Dummy Load Heats Like a Mofo...

[rg12]

Hey Guys,

Built a dummy load to discharge lipos.

Connected x4 100w 20ohm resistors in parallel, screwed them with thermal grease to a heatsink with a fan and connected a 7s lipo to it along with a watt meter to monitor, amps, volts, amps hours and watts.

The resistors heated up to a level I can't touch them within about 20 seconds discharging the pack at only 5A (130w).

How come 4 big a$$ 100w alluminum resistors (on a heatsink) heat up so bad from just 130w? I thought that the watts are spread onto 4 resistors making each resistor take about 33w.

Any ideas?

 

[amberwolf]

Soldering irons are around 15w on up, and they melt solder....

 

[rg12]

I don't mind them heating up, I just don't want them to go up in smoke and that temperature doesn't seem healthy for them...
What is normal temprature for them?

Also, if it heats up so much on 130w then how can I crank them up to 400w? (4 in parallel)

 

[Hillhater]

what voltage are the resistors rated at ?

 

[Sunder]

130w is a massive amount of heat to dissipate. Think about a PC, which is usually only 90w, and has a 85*C limit. Even that has a bigger and finer heat sink than most heat sinks you buy in electronics stores, and 85*C is hotter than you can safely touch.

Water has a very high specific heat. Perhaps you could put the non electric end of the heatsink in a tray of water.

 

[markz]

Other members have used stove top coils and some cheap heaters that were probably modded.

 

[Overclocker]

that's 2000w. i blow a fan to keep the coils from going limp. nichrome wire btw

 

[rg12]

Their voltage is not rated:
http://www.ebay.com/itm/General-1-5...var=650746519195&_trksid=p2057872.m2749.l2649
* I don't remember seeing voltage ratings on resistors...

I don't like those scary heat elements...

So what is the max safe operating temperature for them?

 

[marty]

Marty's battery load tester.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=1170&hilit=resistor#p36647

From 2007
Battery tester is done. We testing. Battery load tester is real HOT! [portable battery powered oven?] Don't try this at home kids. After charging voltage was 54.8V It's slowly going down now.

Don't trust the BMS's cut-off. If it doesn't cut-out before 48.1v -- 3.7v/cell -- stop the power there. Depending on the BMS cell balance function, some cells could be substantially lower, and well into permanent damage territory by that point.

Sorry Xyster, I would rather sleep then watch battery discharge. Do have voltmeter hooked up. Keep looking at voltage. Don't have time for battery baby sitting. According to chart on page 1 of this topic. BMS cut off is around 2.75V per cell or about 35 volts for 13 cells. Will hope and pray that it does stop around 35V ?

For those of you who care? Here is parts list for battery tester. WARNING! it gets HOT!

3/16" x 12" x 12" aluminum sheet
RESISTOR Alcol 100W 50 Ohm, part # HS100 50R 1%
LAMP HOLDER SPC Technology, Miniature Bayonet socket, part # 21722
BULB SPC Technology, Miniature Bayonet lamp 55V, part # 1835
FUSE HOLDER Cooper Bussmann, 1/4 x 1-1/4, Panel Mount, part # S-8002-1-R
FUSE Cooper Bussmann, 5A, 1/4 x 1-1/4, part # AGC-5
CABLE CLAMP Thomas & Betts, Catamount, .125 Diameter, part # N4NY-002-0-M
FEET Plastic Bumper, 5/8" McMaster-Carr part # 9405K18

Anyone wana buy 98 small cable clamps, or 96 plastic bumper feet? Will give you good deal!

 

[markz]

nichrome wire

Ah yes, I remember that method. Fast Tech sells that stuff, remember to select ePacket for faster shipping.
Now I wonder if you coiled that nichrome wire.

 

[Hillhater]

130w is a massive amount of heat to dissipate. ...
...Water has a very high specific heat. Perhaps you could put the non electric end of the heatsink in a tray of water.

At that voltage you could safely dunk the whole assembly in a bucket of water !
Personally, i use a selection of 100w rated , 24 v bulbs ( $2 for 10 from ebay !). Arranged to suit whatever voltage and current i want to dump. Lots of light , no issues, but you wouldnt want to grab hold of them !

 

[fechter]

Any ideas?

Use a fan. A hair dryer dissipates over 1kW.
130W isn't a crazy amount of heat but for sure you need some air moving or things will get really toasty. I'm sure those resistors are good for 200C or more.

 

[Inwo]

Don't make it hard. Use a real power resistor.
As little as $15
500 watts. Tap it with clip lead. May only need one for wide range.
If need cheaper. I may have some damaged ones pulled from the dumpster from time to time.

 

[markz]

Tried finding - 10pc 10x 24V 100W bulbs - Lowest is 30W I could find, but my search skillz suck.
I haven't really needed a dummy load, but I can see how I could need one.

 

[Hillhater]

Tried finding - 10pc 10x 24V 100W bulbs - Lowest is 30W I could find, but my search skillz suck.
I haven't really needed a dummy load, but I can see how I could need one.

i couldnt find the same deal again, but these are plentiful.. 4 for $5,..free post
http://www.ebay.com/itm/4PCS-H3-24V100W-Car-Headlight-Lamp-Fog-Halogen-Bulb-Hight-Power-Light-Warm-White-/391513373466?hash=item5b2803fb1a:g:9zgAAOSwyLlXoreK

 

[marty]

Best place for light bulbs on planet earth.
donsbulbs.com