Author Topic: Current limiting device  (Read 421 times)

veggie

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Current limiting device
« on: May 11, 2024, 03:37:22 AM »
Can anyone help me with a recommendation for a device that can limit DC current from a battery bank to a device?
In this case the 24 volt battery bank is connected to a 1000 watt grid tie inverter.
When connected, the inverter immediately draws the full 1000 rated watts and runs at it's maximum capacity (and heat) with no restrictions for longer life or safety.
My goal is to restrict the supply current to 800 watts (33.3 amps) by installing some device between the battery and the inverter.
For low wattage circuits a light bulb can sometimes suffice, but for 800 watts, I have no idea what would work.

If a big resistor is a possible solution, I need some help sizing it.

Any recommendations or instruction ?

thanks
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- JiangDong R165 Air cooled - 2 kw
- Changfa S195 (Waiting for a project)

CS6_owner

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Re: Current limiting device
« Reply #1 on: May 11, 2024, 09:33:07 AM »
Some (most/all/?) inverters have a setting that limits the max power output (ie: limit a 4 kw inverter to max 3.2 kW output).

This would be the best solution. Don't know if your inverter has this possibility...

veggie

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Re: Current limiting device
« Reply #2 on: May 11, 2024, 03:02:04 PM »

CS6_owner,
This inverter has no settings. It's a small 1000 watt unit.
Just plug-and-play.
I want to limit the power going to it at 800 watts.
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- JiangDong R165 Air cooled - 2 kw
- Changfa S195 (Waiting for a project)

CS6_owner

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Re: Current limiting device
« Reply #3 on: May 11, 2024, 10:00:51 PM »
I think this is the only practical solution possible.

I think that if you put a resistor in series with the inverter that would consume 200W, that you will draw 1200W from the battery. The inverter will increase input current to get to its 1000W (actually VA) rating (up to its max input current).

You would also lose constantly 200W, not the most efficient solution.

It is maybe possible to build a DC chopper that would limit the input power to 800W, but I don't think you could buy something like that. You would have to design/build it yourself (you would turn a switch (mosfet or something like that) quickly on and off while you constantly measure that you take only 800W from the battery).

Your best bet is to buy a inverter that has the possibilty to limit max output. Sell the unit you have now...



Why do you want to limit the 1000VA unit to 800VA?

« Last Edit: May 11, 2024, 10:04:50 PM by CS6_owner »

BruceM

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Re: Current limiting device
« Reply #4 on: May 12, 2024, 05:55:02 PM »
Hi Veggie,
I always enjoy your interesting and well done projects!

I do concur with CS6_owner.
Without more info on the system, it is hard to see the value in reducing the draw on the 24v battery to 800 vs 1000W via dropping resistor since roughly half the savings are wasted as heat.  But do drop me a note if you'd like to discuss it in more detail.

If the specific model inverter is doing MPPT, adding a dropping resistor may do the job, but as CS6 points out, you then have added an expensive power eating and heat generating element so that battery draw will still be perhaps 900W while the inverter is stuffing 800W into the grid.  You may also achieve the desired result by adding resistance in the AC output feed, again, depending on the inverter design; it will see line voltage getting too high and may back off- or for some designs may just shut down, thus thwarting your plan.  The specifics of the inverter design matter and it can be hard to find such detailed design info. 

Calculating resistance is as simple as ohms law and there are plenty of online calculators.  The problem is that the specific characteristics of your inverter's mppt circuit or output voltage regulation scheme matter.  You will be at something under 40 amps on the 24v side or under 4 amps if on the 230VAC side. If the MPPT circuitry would cooperate appropriately with a drop of 4V, the R=V/I or 4/40 or 0.1 ohm and the needed wattage is 4x40 or 160W minimum (better 250 to 300W).
Here's and adjustable one ($33 US):

https://www.digikey.com/en/products/detail/vishay-huntington-electric-inc/AVE030020ER100KE/269995

Since there is no guarantee of how this may work out with your inverter I would consider testing with a coil or length of  steel fence wire before popping for a power resistor.  Here's a calculator (select iron):
https://www.omnicalculator.com/physics/wire-resistance

A battery jumper cable to make the steel fence wire connection will allow adjustment. Plated wire will change the value somewhat.  A milliohm meter would help.   I use both 3/16 mild steel rods and galvanized steel strip resistors as adjustable current limiter for my 36/24V battery powered DC stick and scratch TIG welder.  I used the 3/16 rods for initial testing, good data is available online for mild steel wire resistance.  SInce your 24VDC side is at only 40 amps, not the 160 amps of my stick welder, you should  be fine with roughly 7 foot of 1/16th inch steel wire for brief testing.  One useful trick is to measure the resistance of a longer length, minus the meter lead resistance, so you can get a reasonable estimate for your needed roughly 0.1 ohms. 1/10th of the length that measures 1 ohm, for example.

If I was trying the resistance  on the 230V AC side (roughly 4 amps), and was shooting for say a maximum of 15V rise,  that would put my target resistance at 15V/4A or 3.75 ohms at 60 W (better at least 100W).  Adjustables are handy for this and not hideously expensive.

https://www.digikey.com/en/products/detail/ohmite/D100K4R0E/4298783

As CS6_owner points out, choosing an inverter with a programmable set point may be the easier and cheaper solution unless you have the spare time to experiment.  There are microinverters that are designed for 350-400W, 24V nominal solar panels. Perhaps a couple of those in parallel...

Best Wishes,
Bruce M






ajaffa1

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Re: Current limiting device
« Reply #5 on: May 13, 2024, 11:21:25 AM »
Hi Veggie, I am a little curious as to what you are trying to achieve. You have told us that you have a 1000 watt grid tied inverter connected to a 24 volt DC battery bank. How are the batteries being charged, solar panels? Are the batteries being charged from the grid during the day and discharged back into the grid at night?  Are you trying to run off grid at night? Much more info required please.

Bob

veggie

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Re: Current limiting device
« Reply #6 on: May 13, 2024, 09:06:50 PM »

Thanks for the comments guys
After seeing your comments, I appears that trying the throttle the output to the inverter would waste considerable energy as heat and reduce a lot of the gains I'm trying to get with the grid tie inverter. I think I will simply leave the inverter connected to the 800 watt PV array and keep things simple.  :)
Driving these cheap Asian grid tie inverters from a battery bank generally ends up with a bunch of burned MOSFETS.
« Last Edit: May 13, 2024, 09:12:01 PM by veggie »
- 6/1 GM90 Listeroid - Delco 33si Alternator
- Changfa R175 - Lease/Neville Alternator
- JiangDong R165 Air cooled - 2 kw
- Changfa S195 (Waiting for a project)