high power dc load - Page 1
Forum section: electronics / projects

author: classified
author_rank: Contributor
timestamp: July 10, 2022, 10:37:10 am
content: hi alli am building large battery packs (96v nominal 111v full charge upto 300a discharge 250 amp hour capacity) and i would like a dc dummy load able to handle this size batterynow i know there are some already made out there but there ether too exspensive or not powerfull enughthe best one i can find right now is onaly 600w wich is onaly about 7 amp discharge wich would take a long long time to discharge the battery i am aiming some ware around a 5kw to 8kw dummy loadand i know heat will be a big issue but i am going to use water cooling to keep everything coolso i was wondering if there is a way i could do this as a diy projectnow i will also say i know i could use a large dummy load resistor and a cheap power moniter but i would like a constent current dummy load not a constent resistenceany help is grately apreciatededit 1: hi there i think there is some confutin going on here i sead i would like to stay away from resistive loads i was thinking of something like this item herehttps://www.aliexpress.com/item/32821877897.htmlonaly made to deal with higher poweredit 2: ok to avoid any more confution here when i say water cooled i mean it as i use for exsample a cpu water block (but bigger oviusly) to keep it cool rather then useing just heat sinks or forced air

author: geggi1
author_rank: Frequent Contributor
timestamp: July 10, 2022, 11:30:26 am
content: You will need something that can handle approxomately 28kW.The simplest and cheepest way is probably to make a water load.You will need a large water container. A 200l plastic barrel will work pretty well.Cut open the top end so its like al lage bucket.Then you need two electrodes these can just be regular steel because conductivity is not very important.You need to make some kind of fixture to keep the electrodes apart. It must be possible to adjust how deep the electrodes goes into the barrel and the distance between the electrodes.Fill the barrel with tap-water and connect the battery to the electrodes.Slowly adjust the depth of the electrodes until you got the wanted current.Another way to adjust the current is to install the electrodes in the barrel and then fill water until you got the correct current.If you dont get high enough current you can add table salt slowly until the current is correct.Be aware!You will produce hydrogen gas and must do this out door or in a place where the electrical installation is explosion proof.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: July 10, 2022, 11:47:34 am
content: And if you add salt to increase conductivity, you will also produce chlorine gas.

author: Psi
author_rank: Super Contributor
timestamp: July 10, 2022, 12:17:43 pm
content: hm.. 28kW constant current load.About the only thing I can think of that would be somewhat easy/quick to build would be a big water container.Inside it you'd have 8 separate wire circuits and each would have a different length based on a fixed length, eg 1x that lenght,2x that length,4x,8,16,32,64,128.You'd then have 8 high current mosfets/IGBT bricks to connect/remove each circuit from the source.Since you have the wire circuits in binary multiples you can combine them in all possible combinations as 8 bits of 1 byte to make 255 possible resistance steps.Add a hall effect current sensors and some MCU code to read current and set the number of wire circuits connected dynamically to seek to your desired current in real time.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 10, 2022, 12:21:06 pm
content: I was on d/d project years ago for a batch of battery test dummy loads probably rated about 30 kW . We used stainless steel dynamic braking resistor banks made by Australian division of a British vendor.I don't see these resistor banks listed now on their web page.The image on this other company "Brake Resistors 120 kW" closely resembles what we used.https://www.danotherm.com/power-resistors/cooling-method/self-cooled/steel-grid-resistorsAbout 3 such banks rated force air cooled were mounted in a steel housing like a "coffin-on wheels"There were about 3 240 V axial fans over the banks, airflow inlet under, vertical upwards and a panel on the side with tap switches and meters.The vendor provided banks assembled using their standard stainless element packs to get coarse and fine taps.I recall doing some adjustments to the tapping configuration on the first off protgotype.At this power level I don't think water cooling is desirable.One reason is if the dummy banks are infrequently used it is a corrosion problem for the water cooing galleries.Also pumps and cooling towers, deionizer etc are needed which will probably at least double the equipment cost.But water cooling may be necessary if user location can not run the banks in adequately ventilated area.

author: eugene
author_rank: Frequent Contributor
timestamp: July 10, 2022, 12:25:25 pm
content: Do I understand that you want a device that will handle 300A with 111V across it? In other words a 111V/300A = 0.37 ohm resistor that is able to dissipate 33300W of heat?33300W is about 115000 BTU. In the northern US, a typical 2500 sq ft home will have a 120000 BTU furnace for central heat. Besides heating my house, I'm not sure how I would get rid of that much heat, even with a low duty cycle.Alternatively, 33300W is about 45 HP. You could setup a 50 hp motor to do something useless, like pump water from a tank up to some height and then let it drain back into the tank. Might as well make it a decorative fountain. The motor would likely be 3 phase AC, so an inverter would be needed to run it from a DC battery.These aren't necessarily good ideas. I'm just trying to express the amount of power involved.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 10, 2022, 01:30:56 pm
content: Quote from: eugene on July 10, 2022, 12:25:25 pmYes it is a consideration for indoor use especially if aircon .It is hard to tell on eevblog, but I think Classified is asking about a design to be used in UK workplace and not a hobby shop?The air cooled load banks about 30 kW I mentioned in #4 were in compliance with Australia standards and workplace regulations, and prototype verified at the time:DC rated tap switches and fusingAll surfaces with the temp rise less than workplace touch limits ( I forget, around 40 C ?)The air flow axial fans were rated to keep the exit air low, I recall the air was warm not hot.These things were on wheels, for use indoors for testing UPS batteries in electrical utilities substations, power generation and telecom.

author: horvat.kamca
author_rank: Contributor
timestamp: July 10, 2022, 10:18:56 pm
content: Axiom is making fairly beefy DC loads, one of them could suit your needshttps://www.axiomtest.com/Electronic-Loads/DC-Electronic-Loads,-More-than-10-kW/

author: horvat.kamca
author_rank: Contributor
timestamp: July 10, 2022, 10:32:11 pm
content: Quote from: eugene on July 10, 2022, 12:25:25 pmIt may seem like a lot, but if you take into account exceptionally high high specific heat capacity of water, it's only boiling off 1/10 of liter of water each second (with starting temperature 20 deg C). Actually, it may be a little bit less, since I didn't factor in heat of vaporization.According to my calculations, to fully discharge 250Ah@111V battery , you'll need only about one third of cubic meter of water.In the end, probably the best way to test such large battery packs may be to build two of these, and then discharge one and use the energy to charge another battery pack and monitor current/voltage during the charging/discharging process.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 11, 2022, 04:11:19 am
content: I remember the idea of converting a car engine into an air compressor to load test another car engine outside of a car. But that's when you're starting from mechanical power so while it might be worth thinking about if you have a use for a really powerful air compressor, it probably wouldn't be the cheapest solution in your case.Most likely the cheapest would be to get a bunch of 120V heating elements and PWM them to fine tune the load.Quote from: horvat.kamca on July 10, 2022, 10:32:11 pmUsually, such batteries would have much lower charge rate limits than discharge rate limits, but it would be an easy way to recover energy if you have at least 2 batteries to test.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 11, 2022, 11:24:03 am
content: a bunch of electric space heaters should do the job. They don't mind whether they get AC mains voltage or DC battery voltage, Except for the switches, but it's not so difficult to get some MOSfet or DC capable solid state relays to switch your resistors.Or use water heating elements such as used in electric boilers or washing machines.

author: CaptDon
author_rank: Super Contributor
timestamp: July 11, 2022, 01:44:00 pm
content: Electrodes in water as dummy load, worst idea I have ever heard. Some of the metal would become sacrificial and you would end up with a murky mess. How big would the electrodes have to be to actually be capable of 300 amps of current at the listed voltage? I know of an aluminum boat with an outboard motor that was mis-wired with the boat being 'hot' at a fresh water marina. It didn't trip the 30 amp breaker (this was far prior to the GFI installations, 1965-ish) and the boat sank after an estimated 1 week. The entire bottom of the outboard was gone with the exception of the stainless steel shafts. I used to have a picture of it for marine safety seminars. This is how swimmers get electrocuted in marinas!!!

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 11, 2022, 02:06:58 pm
content: I agree with Capt that for this application water cooling might be too costly.The following issues might apply for battery DC resistor banks unless suitable electrically isolated resistors can be used:In the larger induction heating systems, the DC rectifier stacks are still contact water cooled although in smaller units the isolated modules are now used.To cool DC cathode side without catastrophic corrosion, entails non conductive hose, and deionisers with conductivity monitors, and periodic water changes. In addition DI water can not be exposed to atmosphere so 2 dual loop water reciculationis used with tower water in the primary.In most countries, workplace water tanks and recirculating systems have regulations against Legionaires .So biocide dosing, periodic cleaning, record keeping and maybe inspection might all add to the cost.Edited to add the UK industrial water cooing Licensing:https://www.gov.uk/cooling-tower-notificationI see that completely closed water cooling systems are exempted, however they are less efficient hence more costly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2022, 02:21:34 pm
content: If you can waive the constant current requirement, I would also suggest resistors. As noted, contact water cooling is a bad idea around DC (it does work alright at AC), but that's fine, there are plenty of heater elements you can get which are isolated and good for multiple kW, water heater elements for example.If not, I have a design which can probably do that cheaper than most, though it's not going to be "DIY"; frankly I wouldn't trust an end user to assemble it, not at these power levels -- you understand, nothing about your capability, just basic liability.For that matter, probably not even a good idea to supply such a unit at all...Which begs the question -- where did you get these batteries from, and what does the manufacturer recommend? Do they recommend this service at all? What tools do they suggest for performing this function? If not... that's your hint. Just, don't.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: July 11, 2022, 02:30:29 pm
content: I used to work for a company that was making equipment for battery testing.......$ for a 12 channel tester, you would need to connect a few of them parrallel , water cooled, connected to a .....$ water/air heat exchanger, accurate like a 6.5 digit multimeter. You need deep pockets. DM if you are interested, I put you in contact with the company.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 11, 2022, 02:32:48 pm
content: Some years ago I designed a UPS test load for the product testing bay of a USA UPS manufacturer.The test load I think for 10 kVA single phase 120 V had to comply with an IEC spec with specific load values at various kVA steps to measure efficiency and transient response etc..I wonder if similar standards apply to the DC side, I think it is likely in the merging PV arena.

author: Psi
author_rank: Super Contributor
timestamp: July 13, 2022, 01:34:06 pm
content: You could buy like 30x 110V home hot water cylinders.Group all the heating elements into some number of parallel and series combinations with an FET/IGBT/SSR module on each to optionally short it out and remove it from the series string it's in etc.. eg increase current flow in that string.If you do the math to figure out the best combination/group sizes you should be able to build a system with enough switchable combinations to do crude current regulation. At least to a degree that should work for your narrow 96V to 111V range. (I assume you want current regulation to keep the current static over a discharge period as the pack voltage falls?)An advantage with hot water cylinders is they're all setup for continuous water coolingYou could have water pumping in a loop through them all and into a big storage tank/radiator before going back into the cylinders. You could probably build a 33kW system that can run 24/7 if you wanted to.Just make sure you leave all the safety systems intact, mythbusters showed just how dangerous a hot water cylinder is if it fails from overpressure.

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 13, 2022, 08:37:28 pm
content: Quote from: horvat.kamca on July 10, 2022, 10:32:11 pmWater heat of vaporization is pretty enormous, so it will be the dominant contributor to the energy input. If I calculate correctly, it takes 226 kW to vaporize 100g of water per second at normal pressure, starting from the boiling point. 33 kW will boil away 13 ml per second starting from 20 C, and the original 5 - 8 kW requirement in the OP would correspond with 2 - 3 ml per second.

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 14, 2022, 04:39:17 pm
content: At this voltage, 120V space heaters are probably the cheapest (used can be bought cheaply). Most of them can dump the heat continuously, passive convection ones will be preferred.Water load if you can keep the mess. I know how dirty water loads for welding power supplies tests get after a while.They can be pretty small.Water load must be kept outside. For heaters, it is recommended if you do not want to be bakedAnother option is a water heater, pump, and "big" radiator, a typical car radiator with a fan can easily dump this power level.

author: shan
author_rank: Newbie
timestamp: July 21, 2022, 12:44:09 pm
content: My first day on the forum and I find someones asked my question just a week or so ago.Longer term I'm thinking of building a variable resister bank.. probably just switching parallel resister sets in and out to control discharge rate. Somewhere in the range of 150kW. In the short term I'll start up with single cell testing. I'd love to hear how you end up solving the problem.(sorry not an answer - just happy to be here.)

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2022, 06:20:30 pm
content: For the resistive part of the load, I would make a water cooled 0.4 ohm resistor using copper or brass tubing. The water could be pumped through the tubing rather than immersing it, saving space and weight.That still leaves making a transistor load which can handle 60 volts at 150 amps or 9 kilowatts, but that is considerably easier than 120 volts at 300 amps and 36 kilowatts. I am assuming that pulse width modulation is not acceptable.