Need help with my first DIY project

[Lorien]

Hi everyone,

I'm working on my first educational DIY electronics project. In this project, I want to charge a lithium-ion battery pack from a 50W solar panel. I have a solar panel, a dc charge controller, and the battery pack. The solar panel has the following specs:

Optimal power [Pmax]: 50W
Working voltage [Vmp]: 18V
Working current [Imp]: 2.7A
Short-circuit current [Isc]: 2.9A
Open-circuit voltage [Voc]: 20V

And the charge controller has the following specs:

Input: 13.0 VDC nominal. 10 to 15.5 VDC, 7.5 Amp
Output: 19.2 V, 2.6 A

Right away, I can see I need to build a voltage regulator to limit the voltage coming from the solar panel. But what parts do I need to construct such a circuit? And, based on these specs, is there anything else I should consider, like what if the solar panel voltage drops below 15.5 volts due to poor lighting?

I'm not looking for a complete answer, but I need something to get me started.

Any and all help is appreciated

 

[Harald Kapp]

Welcome to electronicspoint, Lorien.

I can see I need to build a voltage regulator to limit the voltage coming from the solar panel

Right. Use a switch mode regulator. Modules with matching parameters (input/output, voltage/current) can be found easily on the internet - don't try to build one from scratch, these beasts require some experience.

On the other hand, your combination of solar panel and charger is sub-optimal. Any conversion of the panel's output voltage to the battery's input voltage comes with unavoidable losses. Adding several converters adds the losses (multiplies efficiency < 1). A good setup will use just one charger directly connected betweeen panel and battery (see also below).

what if the solar panel voltage drops below 15.5 volts due to poor lighting?

With a simple step down converter, there will be no charging in this case.
A buck-boost converter ( abit mre complex) can step the input voltage down and up, depending on input conditions. However, with lower input voltage it will require more input current to keep up the same output voltage. This will quickly lead to the voltage of the panel dropping even more - a vicious cycle. It may be better to shut down the regulator and the charger when the panel's voltage drops too low.

If you want to optimize power output from the panel, use a Maximum Power Point tracker (MPP tracker) which will adjust the load current such that the output power (V*I) is maximized. Ideally you find such a controller with integrated charger (I'm not aware if that exists, though).

 

[dorke]

What you need is a DC to DC step down converter
You need to set the output to 13V it can handle 3A (more than the solar pannel can supply.

 

[Colin Mitchell]

What is the battery voltage?

 

[Lorien]

@Harald - Ah yes, I've done a lot of research on charging 12v lead acid batteries from solar panels. And you're absolutely right about using an MPPT charge controller. But I should have added the battery specs and the ultimate goal of this project.

The battery is a 14.4v 5.2AH Lithium-Ion battery pack consisting of 8 LG ICR18650 3.6v cells in a 4S2P configuration. The charge controller is made specifically for this battery pack. I would like to double the capacity buy building a 4S4P pack, but thats another project. Unfortunately, Lithium batteries don't charge the same as lead acid batteries and voltages supplied by the charge controller are very specific.

The goal of this project is to create something that can be used while backpacking. Weight is the biggest consideration, but so is size. That's why I was thinking about building my own voltage regulator. I saw a video on youtube that made it seem easy enough using an LM317 adjustable voltage regulator, but I just didn't understand the math enough to be comfortable designing something to fit my needs.

@dorke - Based on the goal of my project (see above), would this product do the same thing? It's waterproof/dustproof...

www.amazon.com/DROK-Waterproof-Converter-Adjustable-Transformer/dp/B00C0KL1OM/ref=sr_1_5?ie=UTF8&qid=1452834783&sr=8-5&keywords=3A+DC-DC+Converter+Adjustable+Step+Down+Power+Supply+Module

There is a note that says "please ensure input voltage is at least 3V higher than output voltage". What happens if the solar panel drops below that threshold? Would I be better off going with a 24v solar panel?

@colin - Please see above.

 

[Harald Kapp]

using an LM317 adjustable voltage regulator

The LM317, while kind of a workhorse, has one serious drawback: It converts the excess power (overvoltage*load current) into heat.
A switch mode regulator has much higher efficiency, thus the excess power can be used to charge the battery which should suit you very much.

 

[Colin Mitchell]

You need a SMPS that will input from 13.5 to 20v and output 12v. You can get some 3 amp versions for $5.00 on ebay
You panel is far too big for a back-pack. Where are you going to fit all your tins of spaghetti?

 

[dorke]

Yes that DROK will do the same .It is adjustable only to 15V(otherwise is probably almost the same inside).

No need to go for 24V(which will also at some point drop,say cloudy day or night time etc.).
If the solar panel voltage will drop to less than 3v above the output the DROK is set to deliver, the output of the DROK will drop below that voltage setting.
That shouldn't be a problem.

 

[(*steve*)]

unfortunately a standard smps has a serious drawback in this case. When the output voltage falls, the smps will try to draw more current. Once it tries to draw more than the panel can supply, the panel voltage will collapse, and the panel/regulator can get stuck in this state.

switching back to your other question about what happens if the voltage falls below what your charge controller requires, typically the charging will slow then stop.

a better regulation for solar panels is a shunt regulator. This simply places a sufficient load on the solar panel to keep the output voltage reduced to what you require. The major benefit is that they waste less energy as your load uses more.