I have looked through some of the current limiting responses, but don’t see any close to what I’m working on for my project.

I have an EV system running in around 200 - 270V DC. I want to build a circuit that will allow me to connect an external say 280V DC LiPeO4 battery bank and charge up the internal DC system.

Obviously, I don’t want to dump the total current possible when the circuit is made. I have a Gigavac high voltage relay that can make the circuit.

I want something that will limit the charge to say 20A at 270V DC to the internal battery system to ‘top it up’ in real time.

I’m an engineer but more in thermal/mechanical with some electrical background, but not in ICs or design.

Any help is appreciated, but remember to keep it simple (e.g. pretend I’m a 10yo) :D

  • 1
    \$\begingroup\$ The simple, but wasteful, approach would be to use a \$4\:\Omega\$ resistor rated for about \$2\:\text{kW}\$ dissipation. That would limit the current from a \$280\:\text{V}\$ system to a "low" \$200\:\text{V}\$ to \$20\:\text{A}\$. (Less, as it charges up.) Any linear (non-switcher) system to perform this will have to dissipate (and waste) a lot of energy. You probably want an appropriate switcher system so that you get the most out of your recharging process. Or maybe I just misunderstand. \$\endgroup\$
    – jonk
    Jan 31, 2019 at 21:12
  • \$\begingroup\$ Is the "internal DC system" also a battery? If so, do you know what type? \$\endgroup\$
    – Nate S.
    Jan 31, 2019 at 21:16
  • \$\begingroup\$ To avoid wasting power typically you'd use a DC-DC converter, but I don't know how expensive it would be to get/make one that can handle that much power. \$\endgroup\$
    – user253751
    Jan 31, 2019 at 22:45
  • \$\begingroup\$ Yes, the internal DC system is a battery - composite NiMH \$\endgroup\$
    – EngMarc
    Feb 1, 2019 at 1:32
  • \$\begingroup\$ what's the maximum voltage difference, these lithium batteries tend to change voltage with state of charge. \$\endgroup\$ Feb 1, 2019 at 3:35

1 Answer 1


If you don't need an adjustable current limit value and you don't need to cut off all charging when a specific terminal voltage is reached, then the classic two-transistor circuit will work with appropriately beefy parts.

Current Limit circuit

At 20 A you'll dissipate about 14 W in Rsense, and much more in T1 depending on the initial terminal voltage. A switching circuit will be much more efficient, but also more complex.

  • \$\begingroup\$ I assume you mean a switching circuit where it allows the voltage to equilibrate and then switch once the current settles?? \$\endgroup\$
    – EngMarc
    Feb 1, 2019 at 1:34
  • \$\begingroup\$ No, I mean a switching regulator rather than a linear one:en.wikipedia.org/wiki/Switched-mode_power_supply \$\endgroup\$
    – AnalogKid
    Feb 1, 2019 at 3:14
  • \$\begingroup\$ Ok, now the challenge will be to find a switched mode power supply that runs a max of 300v (typically 270-280v) to change a 180v (prob lowest would be 100v) DC system. Overall charge rates can be as high as 80a but on initial connect it should slowly ramp up and then slowly ramp down/off to target voltage. \$\endgroup\$
    – EngMarc
    Feb 1, 2019 at 14:14
  • \$\begingroup\$ Any idea if it might be beneficial to use something like the TL431 (LM431) shunt regulator to turn on and off the charge based on some target voltage start/stop values? \$\endgroup\$
    – EngMarc
    Feb 2, 2019 at 14:36

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