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In Reply to: RE: I just looked up the internal impedance posted by Maxamillion on November 17, 2015 at 12:41:03
It seems to me that the battery does not care about its internal impedance; it cares about the impedance across its external pos and neg terminals. Since an intact capacitor should have near infinite impedance at DC (although large value electrolytics do leak a bit, on the order of micro-amps), there is essentially no load or very little load. Uncle Stu wrote about this, at least tangentially; if the capacitor or the battery get hot or if your battery wears out prematurely, that means you've most likely got a leaky capacitor. Maybe I am still misunderstanding the issue. Sorry, if that's the case.
Follow Ups:
until it charges up, then it becomes a near infinite impedance to DC. The current required to charge an empty cap is called "inrush current". Think about it, AC can pass through a cap because the cap is alternately partially charging and discharging at the AC frequency, and if the voltage stops alternating it becomes DC and then cannot pass because the cap is neither charging nor discharging. That's why a cap-coupled amp produces a thump in your speaker at startup as the voltage ramps up and the coupling cap charges. Inrush current.
The battery's internal resistance matters exactly as much as the cap ESR, it sums with it plus the resistance of the wires to form the total resistance of the circuit.
All that said, perhaps 4.5 - 9 amps for a short period of time is not enough to destroy the cap or the battery, but I think I'll try some resistance anyway just to be on the safe side.
Technically, you're correct. My language was sloppy. When I wrote that the battery does not "care" about its internal impedance, I only meant that the external impedance, which is by far the larger term in calculating total circuit impedance, is much more important. Further, for a fresh battery, its internal impedance is a constant low value, regardless of what is strung between its external terminals. Note that the X-axis in your graph shows full charge in around 80 milli-seconds. So, in terms of amp.hours, the shock to the battery is not so great. I wonder whether the resistor would in some way alter the "performance" of the BGT, which I don't understand in the first place. Also, the spike you show would occur only once in the lifetime of a BGT, at start-up. Could you even charge the capacitor a priori (using, say, a larger battery dedicated to that purpose), so as to avoid the issue entirely?
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Yes, I thought about a pre-charge using a resistor as an alternative to wiring it permanently into the system, or perhaps a switch so that one could switch the resistor into the circuit for charging then switch it out for normal operation.
Don't know if it would affect the operation as there's not a complete circuit to the connected gear so no current should flow to the gear anyway, so added resistance in the + leg should have no effect.
I suppose I could try it with and without the resistor and see if I hear a difference. It's a simple enough experiment, and one I can probably perform in the next week or two when I get time.
Also, I think I was wrong in feeling comfortable that the total time to full charge in your LTSPICE simulation is ~80msec. The simulation appears to be for a single .047uF capacitor, and I suppose you are using thousands of microfarads across the battery. So charge time could be considerably longer.
So that would be reflective of what seems to be the most popular configuration. However, if you look closely at the graph it doesn't actually approach zero current until 300msec or so. At 80msec it's still pulling almost 2A through the cap.
I'm glad you wrote that, because last night, while I was away from any computer, I was telling myself that the charge time would be much much longer for the typical amount of capacitance people are using in the BGT, if your figure was actually valid for only .047 microfarads. Then, I said to myself you must have meant .047 Farads.
Note the transient current flow upon startup (connection of the 9V battery). I assigned a 1 ohm resistance to the battery and .02 ohm to the cap. In about 1/3 second the current is approaching zero, but it gets up near 9A initially. Cap is 47000uF.
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