# Advice for a Solar Charger and Battery Project

I am a high school robotics teacher (no formal engineering training) and I am taking a group of students to Kenya this summer to work with high school students there. The organization we are travelling with has asked us to help out with a side project at a small rural village that has no electrical power grid. They have said that most of the households have no lights in their homes but they all have cell phones. The problem is that along the equator days and nights are essentially 12 hours each all year long. This makes it difficult for students to study at night and for parents to work much after sunset. In addition, only a few individuals have the ability to charge phones which means others must wait in line for charging.

The proposed solution is a solar system for individual homes. I am trying to design something to test with the idea that the more we can purchase with the funds we have will mean we can help more families. Here is what I have so far.

1. We are assuming that the system will have an LED light which needs 10.5 W and a USB port which needs 18W. I have drawn up a simple schematic for everything but the USB. I am not sure where the USB would go. I am thinking between the LED and the battery. I am also wondering if this product would work.

1. I am also trying to calculate battery requirements. I have used the following formula to estimate storage capacity:

What am I missing with these estimates? Once I have this figured out, how do I calculate the minimum power requirements for the solar panels?

• So, is it the case that you want to supply sufficient power during the day to provide both complete recharging the cell phones and also charging of some battery system that will provide night-time lighting for homework and studying? If so, you have to total up the energy required for cell phone charging plus the total energy required for reasonable nighttime use of lighting for study. Also, you have to take into account the relative angle of the sun during the day to the solar panel (cosine function.) They will need energy storage (batteries.)
– jonk
Mar 31, 2019 at 6:34
• Have you searched for this? It has been done many times and people posted blogs, schematics, videos etc Even one full write-up in the HomePower magazine... Mar 31, 2019 at 7:04
• Which edition of HomePower has the write up? Their site does not appear to be very serachable. Mar 31, 2019 at 16:23
• I was trying to calculate for charging a cell phone for 3 hours during the day and then powering one LED bulb for 4 hours at night. I am trying to keep the battery discharge above 50% and account for the possibility of only being able to charge every other day. From that, I came to the conclusion that it will need a battery with 43 Amp Hours storage. Do those calculations look reasonable for the battery size? Mar 31, 2019 at 16:30
• My next step was to find the solar panel appropriate for the design. It looks like I am going to determine the average azimuth of the sun at that latitude and also consider the roof design of the houses. Anything else I might be missing? Mar 31, 2019 at 16:30

To answer one small piece of your question, the component you have linked in your question will not work in the circuit you have shown. USB power is 5VDC while your calculations show you're planning on a 12VDC system, so you can't just connect the wires directly. You will need to include a 12V USB Adapter similar to this item USB Adapter.

The azimuth of the sun and roof design are more about installation than component selection. The factors for selecting the solar panels would be the average or seasonal intensity and duration of the sunlight, and the power generation efficiency of the solar panels are the factors that drive the solar panel selection. You will also need to consider the charging efficiency of the batteries since some of the power applied to any battery for charging is lost.

There are a few issues here with your design that will cause you grief.

Generally the output of the Solar panel will usually be more than your battery can handle, (depends on the make, model and number of panels used) I would look at using 12V AGM batteries connected to a MPPT Charge Controller to regulate and control the charging voltage and current to the Battery.

Connecting a LED across a battery like this will likely destroy it unless its using resistors in the path, ready made indoor LED strip lights are perfect for this, and come in 5 metre lengths, you just cut it however long you want it but I suggest 1m as a start.

MPPT charge controllers vary in price from cheap to relative expensive depends on your budget, you can get them on Ebay, VERY important point if it doesn't have an "Inductor coil" it is not an MPPT charger. You can get Non-Mppt charge controllers but these aren't as efficient. remember MPPT is a smart controller and manages the whole process. Lots of internet link describing how it works.

The biggest cost will probably be in the batteries themselves, capacity equals how long they keep the lights on. Also there are different types, the MPPT controller will need to be set to the specific type of battery, i.e lead acid or AGM other..... always read the manuals.

You will need to find suitable solar panels these can vary in output voltage and current ratings same deal MPPT is smart and makes the best use of available power. Panels can be connected in Series or Parallel but a good rule of thumb for these type of things, think 30V from the panel into MPPT, then into 12V batteries then in your 12V led strips.

I used a similar setup to connect 12V LED light strips for use in camping. I made some brackets 1m long and connected 2 strips per panel. Search for 12v Camping battery boxes, these are great to house the battery and protect it from the elements.

No need for D1 if using mppt controller, but right idea. The camping battery boxes have built in switches and meters to show voltage etc. Look into them and you wont go wrong

All the best....

B

Other points to remember Volts time Amps equals power, power in is always more than power out, due to loses, Led strips are polarity sensitive, plus to plus and negative to negative and are very low power usage. Battery boxes often have USB chargers built in. Remember you get what you pay for. A good mppt controller can cost around \$100.... LED strips is dirt cheap, use good connectors and cable, suggest Anderson connectors to crimp cables, good quality wire and keep the lengths short as possible to minimise loses due to resistance... depending on what you have it might be possible to power a couple of houses from the same batteries. Use LED over incandescant bulbs lower power requirements. Your 43 amp hours per day seems a little high, I used a 120AH battery for my camp site and it powered the LED strips for near on 5 days before It needed charging, on the equator you've got plenty of sun. you'd get more charging capacity each day...… lots of ebay parts also check out youtube videos...… happy off-grid solar power...

• Welcome to EE.SE. Please read the tour (electronics.stackexchange.com/tour) on how to write good questions and answers. Your post is poorly formatted and you gave three different answers. This is not good practice. I recommend you to edit your first answer and rewrite it in a better structured way. Mar 4, 2020 at 10:21

Lastly remember the safety mantra "Volts can jolt but mills can kill" always endeavour to keep currents as low as possible, battery charging currents can be quite high 5-10 amps this can be normal..... use good wire here and proper connections. don't mix and match batteries and panels always try to have the same type and capacity per setup, if they are mixed you will get issues. LED light strips draw about 1-1.2 amps per meter at 12V = 15-20watts.... they are very bright, use Cool White light not the orange warm light.

• what influence does the CCT of LED strips have on OP's problem? Mar 4, 2020 at 10:22