Beginners • Re: off-grid project:: solar -battery-driven - some calculations

a 12v 8ah battery provides 96 watts of power

Stop. Right. There. Your dimensional analysis is flat out wrong.

12v, 8Ah is 96Wh. It tells you absolutely *nothing* about how much power can be provided. Just for FYI... Wh is *energy*, not power.

5v at 96 watts gives approx 19ah

No, it doesn't a 96W source providing 5v is--theoretically--capable of providing 19A. None of watts, volts, or amperes has a time component.

running 1000ma from 19ah gives approx 20. hours running

Go back to your battery *energy* capacity of 96Wh. 5v at 1A is 5W, so--assuming that the battery can provide 1A (which is almost a given for that capacity)--you have a theoretical run time 19 hours. You won't get that in practice because of (1) voltage conversion losses (your battery doesn't supply 5v) and (2) it's bad for batteries to be run down to no energy left. You'd probably--as a practical matter--get about 15 hours or run time.

to run a pi for 24 hours would require 3 x 8ah 12v batteries (24ah)

You're still using the wrong units. To run a Pi that requires 1A at 5v (5W) for 24 hours require 120Wh of energy. One would normally run a set of batteries in series, so your capacity 3*12v*8Ah = 36v*8Ah = 288Wh. (And, for what it's worth, two such batteries in series would give you 24v at 8Ah, or 192Wh.)

Lets say we have 14 hours of sunlight to replenish the batteries

24ah ÷ 14 hours gives us 1.7a per hour charge required

12v at 1,7a is 20. watts

A 12v 20w solar panel is needed to maintain the battery for permanent non stop power supply

You got some suspect assumptions there. You may get an average of 14 hours per day of *light*, but you aren't going to average over 12 hours of actual sunlight falling on your panels. Plus, unless your panels are tracking the sun (both altitude and azimuth), you're going to get less--and often significantly less--power from the panels than their rated output.

This also overlooks the fact that, to charge a battery, you need to have a supply that provides a higher voltage than the current battery voltage. While a 6-cell Lead Acid (or sealed lead acid) battery is nominally 12v, when fully charged it will be at more like 14.4v. So the charging source really needs to be able to supply at least 15v. And if you have your three batteries in series, you need about 45v.

The good news in all this is that you aren't likely to draw enough current for the output voltage of a lead acid battery to collapse. When a 12v lead acid battery is under heavy load--such as starting a car engine--the output drops to about 7v. Car starter motors are built with that in mind.

i hope my calculations are correct and if not please advise before i go buing batteries and panels

I think you need a course in basic electricity before you start spending money. You've got several misunderstanding of units and how they work, plus a bunch of unwarranted assumptions.

Oh...and something else to consider...what are you going to do if you get several days of heavy overcast, especially in mid-Winter? You really can't count on having just enough capacity to run your Pi for 24 hours.

Statistics: Posted by W. H. Heydt — Wed Nov 27, 2024 3:33 am

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