Help, I need assistance to power my raspberry pi computer

claverest

Hi, I'm new here and need guidance. 

I have 3 computers which I need ro keep running 24/7 because I'm using them as a server cluster. I have purchased a 100amp pwm solar charge controller and I have two batteries which I can use for this project. Details of my load and batteries are below. 

1. Raspberry pi 5 drawing 5 volts 4 amps 

2. Raspberry pi zero 2w drawing 5 volts 2 amps 

3. Mini ethernet switch drawing 5 volts 1 amp 

4. One single 12 volts 90 ah lead acid battery

5. One single 3.2 volts 72 ah CalB LifePo4 battery. 

Those are the items i have on hand right now along with the 100 amp pwm charge controller. Please I need to know the right size of solar panels that I can purchase and wire up which would be able to provide enough power to directly supply current to the three devices I have as loads above during the daytime and then be able to recharge any one of/both of the batteries I have to be able to power the loads during the night when the sun goes down. 

I am a complete novice in electrical matters but I am willing to learn. I do not have any cash right now to purchase an additional battery so I need to make do with the setup I have on hand right now and plan for an upgrade soon. I just have to purchase the right solar panel size and type to give me the juice I need to at least power my devices during the day and then I can depend on whatever my battery backup can give me at night for the time being. 

I am in Africa, we have good sunshine most of the day. All my devices arr rated at 5 volts max. I fear I bought an oversized pwm charge controller for my setup (100 amps) hence my worry that I might have to splurge on solar panels to feed such a hungry controller.  

My humble questions are thus, can I use a lower rated solar panel with the 100 amp controller? Is there a minimum wattage of panels that can be used on the 100 amp controller? Will it hurt the system if I under-feed the controller power from the panels? I don't think I have the cash to buy anything more than a 150 watt panel at the moment but I desperately need this system up and running one way or the other. 

Tha is all. I appreciate your advice. 

BB.

Welcome to the forum Clarverest.

Here are the basic set of calculations:

1. Raspberry pi 5 drawing 5 volts 4 amps: 5 volts * 4 amps = 20 Watts
2. Raspberry pi zero 2w drawing 5 volts 2 amps: 5v * 2a = 10 Watts
3. Mini Ethernet switch drawing 5 volts 1 amp: 5v * 1 amp = 5 Watts
===========================================
35 WH total load * 24 hours per day = 840 WH per day loads

Battery capacity, suggested 2 days of "no sun" (poor weather) and 50% max planned discharge (for longer battery life):

840 WH per day * 2 days of storage * 1/0.50 max planned discharge * 1/12 volt battery bus = 280 AH @ 12 volt battery bank suggested minimum battery capacity.

Note: you usually cannot mix Lithium Ion and Flooded Cell Lead Acid batteries together on the same battery bus. Each has its own Voltage needs for charging. And a single cell Li Ion battery--Would make things more complicated (need a 3.2 volt Li Ion solar charger, and a boost regulator to convert from 3.2 volts to 5 volts USB power).

For the solar array, two calculations, one based on the minimum flooded cell lead acid battery capacity, and another based on loads and hours of sun per day... First on battery AH capacity.

280 AH FLA battery (enter size of "your bank here") * 14.75 volts charging * 1/0.77 panel+controller deratings * 0.10 minimum suggested rate of charge for full time off grid system) = 536 Watt solar array based on 280 @ 12 volt FLA battery bank

Based on Lagos Nigera, fixed array facing north, tilted 6 degrees from flat:
https://pvwatts.nrel.gov/pvwatts.php

840 Watt*Hours per day * 1/0.77 solar panel+controller deratings * 1/0.80 FLA battery capacity * 1/4.55 hours of sun (average July) = 300 Watt array for "July break even"

Suggest derate actual solar harvest by 50% to 65% to minimize generator usage during poor weather:

300 Watt July break even array * 1/0.65 solar fudge factor for Weather = 462 Watt minimum solar array for year round running your Pi cluster.

The minimum solar charge controller needed based on 462 Watt array (enter your actual solar array purchased):

462 Watt array * 1/0.8 PWM controller derarting * 1/17.5 volts Vmp array = 33 amps minimum controller based on 462 Watt array

Your 100 Amp PWM controller should work fine. The loads+battery bank will only take the solar energy they need--Assuming the solar charger is setup correctly (typically 14.75 Volts charging for FLA battery).

In all things electrical, details matter... You have to pick the correct solar panels (typically Vmp-panel = ~17.5 to 18.0 volts or so--Voltage maximum power).

You probably need to pick a different (larger) battery bank (several batteries in series/parallel depending on what is available to you).

Assuming you will be using 12 volt (or 12/24 volt) USB chargers for your system.

These calculations are all based on your assumed location, loads, and clear access to sun (any partial or total shading on solar array will usually kill the output to 50% or even 0% during times of shading).

I will stop here--Please feel free to ask more questions as needed. But this should be enough to get you started.

Bill

Photowhit

Bill's right on using your numbers...
...but I suspect your numbers are the maximum draw from each of those units.

If you could measure the actual use that would be tremendously helpful. I would guess you might not need more than half that storage, but it might be used most often on overcast/cloudy days.

A 100 amp PWM controller should work nicely, but since you have to replace your battery, you might consider a LiFePo4 battery bank, which would have an integral battery charger and would already be designed to output a USB bus. 

Remember to charge with solar you need direct access to the sun throughout the solar noon hours!