DC-DC Current Limiting

Thread Starter

musclesbenz

Joined Oct 14, 2016
11
Hello,

I'm looking to design a stupid simple DC-DC converter with two goals only:
  1. Current regulation to no more than 90 amps
  2. Lowest possible cost (total cost sub $50)

Can it be done?

The power supply is a Prius. The Prius Gen 2 DC-DC converter (used instead of an alternator to charge the onboard 12v battery) is capable of supplying up to 100 amps with an output of 13.8v.

The load is charging a 12v deep cycle Battery connected to a 3000w inverter.

Some potential strategies might include:
depletion mosfets as current limiting devices

Buck Boost Converter

Power Jfets?


Further info for those who are interested:

The reason for having the deep cycle battery is that I have intermittent loading of the inverter that will exceed the 100 amps that the Prius can provide, and to keep from ruining the electronics on the Prius I need a current limiting circuit.

I'm not terribly concerned about power consumption of the current limiting circuit, as the Prius will provide ample power, but obviously efficiency is always better.

If the solutions you come up with supply less than 100 amps, I'd be willing to consider them, especially if the price is right, as the inverter has a low-voltage cutout which would notify me that the Prius isn't able to keep the deep cycle battery charged sufficiently.







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LowQCab

Joined Nov 6, 2012
1,619
90-Amps and ~$50.oo don't mix well.
Why do You believe that You will damage the Car's Electrical-System if You exceed ~90-Amps ?
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Thread Starter

musclesbenz

Joined Oct 14, 2016
11
90-Amps and ~$50.oo don't mix well.
Why do You believe that You will damage the Car's Electrical-System if You exceed ~90-Amps ?
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The Toyota spec sheets for the Prius state that the DC-DC converter used to charge the 12v system can supply no more than 100 amps at 13.8v. I tend to believe engineers when they tell me the specs.


What about 30 amps?
 

LowQCab

Joined Nov 6, 2012
1,619
"can supply no more than" is not a "Specification" and indicates nothing.
This may simply mean that the Voltage Output will "sag" when drawing more than 100-Amps.
It "may" be perfectly Self Regulating and Self-Protecting, and
be ideally suited to doing exactly what You want.

What is your idea of "intermittently" ?

Are You sure that your Inverter is actually capable of drawing over ~200-Amps without starting a fire ?

Do You have the appropriate "Welding-Cables" for the input of your Inverter ?

This is a hideously inefficient way to get lots of AC power.

Every time You make a "conversion" of energy,
You will loose around ~15% to 30% of that energy to Heating the surrounding Air.
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Thread Starter

musclesbenz

Joined Oct 14, 2016
11
"can supply no more than" is not a "Specification" and indicates nothing.
This may simply mean that the Voltage Output will "sag" when drawing more than 100-Amps.
It "may" be perfectly Self Regulating and Self-Protecting, and
be ideally suited to doing exactly what You want.

What is your idea of "intermittently" ?

Are You sure that your Inverter is actually capable of drawing over ~200-Amps without starting a fire ?

Do You have the appropriate "Welding-Cables" for the input of your Inverter ?

This is a hideously inefficient way to get lots of AC power.

Every time You make a "conversion" of energy,
You will loose around ~15% to 30% of that energy to Heating the surrounding Air.
.
.
.
You make a good point about Toyota's "Specifications". I'll see what information I can find about the Prius DC-DC converter. I'm a little hesitant to experiment, the cost of repairing that part is more than I paid for the entire car, but if I can find evidence of someone else doing something similar, or a schematic of some sort that I could analyze, I certainly would give it a shot. I have done some searching, but haven't yet come across anyone in any of the Prius forums that has successfully placed a significant load on the Prius low voltage battery.

"Intermittently" to me would mean I'd be loading the inverter to 1800 watts for no more than 30 minutes at a time, the rest of the time it would be 1800 watt draw for 30 seconds, with 5 minutes of rest (~200 watts) in between each use. Up to 12 hours of total run time in a day. Honestly I was planning to set my usage characteristics based on the outcome of this project. I can always make use of more power (additional lights/heat).

Also, I realized that I mistyped the output of the inverter. It is actually a 6000 watt modified sine wave inverter (produced by Durified & provides up to 12000 watt surge). I purchased that size in order to start the larger (15 amp) motor loads such as a table saw and chop saw. I have not yet tested the inverter, as I plan to resell it if I can't come up with a DC-DC current regulation scheme.

I've attached 2' of 2/0 welding cables with crimped and soldered lugs, I believe that will be sufficient for my needs.

Incidentally, small gasoline engines are also a horribly inefficient way to generate power, particularly in a scenario where they are running all the time, and only supplying power 1/10th of the time. A Prius would automatically kick off when the main 200v battery is charged, and then kick back on when the 200v battery needed more charging. Effectively eliminating unnecessary idling that a regular generator would do. According to what I've read on various forums, it approximately matches most non-Honda generators in fuel use for standard backup power.
 

LowQCab

Joined Nov 6, 2012
1,619
So ....... You're using a Prius for a worksite Generator.

Did You realize that most Power-Tools will run on DC, directly from the Main-Batteries ?
( be very careful, 200V DC is extremely dangerous )

In any case, a 240VAC Inverter with a 200VDC Input would be far more efficient than
converting it down to 12-Volts, and then back up to 240V.
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Thread Starter

musclesbenz

Joined Oct 14, 2016
11

Thread Starter

musclesbenz

Joined Oct 14, 2016
11
So ....... You're using a Prius for a worksite Generator.

Did You realize that most Power-Tools will run on DC, directly from the Main-Batteries ?
( be very careful, 200V DC is extremely dangerous )

In any case, a 240VAC Inverter with a 200VDC Input would be far more efficient than
converting it down to 12-Volts, and then back up to 240V.
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.
.

Yes. Worksite generator.

Thanks for the heads up & warning. I guess I hadn't considered that my two main loads (Dewalt 120VAC Chop saw and Table Saw) might function off of the 200v battery.

Efficiency isn't terribly important to me, as this is a short term solution, but keeping low cost and simple is.
 

Thread Starter

musclesbenz

Joined Oct 14, 2016
11
The fact that the output from the DC-DC converter appears to self regulate when supplying loads over 100 amps makes sense from a reliability standpoint; taking the worst case scenario, a completely dead lead acid battery could potentially place a load of that magnitude.
 
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