Solar panel for charging electronics

Thread Starter

quique123

Joined May 15, 2015
404
A while back I set up a project for temperature monitoring in a remote warehouse. It was powered by 2x 18650 rechargeable batteries thru a small solar panel(3W). It worked, more or less, but at times it powered down due to lack of sun. Alas I decided to take the project down but not before adding a second identical solar panel to the power part of the setup (bms module, battery holder and solar panel). I figured I would just use it to recharge my batteries for any project.

Then I remembered I have a large 250W panel sitting in that same warehouse. It's a 17V/14A panel left over from my grid tied setup.

So I'm wondering what would I need in order to use that panel and make a few different taps to have a 5V charger for devices, a 6-9V charger for my 18650 batteries and maybe a few Leds for basic lighting?
 

Thread Starter

quique123

Joined May 15, 2015
404
ok yeah ive got one of these. So this will step down the voltage but what about the Amps? My idea is to take the electricity from the panel and use it to charge 18650 batteries, light up a few LEDs...
 

Irving

Joined Jan 30, 2016
952
You cant charge Li-Ion batteries directly from a big solar panel like that, ideally they should be charged via a proper solar mppt charger which not only protects the batteries but maximises panel efficiency by matching its output to the load. The small panel had barely enough output to partly charge the battery, which is good because if I understood right how you'd wired it there's a strong possibility the bigger panel could have set them on fire - the baby BMS sold for those types of cells are generally for 5 - 9v input (but provide a link to your BMS, it may be suitable).

The current drawn by whatever devices you attach to the panel is determined by the devices, as long as the panel can supply the total amount including conversion losses. There's no 'stepping down' the current. Of course, if the panel is capable of generating more power than you need then thats wasted, and of course, anything directly conneced to the panel will stop working when the sun isn't shining.

Suppose you have a device needing 5v at 1A and another needing 9v at 2A... Connect those up using dc-dc converters 17 -> 5V and 17 -> 9v. The 5v device uses 5v x 1A = 5W and the 9v uses 9v x 2A = 18W, total 23W. The efficiency of the DC-DC converters is around 90% so the panel needs to supply 23/0.9 = approx 26W. At 17v thats 26/17 = just over 1.5A, so the panel may supply that even if lightly shaded. Of course the panel doen't supply a fixed voltage, it varies with the current drawn and the amount of sunlight. If you don't draw enough current the voltage collapses, so you may find that just powering a few small devices doesn't actually work as you're not loading the panel sufficiently.

Of course, as said before, when the sun stops shining everything stops, as you discovered, so really everything needs to be powered from batteries that are charged by the panel. Firstly you need to decide how long the battery should last - lets say you have a 12v battery then the example above needs 26/12 = 2.2A. A 40Ah battery will last about 40/2.2 =18 hours, i.e. overnight plus some slack. A 12v, 40Ah battery stores 12 x 40 = 480Wh of energy, your panel generates (with a suitable mppt inverter) 230W of energy, so on a good day will supply your devices (approx 30W) and charge the battery (200W) - it'll fully charge the battery in approx 480/200 x 1.25 hours or about 3h (the x1.25 is because the conversion is typically about 80% efficient). On a bad day it might not fully recharge the battery....

You need a proper solar mppt charger to achieve this. As said before, a standard BMS simply wont extract power from the solar panel effectively as it wants to draw a fixed current at a fixed voltage (thats probably why your original setup didn't work so well - solar panels are finicky things).
 

bassbindevil

Joined Jan 23, 2014
100
There are various chips for charging lithium-ion batteries that operate like switching regulators from a wide range of input voltages. I've used a module based on the SY6912 to charge a 3S4P 18650 pack directly from a PV panel. The SY6912 can be configured for 1S to 3S packs, and up to 2A by changing jumpers and shunt resistors; I limited mine to 1A charging because it got unpleasantly hot at 2A (supplied from a DC power brick, not PV). Allowable input voltage range is a good match to 12V nominal PV panels. If you wanted to run a 4S battery, there's a MAX745 charging module on ebay; I haven't used those.
Some car USB chargers are rated to run from both 12V and 24V (nominal) vehicle power; those should run OK directly from a sufficiently powerful PV panel, if you only expect to charge your phone or whatever when the sun is shining. If you need the option of charging anytime, a correctly chosen regulator could deliver 5V from a 2S pack, 3S or 4S could run a normal car charger, or you could wire up a power bank boost converter to a 1S pack.
For lighting... if you wire up your own things, 1S LEDs are a good match to 1S lithium packs (just use resistors to limit current); at higher voltages, there are various switching drivers that can run 1 or more LEDs at constant current, or use series LEDs or COBs with resistors. The MR11/MR16 two-pin lamps that run on 12V nominal have switching regulators inside that can operate over a wide voltage range, and sockets and fixtures are readily available. You don't even have to worry about polarity since they were designed to replace halogens in track lighting running from 12V AC transformers, so they have bridge rectifiers inside.
 

Irving

Joined Jan 30, 2016
952
There are various chips for charging lithium-ion batteries that operate like switching regulators from a wide range of input voltages. I've used a module based on the SY6912 to charge a 3S4P 18650 pack directly from a PV panel. The SY6912 can be configured for 1S to 3S packs, and up to 2A by changing jumpers and shunt resistors; I limited mine to 1A charging because it got unpleasantly hot at 2A (supplied from a DC power brick, not PV). Allowable input voltage range is a good match to 12V nominal PV panels. If you wanted to run a 4S battery, there's a MAX745 charging module on ebay; I haven't used those.
Some car USB chargers are rated to run from both 12V and 24V (nominal) vehicle power; those should run OK directly from a sufficiently powerful PV panel, if you only expect to charge your phone or whatever when the sun is shining. If you need the option of charging anytime, a correctly chosen regulator could deliver 5V from a 2S pack, 3S or 4S could run a normal car charger, or you could wire up a power bank boost converter to a 1S pack.
For lighting... if you wire up your own things, 1S LEDs are a good match to 1S lithium packs (just use resistors to limit current); at higher voltages, there are various switching drivers that can run 1 or more LEDs at constant current, or use series LEDs or COBs with resistors. The MR11/MR16 two-pin lamps that run on 12V nominal have switching regulators inside that can operate over a wide voltage range, and sockets and fixtures are readily available. You don't even have to worry about polarity since they were designed to replace halogens in track lighting running from 12V AC transformers, so they have bridge rectifiers inside.
Yes, you can charge small batteries like that, but it would be very inefficient from a large 250W PV Panel. To get the most power out of the panel needs something that can actively adjust its load point, an average SMPS will only passively take what's given. Of course if you're only charging a 3S pack at 1A that's ~11W its a moot point for a 250W panel.

Your 3S4P pack, approx 15Ah, should charge at 7A no problem given a competent charger . The SY6912 chip is easily good for 2A, unfortunately the Chinese manufacturers of these cheap boards never put enough heatsinking on them, they are made to a low price but advertised at the limit or higher of their spec. (a case in point are audio amps advertised at 300W which require some significant and expensive extruded heatsinks but ship as a board with a couple of sq in of copper as shown in the application note for the 10W evaluation board).
 

Thread Starter

quique123

Joined May 15, 2015
404
Ok well i was just trying to put that panel to good use and solve my power charging needs at the same time. I need have a small shed where i do some work sometimes, not even all the time, but we charge lots of things at home; mobiles, battery packs for phones, gopro, my 18650s for projects and since i need a lightbulb in that shed. I just dont have or particularly like having a battery out there to store energy, first because id have to buy one and second because id have to keep an eye on it for maintenance.

But it might just be what I need to do, get one of those sealed (deep cycle) batteries and keep it topped off and use it for lighting up the place, actually keep the gopro on for monitoring the place and charge my 18650s via a small inverter.
 

Thread Starter

quique123

Joined May 15, 2015
404
So basically if i take the power from the solar panel (17V/14A) the best thing to do with it by far, is just stick it into a 12v lead acid or sealed battery?
 

Irving

Joined Jan 30, 2016
952
Not if you want the battery to survive. Charging it at anything above 14.4-14 8 volts, depending on the battery type, will kill it.
 

Irving

Joined Jan 30, 2016
952
Yes, it's that simple, there are stacks of them on eBay for peanuts, but I'd go look at some off-grid sites and see what they recommend. It's much easier if you're not looking for PV charger <-> grid tie, that's what makes it complicated.
 

Thread Starter

quique123

Joined May 15, 2015
404
Wait so i have this pwm charge controller:

It has 2 terminals for lights directly, assuming a 12VDC light bulb. I wonder if those terminals are simply pulling charge from the connected lead acid battery and therefore lighting it. So this might mean I still need the lead acid battery to power that bulb? Because thats what I want to avoid.
 

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Irving

Joined Jan 30, 2016
952
looking at the manual, it connects a PV panel, lead-acid cells and a load. So the load you have on it are just some 12v bulbs? What bulbs are they & how many?
 

Thread Starter

quique123

Joined May 15, 2015
404
no, you didnt understand my post. I dont have anything connected right now. I am thinking of connecting the controller to the panel and using those 2 terminals on the controller to get power, which would obviously be 12vdc power. But my doubt is, it probably needs the lead acid battery to put out power on those 2 terminals, right? Because I dont have one.
 

Irving

Joined Jan 30, 2016
952
Well, yes, it would be pointless powering the lights when the sun is shining... the whole point is to capture energy when its shining, to use when its not.... and anyway, sunshine is variable, you always need some storage capacity backing up a PV panel. You never power anything useful directly off a panel, except those little executive desk-toys...
 
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