for 3 hours at dusk, then is off until morning when it “resets”.
Sorry I think I indeed tested it with two regular 1.5v batteries and then bought rechargeable batteries. It was a discovery that they are 1.2v :(. Sorry for confusion. So I need 3. Or one 18650 cell instead
Chat gpt suggest this one. But I am a bit unsure and want to ask real engineer, as I have built something with Ai already and it was horrible experience…
