Advice needed for a solar powered LED light (Energy Harvesting) - Page 1
Forum section: electronics / beginners

author: RHSol
author_rank: Newbie
timestamp: October 05, 2023, 12:16:46 pm
content: Hello,I've been trying to find suitable components or even an approach for my tinkering project for a few weeks now.It is about a LED grave candle.It looks like this:-Solar light with continuous operation (LED light Day and Night).-1-4 Ni-MH AA (eneloop or similar) or lead battery (small Cyclon cell 2V/2.5Ah) suitable for low temperatures (outdoor use)-discharge and overcharge protection for the battery-a white LED (3.2V) should be operated with 5-10mA (current may vary)-I am flexible with the solar panel, I already have panels with 0.6V, 4V and 6V lying around.My first search was in the direction of an energy harvesting chip which can handle MPPT for the solar and if possible has adjustable battery charging voltages. I already had a BQ25570 module, this would be just right for my application, but the battery voltages only fit lithium batteries.Another option would be to let the solar panel charge the battery directly via a Schottky diode, but here again I can't find any battery protection circuits which are suitable for Ni-MH or lead-acid.Ready-made solutions like the Prema PR4403 I have also looked at, but I do not understand how their deep discharge protection should work, at 1V battery voltage continues to flow 100µA through the LED. If I understand it correctly there is also no protection against overcharge. The datasheet also doesn't give any leakage current through the component itself. Since the LED candle will be unattended for several weeks, even such a small current can deep discharge the battery.Datasheet:https://www.prema.com/images/downloads/PR4403.pdfDoes anyone perhaps have a new approach for me?Thanks!

author: Buriedcode
author_rank: Super Contributor
timestamp: October 06, 2023, 10:08:47 pm
content: NiMH are pretty difficult to detect charge termination especially at low charging currents. I'm unsure how much you can get away with trickle charging them - I've heard whilst the older NiCd didn't mind it too much, NiMH require a lower trickle charge. Lead-acid might be easier in this respect.MPPT is nice for larger power systems were you want the maximum power available from the cell for storage, but for a single cell small LED light IMO its completely overkill. Unless you're mass producing these, if you're worried that the cell won't fully charge - or charge enough to power the LED over night - then just use more than one solar panel, or a larger one rather than trying to get the maximum power out of one.There are thousands of garden lights that do this and whilst they aren't exactly stellar in terms of efficiency - or longevity they are pretty simple affairs, often with a dedicated IC for controlling the LED based on ambient light, just like the one you linked. I considered making my own a while back as a neat little project - just "nicer than store bought ones" but I too wasn't sure about charging the battery and how many charge/discharge cycles it would take before needing to be replaced (with solar power - every day is a charge/discharge cycle).For that chip, I'd say buy one and measure the leakage yourself.