Current source

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dyarker
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Re: Current source

Post by dyarker » Fri Oct 29, 2004 7:57 pm

Missed saying something about your 27 Oct reactance guestion. I don't see reactance in the battery or the circuit as causing the oscillation. As soon as charging stops the battery voltage starts dropping to toward 12.6V resting. When charging restarts voltage starts increasing to 14.6V again. (Actual Voltages depending on temperature, percent of full charge, etc.)<p>I think the gain of your circuit is so high, that the current is being controlled by microVolt changes in battery Voltage.<p>U1a gets negative feedback by collector of Q3 connected to non-inverting input.<p>U1b only gets feedback via C1. No DC feedback! U1b is open loop at DC. Without getting out my calculator, I think doubling C1 will reduce the oscillation frequency by a third.<p>Making the "window" bigger should fix it. With a trickle charge, the charger will stay off till the battery is actually used. The problem with this is when the discharge stops before the battery dets down to 12.6V. Then only the trickle is recharging. (Trickle takes care of self-discharge that happens even if the battery isn't connected to anything.) So if trickle is set to hold the battery at say 13.55V, set the bottom of the window to 13.3V.<p>If the charger is going to use a microprocessor anyway, do it firmware. Avoid the headache of yet more analog circuitry.<p>"Float", I've been led to believe, is for equallizing the voltage of cells in a battery or of series batteries in a bank of batteries. I think it is a controlled over-charge. The fully charged cells can't absorb any more energy, so it (the energy) is converted heat. "Weak" cells get a chance to reach full charge.<p>"Is there an industry association or government body that develops and maintains definitions of terms associated with lead acid battery charging?" None I know of.<p>"Or is disagreement among writers something I should expect?" Nah, I think it's pretty standard.<p>C U L -
Dale Y

fsdenis
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Re: Current source

Post by fsdenis » Sat Oct 30, 2004 4:30 am

Gentlemen:<p>Looking at the battery as a reactance question:<p>I am attempting to understand the behavior of a lead acid battery in language I'm accustomed to using in electronics.<p>I've used the term "reactance" to describe the spring-like behavior I'm observing. I'll use Dale's "12.6V resting" notion to compare a battery to a spring:<p>If I connect my current control circuit to charge the battery (as in drawing Ed-19 on page 4) and set it at, say 300mA, then voltage will gradually rise above 12.6 volts until I turn it off.<p>When I turn off the charging current then battery voltage will gradually drop back to 12.6V, even though energy has been added to the system.<p>If I were to start at 12.6V and discharge the battery for a time the battery voltage will drop below 12.6V during discharge. Then, when I stop the discharge, the voltage will tend back up to the neighborhood of 12.6V, even though energy has been removed from the system.<p>In electronics, as far as I know, there are only resistances and reactances.<p>Resistances dissipate energy only. Do not store it when current passes through them.<p>Capacitances store energy when currents pass through them (or reduce the amount stored).<p>Inductances store energy when currents pass through them (or reduce the amount stored).<p>Capacitances and inductances are called, named, reactances as a class name. And it is this property of energy storage that seems to classify them as reactances.<p>Lead acid batteries store energy when currents pass through them (or reduce the amount stored).<p>They also dissipate some, so there is a resistive component in batteries as there is in practical capacitors and inductors.<p>The reason I'm inclined to call a battery a reactance is this energy storage property.<p>The reason I'm inclined to call battery reactance distinct from inductance or capacitance, at the moment, is that it behaves so differently from inductors or capacitors. All three behave like mechanical springs, though.<p>The bottom line here is that I need to call this springy battery behavior something for MY design convenience. So, until some better language comes along, I'm calling it battery reactance.<p>However, part of the convenience of language is shared meaning among people. It would be nice to have agreement among us of some sort so we can communicate. I don't insist on my language being standard. If you have something better, I'll go for it.<p>It could turn out that battery "spring like" behavior can be usefully described as a combination of inductance, capacitance, and resistance. That would please me.<p>Just now, I've been taking a beating on the bench with this surprising battery behavior causing oscillations in my control circuitry. I had always before thought of batteries as huge and very stable capacitors.<p>By the way, my circuits involve no explicit capacitances or inductances (drawings Ed-19 and Ed-20 on page 4). My 24V "charger" is two 12V batteries in series.<p>I agree that much of this battery "springiness" can be dealt with to reduce or eliminate oscillation in my various attempts at controls referenced to V- by use of capacitances here and there. I've done it a couple of ways.<p>And Dale's window comparator approach to practical charger design looks like a winner.<p>What's going on with me just now is that I've had a huge surprise contradicting what I thought I knew about batteries and I'm trying to re-structure my thinking and language to get it closer to reality I now see.<p>Thanks to both of you gentlemen for your help.<p>[ October 30, 2004: Message edited by: windmiller ]<p>[ October 30, 2004: Message edited by: windmiller ]<p>[ October 30, 2004: Message edited by: windmiller ]</p>

dyarker
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Re: Current source

Post by dyarker » Sat Oct 30, 2004 7:42 pm

A battery certainly has capacitance and a little inductance, but it is not an electronic device as much as it is a chemical device. Trying to model it (build a mental picture) electronically will just confuse you. Voltage and current flow are beneficial side effects of chemical reactions in the battery.<p>The component atoms of acid in water want combine differently with the positive and negative plates. As acid molecules, the atoms where put in the battery with equal numbers of protons and electrons. When the atoms go their separate ways; one has one or two extra electrons, the other is short one or two electrons. This causes a charge (Voltage) on the plates. The charge on the negative plate stops more atoms with extra electrons from approaching. The same for atoms missing electrons and the positive plate. Likes repel. For lead acid the voltage is about 2.1V per cell.<p>When an electrical path is provided between the plates (external circuit), the excess electrons on the negative plate flow to fill vacancies on the positive plate. This is the electricity we want.<p>The battery is discharged when all the acid atoms are combined with plate atoms, or all surface surface atoms on the plate have an acid atom. A graph of voltage during discharge is a curve because fewer and fewer acid atoms are trying to find fewer and fewer plate atoms. The average travel distance of the atoms increases.<p>Remove the external path and the voltage increases. But, not all the way to 2.1V because less voltage is needed to repel the fewer acid atoms from the fewer plate atoms, and there are fewer acid atoms in the water repeling each other toward toward the plates.<p>Recharging is applying enough voltage to force the acid atoms off the plates back into the water.<p>All this involves atoms physically moving. Moving takes time. Atoms don't move as fast as electrons.<p>Electricity it not stored in a battery. Energy is stored by changing chemical bonds between atoms. Chemical bonds can have more energy per volume than the electrons on the plates of a capacitor or the magnetic field of an inductor. The battery is more energy dense. The energy in gasoline when chemically reacted with oxygen (burned) is denser than batteries. (That's why we're not all driving electric cars.) Going up the density scale is fision (splitting large atoms), fusion (combining small atoms), and anniallating(sp?) atoms with antimatter atoms.<p>Does that help?<p>Cheers,
Dale Y

fsdenis
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Re: Current source

Post by fsdenis » Sun Oct 31, 2004 3:25 am

Thanks, Dale.<p>Yes. Your explanation does seem to be helping. <p>It indicates that I might go two ways in the attempt to improve how I do engineering in electronics:<p>1)Look into the chemistry of important devices in electronics for an understanding of real structure that produces the effects I interpret as resistance, capacitance, and inductance.<p>2)Retain the mechanical models as well, like capacitors as two plates separated by a dielectric, and play with this as a model of the behavior of batteries.<p>Engineering is creating a mechanism that transforms a situation from as it is to as I wish it to be. <p>The creation process seems to be provoked to work to accomplish good "cat skinning" when:<p>1)I have a clear understanding of the situation as it is.<p>2)I have a clear understanding of the situation as I wish it to be.<p>3)I have language that permits me to relate the two situations, preferably in more than one way.<p>And I understand your suggestion is to look at batteries at the atomic level to understand the effects I'm getting on the bench. A good idea.<p>I might also play with the mechanical model of capacitors and ask what mechanical structure would have to be to give me behavior equivalent to what I see in batteries.<p>For instance: If I charge a capacitor to a certain voltage, then stop charging and the voltage drops by, say, two volts with no heating.<p>The equivalent in a mechanical structure would be that the capacitance GROWS when charging stops.<p>I might have use for such an effect in a future design that would be difficult or impossible to
achieve with normal capacitors.<p>I have already discovered, inadvertently, that I can build a new class of oscillators with this effect (capacitance grows very quickly when battery charging stops and shrinks very quickly when battery discharge stops). <p>These oscillators can be extremely simple to build and if I apply your window comparator idea to oscillator design (which is just another way to describe how a battery charger would be controlled) then I have the capability of frequency control (tuning) to quite low frequencies. Even to frequencies of one cycle per month or less. In the other direction I have already discovered I may have oscillation frequencies to 125 khz.<p>So? Yes. Your thinking is quite helpful. I will ease further into an understanding of lead acid batteries at the atomic level.<p>Thanks again,<p>Fred

fsdenis
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Re: Current source

Post by fsdenis » Mon Nov 22, 2004 12:28 pm

guitronics:<p>yitiger's basic battery charging circuit as developed in this thread seems similar to what you are asking for.<p>The current regulator can easily be pulsed at 1khz, probably by your 555 with frequency and "ON"
and "OFF" time (with known accumulated charge) easily controlled. <p>If you are looking into desulfation with this scheme, I will be interested in your thinking.<p>Let me know if any of the stuff here seems in line with and possibly useful for what you are attempting. I can move just the parts you are interested in to your thread, that is, of those things I put into this thread.

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