AC -> DC Amp Conversion and Other Observations

SolaRAC
SolaRAC Solar Expert Posts: 52 ✭✭✭✭
in Solar Beginners Corner #1
So I've been experimenting with my first solar setup for about two weeks now. Below is a list of observations I have made and a few questions I have regarding my findings.

Firstly, the experiment consists of the below components:

- 1 Synthesis Power 75 watt panel. VMP 17.02 / IMP 4.41 / VOC 21,85 / ICS 4.72
- 1 Generic 400 watt vehicle power inverter
- 1 Sunforce 30 amp Digital Charge Controller
- 1 12v Deep cycle 79ah Marine battery

Observations
One of the first things I observed is that the best way to calculate how much I will be drawing on my battery is to multiply my AC Amps by ten. I tested this by comparing the amp reading in my Kill-a-watt meter to my DC amp meter.

Secondly, It seems to be a much wiser plan to use anything higher than 12v system. I learned very quickly that wiring can be very expensive on a 12 volt system.

Lastly, I've learned that batteries capacity can be somewat hard to read. A battery would need to be withoutno load or charge for a few hours to tell its true state. Thing is, in a solar setup, a battery is always under load or charge.

Pending questions:
How do you calculate your remaining battery capacity given your draw rate while under load?
I found somewhere that battery capacity can be calculated by using the folowing formular: (V - 11.4) / 1.5
V is the voltage at any given point in time, 11.4 is 0% charge and 1.5 is the full charge range (i.e 11.4 + 1.5 = 12.9 or max charge).

As stated ealier, it is impossible to calculate state of chatge while under load or charge. Let's speak hypothetically for a second; If I am drawing 10 amps DC and my voltage at the moment is 12.6. With a battery of 79ah, does this mean that my remaining capacity is 63.2ah and I have alittle over two hours before I reach 50% of my battery capacity?
State of charge = (12.6 - 11.4) / 1.5 = 80%
Estimate runtime before 50% state of charge = (80% * 79ah) - 39.5 / 10amps = 2.37hours?

39.5 is 50% of my total capacity by the way.

Your thoughts?
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Comments

  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    #2
    Re: AC -> DC Amp Conversion and Other Observations

    it's not impossible, just complex. :D

    You need at least two current devices to see all the details. One to measure the total input current to the battery plus load and another (in the return wire from the battery to ground) to measure the net current (positive or negative) into or out of the battery. A controller can monitor the voltages and currents to track total the total Ah/kWh in and out from the battery with CEF corrections for energy flow in depending on the State of Charge (this is tricky) and corrections using a simple Peukert/electrical/analytical model for energy flow out. The concept is simple but the devil is in the details of keeping everything in sync with reality. A single sensor can track the battery but it misses information needed to see how power is being used in the whole system.

    Charging the battery: Net current flow into the battery. circuit
    Loading the battery: Net current flow from the battery. circuit2

    Some time tracking data from my solar energy monitor. (The time scale numbers are not in hours)
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  • SolaRAC
    SolaRAC Solar Expert Posts: 52 ✭✭✭✭
    #3
    Re: AC -> DC Amp Conversion and Other Observations

    Very interesing and useful data.

    What are you using to monitor this stuff?
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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    #4
    Re: AC -> DC Amp Conversion and Other Observations
    SolaRAC wrote: »
    Very interesing and useful data.

    What are you using to monitor this stuff?

    It's a custom battery monitor/energy management system I'm building using PIC 8bit and 32bit micro-controllers. The commercial battery monitors sold here work well to give you a ballpark idea on SOC but I needed something that generated the detailed data state of a entire small system to program an expert system to optimize energy storage and use. http://code.google.com/p/solar-monitor/
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #5
    Re: AC -> DC Amp Conversion and Other Observations
    SolaRAC wrote: »
    How do you calculate your remaining battery capacity given your draw rate while under load?

    this paper may be of some help:
    http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

    Here is the discharge graph from the paper:

    Attachment not found.

    Example: if you were discharging 30 amps from a 300 AH battery you would use the C/10 curve. At that discharge rate if you were reading 12 volts at the battery, you would be at a 50% state of charge.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    #6
    Re: AC -> DC Amp Conversion and Other Observations
    vtmaps wrote: »
    this paper may be of some help:
    http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf
    --vtMaps

    That's a good paper for simple static load profiles from a fully charged starting point. But unlike a conventional electrical circuit, electrons are not the only charge carrier. Slightly more detailed papers:
    http://ecee.colorado.edu/~ecen4517/materials/Battery.pdf
    http://www.eecis.udel.edu/~honsberg/Eleg620/Storage%20chapter-v6-printout-for-class.pdf

    Of the main mechanisms that affect terminal voltage only some can be modelled electrically which limits accuracy (sometimes by large amounts) under dynamic conditions.
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  • SolaRAC
    SolaRAC Solar Expert Posts: 52 ✭✭✭✭
    #7
    Re: AC -> DC Amp Conversion and Other Observations

    Very interesting read. I must admit though, most of this stuff is waaay over my head. I will have to read some of this stuff more than once.
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