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I have an incandescent bulb in series with a battery and a constant voltage source (a charger), and would like to work out the current drawn for a given input voltage from a constant voltage source (the charger). Circuit attached.

I have the VI plot for both the bulb and the battery (in discharged state). Attached.

Both of these components exhibit a non-linear VI relationship. I understand the VI curve will vary for the battery as it charges, but I'm only interested in the initial battery state (discharged) which means I can work with one VI curve for that component.

Is it possible to use the Load Line technique to work out what the circuit current draw will be?

It's possible to use this technique for a resistor (linear VI relationship) in series with one dynamic element. See best answer below, but I can't figure out how to plot two non-linear curves on one chart.

Why does an LED have a maximum voltage?

Thank You.

VI Plots Circuit

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  • \$\begingroup\$ I think there is something very wrong with your SLA curve. Just saying. \$\endgroup\$
    – periblepsis
    Commented Oct 21, 2023 at 22:12
  • \$\begingroup\$ How so? Batteries pull almost no current below their resting voltage and it goes up quickly from there. How quickly depends on the size of the battery (Ah) and it's internal resistance. The 'resting voltage' increases as the battery charges. \$\endgroup\$
    – killingtime
    Commented Oct 21, 2023 at 22:15

1 Answer 1

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Using the best answer from the example I linked to in the question, the resistor load line is just mirrored on along the x axis, which means we're plotting (Vsupply-Xaxis)/R against the Yaxis (current). If I do this for the red battery curve I get the green plot below given: V Supply = 14.4v, Ibatt = 0A @ 11v, Ibatt = 1.4A @ 11.7v. Calculations below: 14.4 - 3.4 = 11.0v (0A) 14.4 - 2.7 = 11.7v (1.4A)

Load Line Plot

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