Understanding and safely modifying this Solar 120 LED Lamp - Page 1
Forum section: electronics / beginners

author: FBMinis
author_rank: Regular Contributor
timestamp: July 04, 2023, 04:35:35 pm
content: After two months of use, I noticed the LEDs weren't as bright, even though there's been plenty of sunlight.I opened up the casing and measured the 18650 battery inside at 4.9 volt. This seemed unsafe, so I sent it to the recycling bin and meanwhile I've been trying to understand the circuit, so I can keep using it in a safely manner.Please note the photos in attachment:- solar panel outputs close to 6V under direct sunlight;- if the solar panel isnt plugged in, the LEDs light up whenever there is movement; when it's plugged it, the LEDs only light up at night.My conclusion is that PIN6 is pulled High when the solar panel is connected, disabling the PIR but charging the battery through the transistor.If the panel is at 6V, is the 18650 exposed to those 6V and will take as much current as the solar panel can provide, considering the base of the transistor is biased by the 100 ohm and 10kohm resistors?Also, the LEDs are exposed to over 4.2V, correct? They pulled 600mA at 3.3V under test...Regarding making charging safer and LEDs last longer, can I connect the emitter to the input of a TP4056 board to charge the battery (output to B+ L+)? Can I solder resistor to the anode of the LEDs to limit the current, when B+ is at 4.2V?

author: tunk
author_rank: Super Contributor
timestamp: July 04, 2023, 05:24:02 pm
content: Voltage from a solar panel depends on solar intensity and load.I guess the 6V is without any load? Did the cell have a BMS?And most likely it uses the solar panel as a light sensor.

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2023, 05:40:33 pm
content: How much current does the solar panel put out, in bright sunshine?The obvious solution is to add a shunt regulator across the battery, to keep the voltage below the maximum rating.

author: FBMinis
author_rank: Regular Contributor
timestamp: July 05, 2023, 09:18:12 am
content: Quote from: tunk on July 04, 2023, 05:24:02 pmSolar panel outputs 6v unloaded, yes. I could check the current when loaded but as far as I understand, it would also depend on the state of charge of the battery, as 18650 arent charged linearly;The cell was unprotected, hence being at 4.9V;The solar panel seems to act as a light sensor but there are 3 pins on the pcb with nothing soldered to them, I wonder it's for an LDR voltage divider.

author: FBMinis
author_rank: Regular Contributor
timestamp: July 05, 2023, 09:21:16 am
content: Quote from: Zero999 on July 04, 2023, 05:40:33 pmAs in short circuit current?

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2023, 12:25:13 pm
content: Quote from: FBMinis on July 05, 2023, 09:21:16 amIdeally it should be measured into a 4.1V constant voltage load, but short circuit is the worst case scenario.

author: FBMinis
author_rank: Regular Contributor
timestamp: July 05, 2023, 01:18:04 pm
content: Quote from: Zero999 on July 04, 2023, 05:40:33 pmI'm unfamiliar with this concept. Would it be a sort of load that tries to pull as much current possible at 4.1V, stressing the power supply? A bit like an LED that tries to pull as much current once it's past its forward voltage.Are you suggesting that the charging current is limited by the properties of the solar panel, instead of relying on a fancy charging circuit?Under direct sunlight, the voltage was 6.5V and the short circuit current 135mA.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2023, 08:26:36 pm
content: Quote from: FBMinis on July 05, 2023, 01:18:04 pmYes, a constant voltage load, maintains roughly the same voltage across it, irrespective of the current.The current from the solar panel depends on its area and the intensity of the light.

author: FBMinis
author_rank: Regular Contributor
timestamp: July 06, 2023, 10:07:35 am
content: Under direct sunlight the charging current was about 80mA. I tested it with one 18650 at 3.8V and another freshly charged to 4.2V (through a TP4056). The freshly charged one kept being charged to 4.23V then I removed it.Any suggestions on how to prevent battery voltage to exceed 4.2V?