This is kind of a side note but how in the world can a resistor maintain + or - 1% with temperature fluctuation? As I was reading the data sheet it indicates the current carrying capacity goes down as the temperature rises but it seems to me in my limited understanding of electrical theory the resistance would go up.
Now I understand what is bothering you.tkc100[quote said:When a resistor is derated it total wattage goes down right?
Is that a result of its resistance going up?
While it is true the resistance may change with temperature the amount is really very small (by design). For example the ones shown in post 62 the change is only 50 parts per million per degree C. So If the temperature goes up by say 200C the resistance of each resistor would change by only about 0.01 ohms.If resistance varies with temperature how can you ever arrive at a norm of say 10 ohms?
If the resistance is 10 ohms at say 80 degrees and higher as the temperature goes up how can you ever calculate resistance in a circuit?
I think if I was doing this for myself and only building one or two I would build the one we have now but "trim" the load resistors to get 25 amps. For example if you add a 20 ohm resistor in parallel with the 4 1 ohm resistors their value will now be .247 ohms. For the 50% capacity value the average voltage is about 6.2 volts (6.3 to start 6.1 at end) giving a value or 25.1 amps or 0.4% high. This turns out to be a 1 minute error out of 180 minutes for a 150 AH battery. The same is true for the 80% discharge number. The 100% number has a larger error of 6%, but I suspect this one is flakey anyway because of the steepness of the curve in this area. Let me know what you think.
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