Calculating Battery for Small Solar Project

 Today is about planning the power side of a Raspberry Pi Zero W and Arduino Nano (Elegoo clone) greenhouse monitor. We need to calculate the battery and solar panel size so that it can run continually in the UK at least over the summer months.

I have measured these figures to give us somewhere to start the calculations :

Pi Zero W power draw low 90ma, med 118ma, high 193ma

Arduino Nano Clone, running 1x ADC and serial 21ma 

Full project usage 280ma :o this includes 12 to 5v switching voltage regulator

 280ma is more than I was hoping for, maybe I will look in to reducing this by using a better voltage regulator (without an LED display etc). For now this is what I have to work with, lets add a small margin and call total current draw 300ma.

We have around 8 hours a day sunlight hitting the panel so the rest of the time it will be running on battery.

24 - 8 = 16 hours absolute minimum battery = 16 * 300 = 4800mah = 4.8Ah

 The cost of a 5Ah lead acid battery from RS Components is around the same as a 7Ah at £17. Seems silly not to and we will have 7000 / 300 = 23.3 hours. I also have a few laying around from old UPS that have been upgraded :)

I have a 10W solar panel that I am not using, its sticker states the following :

 

Voc = voltage open circuit = 21.8v
Vmp = voltage maximum power = 18v
Isc = short circuit current = 0.6A
Imp = maximum charge current 0.55A

 

I actually checked the panel with a meter and I was happy to find 0.57A output at the charge controller output in full sun... not so much with the cat sat on it, it seems that the sun does not in fact shine out or her rear !

 

The panel is putting out Power(W) = I(Current) * Volts, 0.57 * 13.9 = 7.9W. Not bad for about £10 from Amazon a few years ago (There are losses in the charge controller as well).

At maximum output this panel will fully charge our 7Ah battery in 7000 / 550 = 12.7 hours.

Unfortunately not all the battery capacity can be used. For lead acid the useable capacity can be from 80% to as low as 50%. I have checked the data sheet for the one I have and it states at least 600mA output for 10 hours. It should be fine and is a lot cheaper (free) than a lithium battery.