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Author Topic: An RF Model for Batteries  (Read 708 times)
K7PEH
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« on: January 29, 2008, 07:05:33 AM »

This is one of those questions that I fear to ask thinking that everyone knows the answer except for me.  Remember, teachers always said that there is no stupid questions but I think the teachers were wrong some of the time.

However, is there a RF model for various kinds of batteries -- or, is the question moot as RF and batteries don't mix.

For example, if you have some kind of small signal source in series with a lithium-ion battery, what happens to that current (amplitude, phase, frequency components, etc) as it passes through the battery.  Are there any "cutoff" frequencies above (or, below) which a battery in series with an RF source is ineffective.  And, same question with regard to your typical lead-acid battery or NiCad, etc.

Second part of above question.  Given that Li-Ion batteries are sensitive to things like current, applied voltage, heat, and so on, is there any concern in using a Li-Ion battery in a circuit that is rampant with RF.

I did a google search and I did find a IEEE paper written in 2003 about the development of models for Li-Ion batteries but my IEEE membership did not allow me to access those particular papers without paying money (which is something that has always bugged me about the IEEE).

But, ideally I would like to read up on the technical aspects of such a model, research papers would be ideal but any specialized "text" book if such exists would be good too.
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IT9JCB
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« Reply #1 on: January 29, 2008, 07:33:54 AM »

Any battery can me modeled as an ideal voltage source plus a complex impedance in series to it. This impedance has a real part and an imaginary part (reactive). The real part splits furthermore into two parts: a pure resistive part Rs (from few 1/100 of Ohms to 10 Ohms or more) depending on how big is the battery and a  RF lossy part which depends on the type of battery and on the frequency. Normally Rs can be measured or deduced from other measurements, while the lossy part is normally < Rs. Then comes the reactive part: normally at low frequencies (say at tenths of MHz) it is like a big capacitance in series to real part, but as frequency increases this reactive part can become inductive as happens in the case of electrolytic capacitors (a battery can be considered a very big capacitor !!). Generally lit-Ion batteries can be considered inductive above ~50-100 MHz. Hope this short memory will help you.....73s de IT9JCB
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K7PEH
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« Reply #2 on: January 29, 2008, 08:13:21 AM »

Thank you for your comments.  I was hoping though for a little bit more technical depth on the model and the factors that lead to the model for different battery types.  Pointers to research papers or other resources are very welcome.

I am guessing that batteries of different types will have very different RF models and characteristics.  It seems logical that the model itself is dependent on the physical construction of the battery and the properties of the chemical exchange (after all, what else is there).  Thus, two batteries employing a different electro-chemical process or with different construction and placement of materials must have different models.
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KE3WD
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« Reply #3 on: January 29, 2008, 09:24:49 AM »

Expect most of it to look like a large capacitance, "large" being relative to the freq, right?  

Tank.



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W8JI
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« Reply #4 on: January 29, 2008, 09:33:46 AM »

A battery, like any other component, looks like a series reactance and resistance (or parallel reactance and resistance), the value of which varies greatly with frequency.

Of course there is a dc source in there too, but for what you are doing it can probably be ignored.

How the reactance and resistance changes with frequency depends greatly on the battery construction and the connections to the battery.

It is so unpredictable no one I have ever known has ever cared about it. The nomal practice is to bypass so RF (and audio) never reaches the battery in any significant amount.
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N3OX
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« Reply #5 on: January 29, 2008, 09:36:17 AM »

"How the reactance and resistance changes with frequency depends greatly on the battery construction and the connections to the battery. "

And probably also changes some with state of charge...
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #6 on: January 29, 2008, 10:58:06 AM »

Just a question - why do you want to pass RF through a battery? In most cases you would just put a bypass capacitor across the battery to route the RF signal around it.
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Bob  AA4PB
Garrisonville, VA
K7PEH
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« Reply #7 on: January 29, 2008, 11:09:43 AM »

>>Just a question - why do you want to pass RF through a battery?

I knew someone would ask this question and I have been trying to figure out how to answer it.

Actually, I don't want to put RF through a battery.  I was actually thinking about what would happen to an RF current flowing through a battery and started researching this question with my own books and googling various resources on the Internet.  Although I came up with a few hits on papers that I could not easily access, without paying money, I didn't find anything useful so I started asking the question here.

They say that life's two biggest pleasures are eating and sex but I want to add a third to that list -- at least for me.  That is the pleasure of the feeling of awe at the discovery of something new.  Not discovery some new hidden truth such as a good answer to radiation reaction on charged spheres but rather just learning something new.  That is the reason for my question.
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K0BG
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« Reply #8 on: January 29, 2008, 11:40:02 AM »

Necessity is the mother of invention. If invention has a father, it is the thirst for knowledge. Once your thirst dies, so do you!

My hat is off to you, Phil. You're still very much alive.

Alan, KØBG
www.k0bg.com
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K7PEH
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« Reply #9 on: January 29, 2008, 01:44:41 PM »

Alan, you are a kind and gentile person.  However, curiosity is also a kind of curse.  I often find myself digging into some minutia of information rather then getting the real job done.  I am forever getting sidetracked and I don't mean for just a few minutes -- days and weeks sometimes.

By the way, I have been strongly connected with the Internet and the general resources available on the Internet for decades.  My work goes way back to the mid-1970s when I was involved with projects on the Arpanet and finding and exchanging information with others.  Then, came the era of the Unix usenet news groups which still exist after so many years.

But, this site, eHam.net, is one of the best resources for the exchange of information that I have ever seen and used.  Indeed, I doubt that I have seen any that are better.

So, Alan, thanks to you and Tom  and Steve and Dan and a plethora of others, this site is a goldmine.
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K4SAV
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« Reply #10 on: January 29, 2008, 01:55:16 PM »

I think the answer that is closest to correct is that the performance is unpredictable.  You can model most any component at any frequency if you know enough about it (and want to go to the trouble of doing it).  In this case you don't.  You might be able to characterize a single battery, but the next one you pick up may be different.  These things are not designed to RF specifications, so any variability in that, is totally acceptable by the manufacturer.  Not only do you have those variations to deal with, the exact mounting of the battery becomes important because of lead length and shunt capacitance to other objects.  Temperature will probably be a significant factor.  N3OX mentioned that the characteristics may also vary with the charge, and that may be possible too.  That could be modeled however, but then it becomes just another variable that is almost impossible to control.  You may even find that the amount of RF current modifies the characteristics of the battery.  

It would be a case of a very poor design if you allowed the RF to go thru a battery.  A more general answer is that it is always a poor design to use ANY component outside of the range in which it is specified to perform.

Jerry, K4SAV
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K7PEH
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« Reply #11 on: January 30, 2008, 09:19:05 AM »

Thanks guys for your contributions and suggestions.  And, a special thank you to Ethan who sent me an e-mail offering to access the IEEE research papers that I could not get access to.  Ethan is pursuing his PhD in EE and as a graduate student has very good access to published papers.  And, I can't let this go without voicing my complaint against the IEEE.  Yes, I am a member of the IEEE and have been continuously for over 35 years but the IEEE has this greedy notion about charging $$ for prints if you do not belong to the particular society that published the paper.


What have I learned from this paper...

1.  First, this paper and several others that I have been able to get my hands on are all from the automotive industry.  The issue or problem at hand is determining the effects, adverse for the most part, of RF leaking into the circuits and passing through the battery whether the automobile is an ordinary single-battery thing or multiple batteries in the case of a Hybrid.  The source of the RF is usually described as the inverters that are sometimes used in these arrangements.

2.  There are some other papers that I have not been able to get prints from the cellular phone industry -- concern about RF in the phone's battery and any damage or change to discharge rate.

3.  From a model perspective, a battery looks mostly like a capacitor in parallel with the battery's DC resistance which is a function of a number of different parameters such as (electro-chemical exchange, temperature, state of charge, frequency of current).   And, there is also an inductance model in series with the battery.  Both the inductance and capacitance models are divided into two separate lumped components for frequency dependent components and DC components.


I am not yet done though -- this is getting more interesting.  By the way, the batteries used in this model were NiMH.  I think another interesting question is the affect of RF (particularly, the alternating aspect) current on the actual electro-chemical charge exchange ions, seems like they might oscillate a little bit resulting in heat.
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