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Old Oct 21, 2002, 03:31 PM
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Mountain View, CA
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Various charger technical questions

-When using a charger is whatever your input voltage is also the output voltage to the battery, or is the voltage regulated? I know you regulate the current to a battery given its capacity but what about the voltage given the number of cells?
-How does the charger regulate the current? Does it use a digital device to send PWM signals to a mosfet/h-bridge/transistor which (through switching) controls the current? Or can that only control voltage and you have to use some sort of pot to control current?

These are pretty broad questions and I'll appreciate any answers
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Old Oct 21, 2002, 03:49 PM
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Ok, so resistance regulates current. I can see how this would work in some chargers, the ones with dials, they probably turn a POT which changes the resistance. But how does a charger like the Super Turbo or Supernova do this? There are no dials, only buttons, does it route the current over different resistors within the device? Is that why there are only certain current values available, usually in steps of 100 mA?
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Old Oct 21, 2002, 06:49 PM
BEC
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The output voltage is regulated by the battery under charge (at least nickel-based chemistries) as long as the charger is capable of putting out a higher one. For chargers that can do more than 7 cells there is a voltage-multiplier of some variety to boost that possible output voltage above the nominal 12V in.

Current regulation...... I don't think there are any other than the cheapest chargers out there which do it by simply dumping power into a (variable) resistor any more - if they did it that way the charge current would continually fall as the battery under charge's voltage increased which plays havoc with reliable peak detection. I presume, therefore that there is some sort of PWM scheme involved though I have never looked into this aspect.

I do know the little Multiplex (also sold by Dymond and FMA) charger with the three charge currents selected by a switch must do it by PWM at a fairly low frequency because you can hear it "sing" under load and the sound changes with the three different current selections (1, 2, 4A).
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Old Oct 22, 2002, 03:36 AM
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Most chargers use a standard electronic circuit block known as a "current source" for regulating current. There are number of variants used but they all allow the current to be defined by an input voltage (not the same voltage as the output to the battery). If you're really interested in this particular aspect of the internal design of chargers I suggest a web search on "current source" or "constant current source" (even though it's really variable).

Steve
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Old Oct 22, 2002, 01:30 PM
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Ah, yes in my searches I keept seeing 'constant current source' come up but thought "Thats not what I want, I want a variable current source.". I'll look back into those circuits. And thanks for pointing out that just a pot won't work, I had actually started looking into digital pots, and someone suggested a DAC coupled with an current opAmp...
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