Battery characteristics and ratings

Can anyone recommend a reference (hard copy of web site) that is good for this topic of battery characteristics and ratings? The sample question 124 in the sample power NCEES exam seems to be a "you know it or you don't" type question. Not having any practical experience with batteries, I'm not sure what to review for this topic.

cbinla said:

Can anyone recommend a reference (hard copy of web site) that is good for this topic of battery characteristics and ratings? The sample question 124 in the sample power NCEES exam seems to be a "you know it or you don't" type question. Not having any practical experience with batteries, I'm not sure what to review for this topic.

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The linke below might be a starting point

http://www.mpoweruk.com/performance.htm

Problem 124 is WRONG. If I have a 2 volt battery (1 cell) and increase the voltage to 4 volts by adding another cell, then I just doubled the number of cells. Therefore, my self-discharge rate just doubled. I sent NCEES an email about this. I will let you know if they reply.

GabeM said:

Problem 124 is WRONG. If I have a 2 volt battery (1 cell) and increase the voltage to 4 volts by adding another cell, then I just doubled the number of cells. Therefore, my self-discharge rate just doubled. I sent NCEES an email about this. I will let you know if they reply.

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I have some bad news for you. The self-discharge rate remains the same. You now just have two cells self discharging at the same rate.

Flyer_PE said:

GabeM said:

Problem 124 is WRONG. If I have a 2 volt battery (1 cell) and increase the voltage to 4 volts by adding another cell, then I just doubled the number of cells. Therefore, my self-discharge rate just doubled. I sent NCEES an email about this. I will let you know if they reply.

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I have some bad news for you. The self-discharge rate remains the same. You now just have two cells self discharging at the same rate.

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Right, but it depends on how you measure the rate. Since the question doesn't specify rate per cell, one has to assume that they are asking about the total rate. They should have said "self-discharge rate as a percentage of the charge capacity of the battery", not just "self-discharge rate".

^Nope. Self-discharge is internal to the individual cells. It doesn't matter how many of them you have in series or parallel.

Besides, the question is which one is it LEAST dependent upon. Self discharge for a lead acid battery is mostly dependent on temperature and age. I have my own thoughts on specific gravity but that's farther into this subject than you will find on the PE exam.

Flyer_PE said:

^Nope. Self-discharge is internal to the individual cells. It doesn't matter how many of them you have in series or parallel.

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That's exactly my point. That's why two cells will discharge twice the charge as one cell. I know it's a semantics thing, but the problem should be clearer.

As to the LEAST verbiage in the problem, the answer in the back of the book states that the voltage does not affect the self-discharge rate at all.

And it is correct. The terminal voltage for the string has absolutely no affect on self discharge rate.

If you're losing 5% in a given period, it doesn't matter if you're losing it in one cell or one hundred cells. You're still losing 5%. The rate doesn't change.

Flyer_PE said:

And it is correct. The terminal voltage for the string has absolutely no affect on self discharge rate.
If you're losing 5% in a given period, it doesn't matter if you're losing it in one cell or one hundred cells. You're still losing 5%. The rate doesn't change.

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I agree completely. But you are assuming that the problem is asking about percentage rate. The problem doesn't mention percentage. DisCHARGE rate is the rate of charge transfer, which is current (amps). The self discharge measured in amps is doubled when another cell is added.

I even thought of a problem that could happen in real life where this would matter. Let's say you work for a facility in Brazil that is looking at changing their electrical system from 120 VAC to 240 VAC. Your manager wants a cost estimate. One of the things you have to figure out is how much the change in self-discharge rate is going to affect future power consumption. Well, according to problem 124, the self discharge rate doesn't get affected by changing the voltage. However, obviously adding 60 more cells will cause your power consumption to go up because you have more cells self-discharging. (Okay, I know you would probably be worried a lot more about other things other than battery self-discharge, but I'm trying here.)

I know that most of the time, people are worried about how long the battery is going to last if you leave it sitting there without discharging it or charging it. In this case it doesn't matter what the nominal voltage is. But you are wasting more energy with higher voltage batteries.

Gnana said:

cbinla said:

Can anyone recommend a reference (hard copy of web site) that is good for this topic of battery characteristics and ratings? The sample question 124 in the sample power NCEES exam seems to be a "you know it or you don't" type question. Not having any practical experience with batteries, I'm not sure what to review for this topic.

Click to expand...

The linke below might be a starting point

http://www.mpoweruk.com/performance.htm

Click to expand...

The above link talked about temperature and age, but I'm not sure if it specifically addressed specific gravity. The following mentions that specific gravity affects standing loss:

http://www.engineersedge.com/battery/speci...ity_battery.htm

I still wonder if these two websites are still enough to cover what could be on the exam regarding batteries.

Self discharge is not measured in amps. It's a measure of lost capacity expressed in amp-hours. There is no current being drawn from the cell, the capacity loss is due to chemical reactions internal to the cell. It is totally independent on the number of cells involved.