Which charge controller & battery to use with DIY solar panel?

Other Hardware General Electronics
1

A while back I rigged together a DIY solar panel consisting of 15 individual solar cells, each of which puts out up to 120mA @6.5V under near-optimal conditions (optimally they can put out a bit more mA's, but not that much and only for a few minutes a day, so it's negligible). These are 110x60mm cells you can pick up on eBay for around $1.30 a piece. I wired them in parallel so the panel puts out up to 1.8A @6.5V. I experimented with various circuit boards until I settled on a switching buck converter that puts out a bit over 2A @5V so I could charge up devices over USB. Under good conditions it can completely recharge a 5600mAH power bank in around 4-5 hours.

However, this circuit board only puts out enough power to recharge batteries when there's at least some direct sunlight falling on it. Early and late in the day, or during cloud cover, or when trees or structures are blocking the sun, it just doesn't put out enough juice. In fact, I think it's actually draining power from batteries (I didn't add a blocking diode as I assumed the circuit had one). The reverse draining issue I can fix with a diode, but what I'm more concerned about is not being able to make maximal use of indirect sunlight so it can extract the maximal total available and usable solar output on any given day.

I tried other circuit boards, all switching for efficiency, including a buck-boost converter, but all had issues and none solved this problem. So it looks like I need a proper charge controller to extract every last bit of power from this solar panel. I'm guessing that MPPT is the way to go. So I'm wondering what charge controller folks recommend for this kind of solar output.

Also, would it be more efficient to rewire the cells to up the voltage? I've designed the panel to be "modular", so it's not hard-wired, so it wouldn't take take much extra work to do this.

And finally, what kind of battery would be best to use for such a setup, in terms of most efficient capture of available solar energy? So far I've been charging LiPO-based USB power banks, cell phones and other devices (including a small fan that makes me smile every time I use it because I'm using the sun to cool me down), but I'm not wedded to that.

Right now this isn't an Arduino project, per se, but I might try to use this setup to power a remote Arduino weather sensor node at some point. In fact that was the original point of putting this solar panel together.

While waiting for responses I did a quick search and came up with this board that might do the trick:

https://www.ebay.com/itm/172785137235

Anyone have any experience with this board and perhaps a link to usable instructions for it given that it has 4 trimpots?

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Much of this has yet to be decided by YOU. You cannot design a system and then decide what to do with it. That's kind-of backwards.

You have to make the decision on what the desired/required output is and balance that with the expected load that will use it, which will in turn determine the battery required, voltage, and current.

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tinman13kup:
Much of this has yet to be decided by YOU. You cannot design a system and then decide what to do with it. That's kind-of backwards.

You have to make the decision on what the desired/required output is and balance that with the expected load that will use it, which will in turn determine the battery required, voltage, and current.

I DID indicate what the use(s) will be. Initially, to charge a USB power bank, which are standard items pretty much everyone here knows about. Later, perhaps, an Arduino--powered by said power bank. Or, if a more suitable/efficient type of battery is suggested, then THAT battery. I'm just asking if anyone can recommend a decent MPPT circuit board to use with the numbers I gave above, for these intended uses.

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Batteries are all roughly the same as far as their efficiency is concerned. They differ in their chemistry and capacity in Ampere hours (Ah) or milliampere hours (mAh).

First you have to decide what your use will be: what voltage and what current will your application circuitry require, and for how long should it run on the battery alone?

Then choose the battery, to provide the required voltage and current, for the desired hours. Then choose the charger/solar cell options.

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I don't think you were following what I was saying in my previous post. Yes, you can make this charge up a powerbank, and yes, you can use it to power a remote weather station. However, one designed for one application may not be well suited to supply the other.

To achieve longer charging, yes, add in more series cells. More current, go parallel. Inefficiency will come when you reach peak charge on the battery in 4hrs, but then it taking 4 weeks to deplete the battery.

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jremington:
Batteries are all roughly the same as far as their efficiency is concerned. They differ in their chemistry and capacity in Ampere hours (Ah) or milliampere hours (mAh).

First you have to decide what your use will be: what voltage and what current will your application circuitry require, and for how long should it run on the battery alone?

Then choose the battery, to provide the required voltage and current, for the desired hours. Then choose the charger/solar cell options.

Ok, for now I just want to recharge a standard LiPo power bank as efficiently as possible using the above-described solar panel. Say, a 5600mAH or 11200mAH one. That IS the application.

So, with the panel in a fixed location and angle and given its specs as described above, what kinds of charge controller boards are recommended? Surely there are certain ones that the community tends to prefer, for relatively low-power applications like this, especially less expensive ($5-$10) ones.

And if a charge controller is overkill, then what kind of DC-DC voltage converter board do you recommend?

I'm not looking for an optimal solution at this point that requires the engineering approach you're all recommending here. I'm looking for a general-purpose solution that would work for this app. I went to engineering school and know how to do things "right". But I'm taking a tinkerer/hobbyist approach here.

I.e. the "fun" approach someone has in their sig line.

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The general rule-of-thumb is use PV panels with a voltage of 1.5 x battery voltage.

In low light conditions the output voltage drops considerably, and the open circuit output voltage
spec of a panel is only for full tropical sunlight conditions.

So for 5V that means 7.5V panel (or more to allow for regulation losses).

The other approach is use a boost-buck converter with MPPT so that the voltages can be different,
and this is what a good solar charger can give you - the MPPT is going to optimize the load the
panel sees.

You could stack your panels in series groups of two, and then use a buck converter to drop down.
(remember the extra diodes to prevent reverse driving of shaded panels if you stack in series).

8

Solar panels are really only good for directly charging batteries where the solar panel voltage is higher than the battery voltage thats being charged.
Solar panels dont work well with switching converters especially in low light, as switching converters need something that approaches a voltage source when they start, and solar panels dont behave like this.
What usually happens is that as the solar panel voltage rises as the light intensity rises, eventually the switching converter will try and start and will try and draw current from the panel , which cant deliver enough so the panel voltage collapses, and the switching converter then goes into an unstable state where it may sit there drawing current but not switching, ie it wont start.
The simplest solution is usually the best, ie use the solar panels to directly charge a battery via a blocking diode, and then run everything else off that battery.

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mauried:
Solar panels are really only good for directly charging batteries where the solar panel voltage is higher than the battery voltage thats being charged.
Solar panels dont work well with switching converters especially in low light, as switching converters need something that approaches a voltage source when they start, and solar panels dont behave like this.
What usually happens is that as the solar panel voltage rises as the light intensity rises, eventually the switching converter will try and start and will try and draw current from the panel , which cant deliver enough so the panel voltage collapses, and the switching converter then goes into an unstable state where it may sit there drawing current but not switching, ie it wont start.
The simplest solution is usually the best, ie use the solar panels to directly charge a battery via a blocking diode, and then run everything else off that battery.

Thanks. A thought occurred to me based on this. Would it be possible to design a circuit that would basically have three states:

One, if the power put out by the panel fell below a certain threshold, say at night, the time from just before to just after sunrise and sunset, and during heavy cloud cover, it would basically shut off, except the part that monitored the current light level, which would use minimal current drawn from the battery otherwise being charged, and of course block the panel from drawing on the battery.

Two, if the power was above another, higher threshold, when there was direct sunlight hitting the panel, the circuit directed the output to the battery, either directly or via some charging circuitry.

And three, if the power was between these two states, such as during light cloud cover, or in the hour or so after sunrise and before sunset (beyond the few minutes after sunrise or before sunset), it would also charge the battery, via charging circuitry, but initially draw upon it to sort of "prime the pump" to get the charging circuitry started, sort of like how a car starts up, drawing on its battery before charging it.

Did I just describe how an MPPT circuit works? I mean, surely it's possible to design a circuit that could take advantage of those times when some solar energy was available, but far from maximal, and also prevent the panel and circuit from draining the battery when there was basically no usable solar energy available.

10

MarkT:
The general rule-of-thumb is use PV panels with a voltage of 1.5 x battery voltage.

In low light conditions the output voltage drops considerably, and the open circuit output voltage
spec of a panel is only for full tropical sunlight conditions.

So for 5V that means 7.5V panel (or more to allow for regulation losses).

The other approach is use a boost-buck converter with MPPT so that the voltages can be different,
and this is what a good solar charger can give you - the MPPT is going to optimize the load the
panel sees.

You could stack your panels in series groups of two, and then use a buck converter to drop down.
(remember the extra diodes to prevent reverse driving of shaded panels if you stack in series).

Your last suggestion did finally occur to me. It's so obvious I'm surprised I didn't think of it earlier. Just double the voltage so that voltage never falls below the buck converter's threshold minimum input voltage. I'm sure that there's a tradeoff in this approach, though. Is there?

My inclination though is to go with the MPPT approach even if it's overkill for this application, just because, ideally, it would put out the max output for any given 24 hour period. Or so I'm led to believe. Is the $5 board I linked to above going to do this? It claims to be MPPT but I'm wondering if that's possible at this price point.

This is the board I meant:

MPPT Solar Panel Controller 5A DC-DC Step-down CC/CV Buck Charging Module

Btw power banks have 5V USB inputs, but generally use 3.7v LiPo batteries. So, we're dealing with two stages of power conversion and the unavoidable extra energy loss that comes from that. So it probably makes more sense to forgo a power bank and connect a battery or battery bank to the solar charging circuit directly. Which might itself call for a different charging circuit than the one used to charge a power bank.

The other person was correct in that you have to start with the intended application and design the circuit around that, and choose components accordingly. That's the engineering (i.e. correct or ideal) approach.

But, right now I'm just playing around with solar to get a "feel" for it, and the various challenges it presents and solutions that exist to deal with them. This might seem academic to old hands here, but when you're learning something for the first time, you basically have to reinvent the wheel by way of "experiments" whose outcomes is well-known, for it to all sink in.

11

Its unclear from the specs what that circuit will do in low light conditions.
It appears to be a standalone MPPT converter which isnt reliant on a battery for its operation, so relies on the solar panel being able to provide enough power to power it , which may not be the case in low light conditions.
If you really want some kind of MPPT, look for a circuit which is specifically designed to charge a battery, and such a circuit wont need a reverse blocking diode as the above circuit does.
Also when looking for a MPPT designed to charge a battery make sure the MPPT is designed for the correct battery chemistry.
BUT, as previous posters have already indicated, define you need, in particular how much power is going to be consumed by whatever you ultimately want to power with your solar setup.