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Old 02-14-2020, 11:24 AM   #1 (permalink)
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Default Understanding external resistive bank discharging in iCharger X6

Basic dumb question whose answer escapes me. There's something I don't quite understand in the manual of the iCharger X6 manual about the Extra Discharge Mode.

Here's the excerpt from the instruction manual:




I'm completely fine and very comfortable with the general formulas they have which are

R = V_batt / I_set
P = V_bat * I_set

These are how resistance is defined and how to find power associated with any circuit element, respectively.

What I don't understand is what info I'm supposed to get from this calculation. So if I have a 20 V pack and I want to set the discharge current to 7 A, are they recommending that I attach in series a resistor of resistance 2.85 ohms? Or is that just the minimum resistance that I should use such that internal power P_i isn't exceeded? What if I attach a smaller or larger resistor but still use I_set = 7 A as the discharge current?

In other words, it seems that the user could independently choose the value of V_batt, I_set, and the external resistance R. So what's the point of the equation R = V_batt / I_set if we could choose any three of the values; the equation won't necessarily be true.
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Old 02-14-2020, 06:27 PM   #2 (permalink)
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Ah okay I'm getting it now. Their value is approximately equal to the required resistance value for the resistor bank in order to fully utilize the iCharge's internal 30 watt power dissipation. Any more or any less resistance than that and the charger might limit the current because there would be more than 30 watts of heat dissipated internally in the charger.
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Old 02-14-2020, 07:02 PM   #3 (permalink)
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Or rather, that is the resistance required such that all of the power is dissipated through the resistor bank. Any less than that resistance would require some of the power to be dissipated through the charger. The charger has a maximum internal power dissipation of 30 W, so if the resistor bank has too low of a value of resistance, the charger would limit the current. On the other hand, if the resistor value is too high, then the current would automatically be limited because of the high resistance.

Sometimes the basic stuff escapes me.

Last edited by xoexoe; 02-14-2020 at 10:09 PM..
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