# 270v DC current limiting circuit

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

• 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. – jonk Jan 31 '19 at 21:12
• Is the "internal DC system" also a battery? If so, do you know what type? – Nate S. Jan 31 '19 at 21:16
• 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. – user253751 Jan 31 '19 at 22:45
• Yes, the internal DC system is a battery - composite NiMH – EngMarc Feb 1 '19 at 1:32
• what's the maximum voltage difference, these lithium batteries tend to change voltage with state of charge. – Jasen Feb 1 '19 at 3:35