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Joined: 03 Oct 2016
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Posts: 62 		Direct Link To This Post Topic: Battery current draw
    Posted: 13 Sep 2018 at 1:53pm
I like pie! (P=IE). Power equals current times volts. Amp-hours is for sizing batteries, power is for sizing systems.
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Posts: 2709 		Direct Link To This Post Posted: 13 Sep 2018 at 10:28am
Originally posted by offgrid

Ok GlueGuy, we will disagree and move on. But ha ha, my 38 years beats your 10 LOL

If you spent your decade designing grid tie PV systems than watts are the way to go. But if you are designing battery based systems, amp hours works better and is much simpler. The nominal voltage is known and is established by the battery, so there is no reason for confusion on that.

If you do use watts and are doing it correctly,  and I certainly agree that can be done, then you must use derates for battery round trip efficiency and PV module temperature. I expect that if you take a look at those derates you will find that they match up very closely with the amp hour approach. If you are not derating when working in watts and watt hours then you will be overestimating system performance.
Well. I did learn electrical engineering in 1969. So that is 49 years. Touche! I just had a brief interlude in the computer business.

We use telemetry on our solar installations. We monitor both voltage and current, and our usage graphs display in watts in vs watts out. Ohm's law rules!
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Posts: 5290 		Direct Link To This Post Posted: 13 Sep 2018 at 10:09am
Ok GlueGuy, we will disagree and move on. But ha ha, my 38 years beats your 10 LOL

If you spent your decade designing grid tie PV systems than watts are the way to go. But if you are designing battery based systems, amp hours works better and is much simpler. The nominal voltage is known and is established by the battery, so there is no reason for confusion on that.

If you do use watts and are doing it correctly,  and I certainly agree that can be done, then you must use derates for battery round trip efficiency and PV module temperature. I expect that if you take a look at those derates you will find that they match up very closely with the amp hour approach. If you are not derating when working in watts and watt hours then you will be overestimating system performance. 
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Posts: 2595 		Direct Link To This Post Posted: 13 Sep 2018 at 9:16am
I think the proper measurement instead of watts or amps is phlogiston.  It's much easier to quantify.  Confused
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Posts: 2709 		Direct Link To This Post Posted: 13 Sep 2018 at 9:07am
Then we will have to agree to disagree offgrid. I have over a decade sizing, building, and using solar systems, and I always work in watts. This is precisely because of the reasons you mention. Because the voltage varies, the amp-hours also vary. Voltage goes down, amps go up (or the voltage goes up, and the amps go down), but the watts used and the watts required will stay the same. I go for simple, and the simplest way for me to plan and use is by using watts.

Amp-hours gives you no clue about the power because it is only a measure of electron flow. It's meaningless unless you know the voltage.
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Posts: 2595 		Direct Link To This Post Posted: 13 Sep 2018 at 9:01am
offgrid and GlueGuy, I really enjoy reading your posts.  I'm anxiously awaiting GlueGuy's reply on watts vs. amps.  I don't know why, but they didn't teach me any of that stuff in law school.  It's really interesting. Reminds me of my litigation days when we had expert witnesses clashing with different analyses of some injury causing event and I had to cross-examine the other side's expert but I didn't understand squat about the science involved.

Speaking of solar, here in NV we have a ballot measure that, if it passes, will require 50 of our energy from renewable sources, in effect solar, by 2030.  There are already discussions of setting up monster solar arrays in our desert which they claim can generate huge amounts of watts or amps (?).  Or, in my book, sending "gazillions" of sparky electrons scurrying all over NV, including to the 30 amp plug at my house where our Pod is plugged in at the moment.  
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Posts: 5290 		Direct Link To This Post Posted: 13 Sep 2018 at 6:19am
That's it exactly lostagain. I'm 100% in favor of choosing solar, if I wasn't I wouldn't have spent my entire career working on commercializing it.

But one thing that I've found turns people off to trying solar or really any other new (to them) technology, is if they feel they are being oversold on it. Nothing in life is that simple, and everyone's situation is a little different.

So consider your actual needs, expectations, and situation, then make the choice that's right for you. For some it will be solar only, others generator or TV charging only, and yet others (like me) will be best off with a hybrid solution. 
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Posts: 5290 		Direct Link To This Post Posted: 13 Sep 2018 at 6:00am
Nope GlueGuy, sorry. 

I've been sizing off grid PV systems professionally for going on 40 years so I think I know a little about it. 

The reason for sizing battery based systems using amp hours is because batteries do not charge and discharge at the same voltage. If you calculate watt hours in and expect to get the same watt hours out of a battery you will be overestimating your system performance substantially. In other words, it is exactly because voltages vary throughout the charge/discharge cycle that watthours are not the way to do the sizing calcs when batteries are involved.  

Batteries do deliver amp hours back out that are fed in. In physics terms they're coulombic efficiency (electrons out/electrons in) is close to 100%.  So if you work in amp hours you don't need to be concerned about voltage differences and changes. 

The reason it is rare to ever get more than 80-85% of rated power out of a PV module is because PV devices vary in voltage significantly with temperature. The manufacturer's ratings are based on measurements at 25C (room temperature). Even if the ambient outdoor temp is 25C, the modules will heat up in full sunlight by about another 25-30C. That will result in a performance reduction of about 12-15%. 

But here again, the max power current (Imp) of the module doesn't change significantly with temperature, only the voltage. So if you are using a PV module designed for 12V battery charging (not one of the large high voltage commercial modules) the Imp of the module can be used to determine the amp hours delivered to the battery without having to run complicated temperature derate calculations.  

I hope that explanation makes sense. 
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Posts: 2709 		Direct Link To This Post Posted: 12 Sep 2018 at 9:09am
Originally posted by offgrid

All, the best way to do your energy budget for off grid solar is in amphours. It is misleading to do it in watt hours because a 100 watt solar module for example does not produce 100 watts for a 100 watt load in one hour of sunshine. its much less.
I disagree. I convert everything to watts and watt-hours because it represents POWER. Amps and volts are variable. The battery is never 12 volts, but always something between 12 and 14.5 volts, and the amps vary accordingly.

Once you convert everything to watts, then you understand how much POWER you need. You don't need to think about whether you have 6 volt, 12 volt or 2 volt batteries, you can treat each battery as a bucket full of watts. Much simpler to convert to watts and work accordingly. Sure panels don't put out 100 watts, but we often see 80% of that on our solar installations. When the sun isn't ideal, maybe between 50 and 80 percent, but still it's power.

That's why appliances, lights, and generators are all sized in terms of watts. Watts rule.
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Posts: 2595 		Direct Link To This Post Posted: 12 Sep 2018 at 8:42am
offgrid, Thanks for taking the time to put together a thorough analysis of energy use and how it relates to solar charging.  What is obvious is that everyone's electrons may vary.  For some, clearly, a 100W solar panel will be hopelessly inadequate, while for others who minimize the use of electricity, they can even lend a few electrons to their neighbors.  It all depends on one's style of camping, access to sunlight, shade, and whether you can move your panels around to optimize energy capture. 
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