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I'm pretty new to stack exchange and I've only just gotten my basic circuit analysis classes done in college. For my school's solar racing team, I've been tasked with designing a constant current load circuit to discharge modules of the battery pack at 20 amps. I've done my research, but am still pretty lost as to what parts to use to design this circuit, and then convert it to a PCB. There are 35 modules, 12 batteries per module, and have voltage of 3.6 volts. The capacity is 36 Ah. Can anyone help me out with designing a constant current load that will discharge the modules at 15-20 amps?

Thanks a bunch, Alvin

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  • \$\begingroup\$ Are these batteries in the modules in series or parallel? \$\endgroup\$ – BeB00 May 26 '19 at 18:26
  • \$\begingroup\$ @BeB00: If the voltage of a module is 3.6V, they would have to be in parallel, giving a system voltage of 126V and a capacity of 4536 W-h. \$\endgroup\$ – Dave Tweed May 26 '19 at 18:52
  • \$\begingroup\$ @DaveTweed I was trying to clarify if the module voltages were 3.6V, or if alvin was talking about the individual batteries. I agree that he probably means the modules, but it's good to check \$\endgroup\$ – BeB00 May 26 '19 at 19:36
  • \$\begingroup\$ Try this answer: electronics.stackexchange.com/questions/437111/… \$\endgroup\$ – Jack Creasey May 26 '19 at 21:46
  • \$\begingroup\$ @DaveTweed they are in parallel, how did you guys know that? I’m such a novice when it comes to all this and I’m just trying to learn as much as possible...I would appreciate any extra advice you guys can provide \$\endgroup\$ – Alvin Li May 26 '19 at 22:01
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Your discharge current range, 15A to 20A, doesn't require a 'constant current' sink. If you want to use it to test battery capacity, then monitor the current as you discharge the pack, and integrate.

The reason you don't want to do real 'constant current' is that it requires semiconductors dissipating a lot of power. That's expensive, and hard work in heatsinks.

One option is simply to use wire-wound resistors, dissipating to air. With '3.6v' batteries, they're obviously lithium, so will vary in voltage between 4.2 when fully charged and about 3v at end of life. Pick a total resistor value that will give you a suitable current across the range.

Another option is to use tungsten filament lamps. They have a relatively 'constant current' effect as the input voltage varies. Whereas the voltage across a resistor varies as \$IR\$, the voltage across a filament lamp varies as about \$I^{1.5}R\$, so the current will vary much less as the voltage varies. The huge advantage of a lamp in a power dissipation application is that they are designed to run very hot, and can dissipate much more power per space or per dollar than resistors.

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  • \$\begingroup\$ Yes, this is a good idea, I think this will involve several kilowatts dissipation, so using lamps will be a huge cost saver. I'd recommend series/parallel combo of 12V halogens, because their thick filament makes them very resistant to shock, 120/230V lamps are more fragile. Since this is for a solar racing vehicle, this "battery discharger" may have to be hauled around to competitions, so toughness is important. If the battery is close to standard mains voltage, a space heater will work fine too. \$\endgroup\$ – peufeu May 26 '19 at 20:11
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In general terms, you'll need

  • a transistor to control the current — at this low voltage and relatively high current, you'll want a power MOSFET mounted on a beefy heatsink.
  • a circuit to monitor the current and control the transistor
  • a circuit to monitor the voltage and cut off the current before the voltage gets too low

Since this is a form of homework, I'm not going to go into details here.

If you come up with a design about which you have doubts, show it to us. Note that we discourage broad, open-ended design review questions here on EE.SE, because the answer(s) tend to become long strings of unrelated edits and/or comments. While this might help you with your immediate problems, it is of no value to the site overall. We DO allow design review questions in which you explain your choices and then focus on a few points about which you still have doubts. To get a better feel of what is or is not acceptable, search for "design review" on the meta site.

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