IEEE 485 Battery Calculation Question 1

[rockman7892]

In reviewing IEEE 485 for calculation of lead acid batteries the process for determining sizing of battery capacity involves evaluating the capacity required for each section of the defined duty cycle and using the maximum calculated capacity from all of the sections to determine battery rating.

The sections start with the 1min rating in Section one and then continue though subsequent sections of the duty cycle by evaluating change in current and battery discharge (k factor) for determining section rating.

What I don't quite understand is why in each of the sections after the initial 1min rating section does it calculate the section by assuming the 1min rating continues through the entire 2nd section and then subtracts the 2nd section current from the positive 1min value by looking at the change in current (usually resulting in negative value).

I cant seem to understand why this process is used as opposed to just taking each section, evaluating the specific rating required for that section and then just adding all the sections together?

Is anyone able to give me a general explanation of reasoning behind how the IEEE 485 calculation process works but examining each section and looking at change in current within each along with Kt factor to derive section capacity?

Thank You

 

[3DDave]

This looks relevant:

https://web.engr.oregonstate.edu/~webbky/ESE471_files/Section 6 Battery Sizing.pdf

 

[Parchie]

@rockman7892 (Electrical),
Remember the word "continuous load". That means that level of current is ever present all throughout the battery loading. That is why subsequent loading always adds over that current level if they draw current for the same period of time. Think of it as the "ampere-hours" drawn out of the battery. This amount of ampere-hours drawn will be exactly the amount of ampere-hours of capacity that you will decide when choosing the battery size.

 

[rockman7892]

@Parchie

Thanks for explanation. I get the continuous loading part and how other loads add to that as you explained but what I don’t seem to get is why the different loads are all evaluated within each section.

For example let’s say section 1 was momentary and continuous load for 1min.

Then let’s say section 2 represented the 8 hrs of continuous load. Why does the section 2 calculation first evaluate the section 1 load again for 8 hrs and then look at change in current (negative value) for the section 2 continuous current to essentially subtract section 2 continuous calackry from section 1 load working the section 2 calculation period.

I guess I’m summary what I’m asking is why each period isn’t just calculated seperately and then added as opposed to how each section calculation considers all periods in IEEE485?

 

[3DDave]

It's a problem with many standards when the theory and argument for the contents are not part of the standard. Maybe it's conservative; maybe it's a mistake; you cannot tell. Contact a battery manufacturer directly as they have the most information and motive to tell the correct story.

 

[Parchie]

@rockman7892 (Electrical),
Trying to digest what you meant, really.
The standard considers momentary loads as occurring not more than 1 minute. These could be motorized valves, OCB inrush, etc. Maybe that is the thing that bothered you. Momentary loads are not included in the calculation for the battery size at first. They are only superimposed on the loading section called the controlling section to have an idea of the worst-case loading scenario.
Say you have computed the whole loading cycle for 3 hours (180 minutes) as 26,080 amp-minutes. dividing by the period of 180 minutes, you get roughly 145A average load. From the loading sections, you see that the section near that value would be the 200A level (the section below it is at the 100A level; see the presentation just below the discussion of random loads).
So, you would expect the worst loading would be 200 + 100A (the random load) = 300A.
Hope that helps.

 

[Sn00ze]

it probably has to do with the actual chemical composition and how the physical manifestations of the battery when it discharges.

you consider a momentary load for the whole duration because it is a spike in the discharge/battery. so this is all the battery sees/cares about. thus, it makes sense that this is why the change rather than the summation is what actually affects the most your calculations. i agree 3DDAVE. the right person to ask is the manufacturer.