small solar panels in parelell - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: March 14, 2022, 10:20:13 am
content: Hi all,i have a small homebrew radio,it runs on a groupe of 18650 cells in paralell,these are charged via usb and a tp 4057 modual,i have 3x small solar panells to fix to the case of it,can i connect these all together in paralell,or do they need diodes between them?,trouble is the panels output 5v max so dont realy want diode drops. TIA.

author: Psi
author_rank: Super Contributor
timestamp: March 14, 2022, 10:33:10 am
content: <Content removed, I misunderstood what you meant>I assume you mean you want to connect the solar panels to the USB charger but also keep the ability to use USB.And you know you need a diode to isolate them but you don't want to lose the 0.8V.You should be able to find a Ideal Diodes module intended for power OR'ing that can do 1A. That will reduce your voltage drop.Ideal diode modules usually have around 0.1V drop.

author: Berni
author_rank: Super Contributor
timestamp: March 14, 2022, 10:35:54 am
content: It is fine to parallel solar panels together.Series connected solar panels are more problematic because if one of the panels in a series string gets into shade then the whole string faces a large drop in power. This is because solar panels are mostly acting like a current source. So in series only as much current can flow as the weakest panel. However in parallel the currents just add up. (Tho there are other concerns like MPPT tracking)Most important thing is to have a charging chip controlling the battery charging process, otherwise the panels would overcharge the cells and destroy them.

author: m3vuv
author_rank: Super Contributor
timestamp: March 14, 2022, 10:56:58 am
content: the tp4057 modual is normaly powered at 5v from a micro usb,it has pads also next to the micro usb connector for a 5v input,it is a complete modual that controls the whole lot,it has the battery input then a load output,it charges the pack at 1a max and cuts off the load when the battery voltage drops to about 3.2v,it controls the cc/cv charge of the battery,the idea is to feed the solar panel output into the modual 5v input,i assume this should work,the panels combined only output about 2 watts total.

author: Berni
author_rank: Super Contributor
timestamp: March 14, 2022, 11:06:55 am
content: That chip is not really designed for solar panels so should be a bit careful with it.The voltage on solar panels can be quite variable. So with not enough light the panels might not produce enough for the chip to run. Remember that the voltage does drop some once current is flowing. Then once the battery is full and no current is flowing it might be possible for the panels to develop too much voltage for the chip and damage it over time. If that is the case you might want to add a low dropout linear regulator before the charger.Just make sure to test things properly in all combinations of battery charge state and light conditions.

author: tunk
author_rank: Super Contributor
timestamp: March 14, 2022, 11:19:05 am
content: On some cheap power banks with a solar panel, it is connected to the cell(s) through a (schottky) diode and a BMS.

author: m3vuv
author_rank: Super Contributor
timestamp: March 14, 2022, 11:20:56 am
content: I thought of feeding the solar panel output into a 7805 but think that needs circa 7.2v input?.maybe a 5.2v 1watt zenner on the 4056 input?

author: tunk
author_rank: Super Contributor
timestamp: March 14, 2022, 11:42:34 am
content: I guess you can't charge it using the 4056, i.e. what will happen when the current is less than 1A?

author: m3vuv
author_rank: Super Contributor
timestamp: March 14, 2022, 11:49:48 am
content: when the current is less than 1amp it charges slower!!!

author: Renate
author_rank: Super Contributor
timestamp: March 14, 2022, 07:08:19 pm
content: Quote from: Berni on March 14, 2022, 10:35:54 amMmm, this is not really so.Most serious panels are already divided into sections with a (built in) reverse diode across each section.So, two panels in series and one of the panels is half shaded with two section dioding, you've lost one quarter of your voltage (and one quarter of your power).Two panels in parallel and one of the panels is half shaded, you've lost one quarter of your current (and one quarter of your power).OTOH, your I²R losses on two panels in parallel vs. two panels in series is FOUR times greater.Count me in for series. (Currently running 68 Volts, 18.6 Amperes, 268 Watts on my two series 300 Watts panels.(It's almost noon but my batteries won't take it any faster.)

author: Psi
author_rank: Super Contributor
timestamp: May 13, 2022, 11:41:11 am
content: When using solar panels there is lots of things you "can" do, as long as you're happy to get less power than the max possible out of them.