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I need to check a 48V battery with dummy loads of 1000-2000 watts for 5-10 minutes.

Perhaps using Nichrome wire, i can wrap some around a fire brick submerged in plenty of water and adjust the load by changing the length used?

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  • \$\begingroup\$ Pretty thick nichrome wire - that's 42A ! \$\endgroup\$ – Icy Nov 3 '15 at 8:39
  • \$\begingroup\$ TE make high power resistors. see alliedelec.com/te-connectivity-te2500b1r0j/70288885 \$\endgroup\$ – Icy Nov 3 '15 at 8:43
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    \$\begingroup\$ 12v headlamp bulbs in series, or 120v/240v halogen floodlight bulbs could make a not too expensive, 'easy to keep cool' and very adjustable load. \$\endgroup\$ – Neil_UK Nov 3 '15 at 8:44
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    \$\begingroup\$ With headlamps you'd need 4 in series but then several of those series groups in parallel to add up to the 42 amps. You only need 1.14 ohms, so you might think of using a fair length of hefty steel ribbon wound on a ceramic base. Connections would need to be very solid, if not welded. \$\endgroup\$ – Nedd Nov 3 '15 at 8:53
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    \$\begingroup\$ It might be worthwhile to consider an Off-the-Shelf 48V DC -> 240V AC inverter. 8A is more manageable, and simple space heaters will easily dissipate 1-2 kW as heat. \$\endgroup\$ – MSalters Nov 4 '15 at 11:35

10 Answers 10

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It's probably cheaper to buy several 120V AC kettles. A single kettle will be about 2 kW when powered from 120V AC but at 48V DC this will be about 16% of the power at 320 watts.

Three kettles full of water gives you 960 watts. Add more kettles if you need more power.

Bonus, after doing the test the water may be hot enough to recycle or even make a cup of tea from.


Addendum

It's worth mentioning that the kettle heating element might be 10% lower in resistance when cold so there may be a a peak in power of 10% falling to rated value over the period of 1 minute or so.

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    \$\begingroup\$ That's certainly a novel approach. \$\endgroup\$ – Nick Johnson Nov 3 '15 at 9:34
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    \$\begingroup\$ Not so novel - reminds of a company i worked for some quarter century. We used heating plates and kettles as dummy load to test solar inverters. Were working in t-shirts in winter and drank a LOT of tea in these days.... \$\endgroup\$ – Gyro Gearless Nov 3 '15 at 11:29
  • \$\begingroup\$ Some Haas CNC machining centres have a stack of what look very like cooker elements as braking resistors for their axis motors. \$\endgroup\$ – user1844 Nov 3 '15 at 17:09
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    \$\begingroup\$ If you want a more professional-looking solution, you could buy water-heater elements (basically the heating element from an electric kettle) or maybe cartridge heaters and screw them into a tank. \$\endgroup\$ – Nick T Nov 3 '15 at 17:13
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    \$\begingroup\$ We used to use banks of incandescent light bulbs to test 400VDC-out power factor correction boost converters. 4 in series x however many strings you need. Certainly a "bright" idea... \$\endgroup\$ – Adam Lawrence Nov 3 '15 at 20:40
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I'd recommend using power resistors.

Basically, you can buy really big wirewound ceramic resistors that look like pipes, and then all you need to do is keep them cool.

For your requirements, try using 5 of these in parallel:

300W 10-ohm power resistor

You can mount them to a sheet of plywood for easy moving-around with some of these mounting kits.

Then, all you need to do is get a reasonable desktop fan and point it at the dummy-load, and you're good to go!

edit: MDF is probably a better choice because it's more rigid. I've used it with pretty good success for this exact application (albeit with a different dummy load).

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  • \$\begingroup\$ Plywood??? I would suggest an aluminum sheet of about 3/4mm thick. It has a far better heat conduction than plywood. Is also less likely to catch fire if things go south fast. \$\endgroup\$ – Weaverworm Nov 3 '15 at 9:00
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    \$\begingroup\$ @Weaverworm - You actually want something with low heat conduction in this case, so that your test jig can be picked up and moved safely even when the resistors are hot. \$\endgroup\$ – Nicholas Clark Nov 3 '15 at 9:01
  • \$\begingroup\$ If you want 42A drawn you'd need at least 8 of the 10-ohm resistors in parallel. Add another heavy adjustable resistor (about 50 ohms) to calibrate the load. \$\endgroup\$ – Nedd Nov 3 '15 at 9:09
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    \$\begingroup\$ @Nedd - True. My calculations were for the low end of the OP's range. For the high end, one would probably want different resistors of the same general type. \$\endgroup\$ – Nicholas Clark Nov 3 '15 at 9:11
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    \$\begingroup\$ @Weaverworm, the heat capacity will be rather large, and you might not want to wait several tens of minutes after turning off before being able to move it. It's also poor design practice to make the parts of the case designed to be handled get hot, and even for personal test kit that's something I'd stick to - you might be working right next to it. \$\endgroup\$ – Chris H Nov 3 '15 at 9:45
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A professional solution to this problem would be to use an electronic load which are available from numerous manufacturers.

They often are programmable via a PC, so you can test your power supply or battery in different ways which are closer to the load-profile of your application.

They also come with different load modes. Constant resistance, constant current, constant power are pretty common, some offer constant voltage as well.

Some of them come with the possibility to dump the power back into the grid (sometimes called "energy recovery") so your tests would be more environmentally friendly than just heating. (if you are allowed to)

Of course these things are expensive, if you do those tests for a living and need your systems to be tested thoroughly I'd say it's worth the money.

(I know this doesn't answer the "How to make part" but is an advice on the "I need to check" part)

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Back in 1987, we needed a load to test a MIDI-controlled stage-light dimmer pack that we developed, which consisted of 6 channels of 1200 Watts each for a total of 7200 Watts.

We used "water heater elements" to test it at full power and it worked.

We weren't sure if water heater elements would "burn themselves out" if we ran them directly in the air, so we built something out of ordinary parts.

We purchased a large metal trash can, 6 water heater elements, 2 garden hose adapters, and various parts. We cut 2 holes along the side for garden hose, one near top and one near bottom, then brazed the fittings. Cut 6 holes in bottom for water heater elements and sealed. Hooked up 6 extension cords to the elements.

We put the trash can on top of bricks to protect the electrical connections at the bottom. We connected an inbound and outbound garden hose. We turned on the water and filled it up, then varied the water rate until the inflow and outflow were close enough to prevent it from overflowing.

I still have it in my attic, but if we had to create one again then likely would mount the heating elements on something that would hang down from the air and dip the elements in the water, because it would likely take less time to construct.

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  • \$\begingroup\$ A domestic water heater usually has two elements. So that's an off-the-shelf solution if you only need two elements. \$\endgroup\$ – markrages Nov 3 '15 at 20:04
  • \$\begingroup\$ You tend to find that immersion heaters fail almost immediately if any part of them is not immersed. So your top-mounting method would probably not work for very long unless you really did arrange to keep the can completely full. \$\endgroup\$ – user1844 Nov 3 '15 at 20:35
  • \$\begingroup\$ Thanks for the immersion tip, it appears our guess in the 1980s was right. The water heater idea wouldn't work for us, because we needed 6 unique heater elements for 6 different loads. A water heater didn't allow us to chose a maximum wattage per element. Buying multiple water heaters might be a solution if we replaced the elements, but far more expensive than our price goal. We chose the element that met our needs then found a solution to mount them. \$\endgroup\$ – Steve Meirowsky Nov 3 '15 at 22:06
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Google for '48V water heater element' - these are now a commodity item thanks to small-scale wind power. These are used as 'dump' or 'diversion' loads to avoid running a wind turbine with no load in high winds, to prevent it spinning too fast.

There's one I can see here on a renewable energy website rated at 48V 1000W, selling for £49.

It'll need a medium-size tank of water to heat: if the tank contains 5 litres of water, a back-of-the-envelope calculation gives < 30 deg C temperature rise after 10 minutes: 1000W * 600s = 600kJ, 600kJ / ( 5kg * 4.2 kJ/kg.C ) = 28 C. So an empty paint can or cooking-oil container would probably do the trick - obviously this must not be sealed under any circumstances.

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I had to create a 2kW load for a project once. It was a bit dangerous but it could be done safely. I used two pieces of copper pipe in a bucket full of water with baking soda to create some ions. The copper was held in place on the top and bottom of the bucket with plastic frames. Also depending on the amount of baking soda, the resistance changes.

You could do the same thing but more accurately using water heater elements. this is much better then a large resistor because water is one of the best thermal sinks.

I have also seen people use metal screening (thicker then what you use for a window, similar to chicken wire) for high power loads, a strip of it can work well.

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Consumer electric radiant heaters are fairly inexpensive, and require no water. They are designed to dissipate 1,500W of heat safely. You'll need a handful of them to meet your needs, since you're running a lower voltage. Put them in parallel until you reach the power dissipation you need. Don't use the smaller heaters that have fans - the fans won't run properly at the voltages you're using, so make sure to select radiant heaters without fans.

The nice thing about this is that you can run them for much longer periods of time, there's no water to replace or spill (possibly causing burns), they are lightweight, easy to move and store, and very inexpensive, particularly if you're willing to buy used.

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From a different direction - so called 'dummy loads' for testing ham radio equipment are readily available. As one example, the MFJ-250 (MFJ Enterprises) is rated for 1kW for 10 minutes and costs about $70. This is a 50 ohm load. No affiliation, just the largest dummy load in the first catalog I picked up.

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  • \$\begingroup\$ An RF dummy load adds the substantial additional complexity of needing to be a matched, hopefully resistive impedance at frequencies of interest. The need in this question is merely for a DC load, so kettles, bathroom heaters, light bulbs etc suffice here, where they would not for RF. \$\endgroup\$ – Chris Stratton Nov 8 '15 at 3:50
  • \$\begingroup\$ True, one needs to be careful. Don't use a microwave dummy load. The most common are DC to ~400MHz nominal 50ohm across that. \$\endgroup\$ – Jon Custer Nov 8 '15 at 13:44
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As Icy points out, 2000W is about 42A at 48V. That's about 1 ohm. If you want to wind your own, I wouldn't use nichrome - too expensive and the resistance is too high.

I'd use galvanised steel fencing wire.

According to http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/ iron and steel have a resistivity of about 1E-7 ohm-metres, and stainless is nearer 1E-6 ohm-metres.

So 10m of 1mm^2 wire should be about the right resistance. Measure the resistance to be sure. To avoid things getting too hot, I'd go for a longer, thicker wire, or several in parallel. You could run big hairpins of wire outside, or wind it round a large concrete former. If you can get plastic coated wire, you can coil it in a drum full of water.

The biggest issue will be terminating the wire without heat affecting the terminations. Solder-on copper terminations perhaps? This is less of an issue if you have them under water, but watch out for electrolysis. Also, be sure you don't short-circuit your load! (I guess it goes without saying that you should have a proper fuse in series with your dummy load.)

I really like the kettle idea. The problem is, it seems from your profile you are in France (?) and kettles in Europe are 240V not 120V, so you'd need a lot more of them for a 48V battery.

I've made my own wide-spaced nichrome coils. They look pretty innocuous and feel pretty cool even when you're 1cm from them. It's only when you brush against them and get burned that you realise how hot they are.

EDIT

I've tried this out on a 3m length of 1.5mm^2 galvanised wire that I happen to have. Measured resistance was 0.2 ohms for a 1cm length and 0.6 ohms for a 3m length, so the resistance of the connection is quite important. Steel's not easy to solder with tin/lead solder. Apparently silver solder works well, but you need a blowtorch. Tightly wrapping the last 1 foot / 30cm in aluminium foil is my latest idea of how to terminate it. I also tried to measure the resistance of my fence that has been outside for years, and found that the connection between my meter probe and the weathered surface of the galvanised wire had a resistance of several tens of ohms.

This shows one advantage of nichrome over galvanised - it's virtually non-corrodable, whereas galvanised might need some scraping.

Anyway, having established that the current would not exceed 30 amps, I carefully connected my 3m of wire across a 12V battery. It was possible to hold the wire for several seconds before it got too hot. I then decided it was time to stop the experiment.

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  • \$\begingroup\$ steel solders well with bakers soldering fluid and ordinary solder, don't use this corrosive flus with your good soldering iron/ \$\endgroup\$ – Jasen Nov 8 '15 at 2:45
  • \$\begingroup\$ Awesome, very interesting. Plus points for finding an easily re used material, it's good for the ecosystem. I would indeed need 12 kettles to check 2kw in the european union. Will have a go at all the suggestions from all the questions if i can get the money for it. \$\endgroup\$ – com.prehensible Nov 8 '15 at 12:06
  • \$\begingroup\$ @Jasen I'd never heard of that - so it's nothing to do with the silver solder and all to do with the flux? Googling for Bakers soldering fluid throws up a lot of plumbers supplies. But it is also available at this UK electronics supplier maplin.co.uk/p/bakers-no3-soldering-fluid-250ml-r32rh (probably cheaper at the plumbers.) \$\endgroup\$ – Level River St Nov 8 '15 at 18:35
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Perhaps the most frugal method would be to submerge 14 meters (45 feet) of enameled 0.5 mm (24 AWG) copper wire in a tub of water, for a resistance of 1.15 ohms. Solder joints to thicker cables should be kept submerged and electrically isolated from the water in order to prevent melting and/or corrosion of the contraption. I have personally done this with a quick coat of silicone rubber over the exposed metal.

I have used this method several times without problems after being inspired by a certain YouTube video from the channel mikeselectricstuff.

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