calculating resistor value for dummy load on 100P 18650 pack - Page 1
Forum section: electronics / beginners

author: carpetpaul2129
author_rank: Contributor
timestamp: November 26, 2022, 06:12:09 pm
content: I had a small resistor lying around, it's a 20WR22J, I wondered if I could connect this to my 18650 pack to slowly discharge it. I think I worked out that using my 3.6v nominal (Li-ion 18650's) it would draw 16.3 amps and consume 59watts (considerably more than what I'm guessing the 20W maximum is ?)got me wondering what kind of resistor I'd need if I theoretically wanted to produce a 100 watt load on this pack3.6v x 8ohm resistor value gives me 28.8 amps3.6v x 28.8 gives me 103.68 watts1, please let me know if I'm more or less correct2, do we always use the 3.6v nominal voltage in these calculations, and assume that a fully charged cell at 4.2 falls within the safe operating range of components ? or should I be thinking that this power and current draw is literally going to change, so I should have this I'm mind and allow for the fact that it'll start producing more power with a fully charged pack ?3, even by my maths, I think I can assume that this load is shared between each cell, so each cell should be producing 1 watt, and current of .28 ampsany help, as always much appreciatedcheersPaul

author: tunk
author_rank: Super Contributor
timestamp: November 26, 2022, 06:18:50 pm
content: Your calculation is wrong:U=R*I -> I=U/R=4.2/8=0.525AEdit:2. 4.2V will give a higher wattage and, I think, should be used for design.3. Yes, assuming all cells are identical. But I guess there will be somesmall variations between them (even if they're from the same batch).

author: TimFox
author_rank: Super Contributor
timestamp: November 26, 2022, 06:54:07 pm
content: Also,P=V2/R

author: bdunham7
author_rank: Super Contributor
timestamp: November 26, 2022, 08:00:30 pm
content: Quote from: carpetpaul2129 on November 26, 2022, 06:12:09 pm3.6V / 0.125 [aka 1/8] = 28.8AYou got it right for the first (0R22) resistor!

author: carpetpaul2129
author_rank: Contributor
timestamp: November 26, 2022, 08:56:37 pm
content: hmmm, back to the drawing board, lolinteresting,the '8 ohm' resistor is an 'eighth' ?I think I got the first one right because I googles what an R22J resistor was, and it told me it was 0.22ohmsso all these are fractions of ohms, I need to go and learn thiswhat a subject, easy once you know it, but when you don't.... OMG ! lolso if m R22 resistor is 0.22ohmhow to I find the 0.125 resistor ? it has to be converted into a resistor value like 0.22ohm became R22 ?Is it R125 ?thanks

author: TimFox
author_rank: Super Contributor
timestamp: November 26, 2022, 09:19:58 pm
content: 0.22 ohms = 7/32 of an ohm, using binary fractions.In the notation R22J, R often indicates the location of a decimal point, and sometimes indicates ohms.J indicates +/- 5% tolerance on the value.For practical manufacturing, there are standard values for components, and 0.12 is a popular value (along with 1.2 x 10N, where N is an integer)

author: carpetpaul2129
author_rank: Contributor
timestamp: November 26, 2022, 09:23:21 pm
content: wow, it's no wonder I failed miserably at maths and now work with carpet is it ?

author: bdunham7
author_rank: Super Contributor
timestamp: November 26, 2022, 11:06:58 pm
content: Quote from: carpetpaul2129 on November 26, 2022, 08:56:37 pmYour math mistake was that you multiplied your voltage by your resistance instead of dividing, which seemed puzzling since you'd gotten the first example right. Yes, I suppose a 0R125 resistor would mean 1/8 ohm, which is what you'd need to make to get your 28.8A and 103.68W.https://www.mouser.com/ProductDetail/Vishay-Dale/RH025R1250JC02?qs=sGAEpiMZZMtlubZbdhIBIO9wXdv5mBIjoAqAxRY3ZJE%3D

author: BeBuLamar
author_rank: Super Contributor
timestamp: November 27, 2022, 12:43:40 am
content: I feel discharge the 16650 battery at 28A rate is way to much.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2022, 12:47:18 am
content: Quote from: BeBuLamar on November 27, 2022, 12:43:40 am100P = 100 18650 cells in parallel, which is scary enough for other reasons...

author: ledtester
author_rank: Super Contributor
timestamp: November 27, 2022, 04:08:55 am
content: Quote from: carpetpaul2129 on November 26, 2022, 06:12:09 pmDoes your resistor look like the one on this page:https://www.yoycart.com/Product/547265134578/The resistor will need to dissipate the 103.68 watts, but a cement resistor like the one at that link is only rated for 20 watts.You could use eight 1 ohm 20W resistors in parallel and get the equivalent of a 1/8-ohm resistor which probably could handle the 103 watts.

author: carpetpaul2129
author_rank: Contributor
timestamp: November 27, 2022, 10:45:52 am
content: after apologising again for being dumb, and thanking you as always for your kind replies :why is 28A spread between 100 cells too much ?is it not 0.28A each ?I've fused the pack pos and neg sides with fuse wire linking to heavy bus bars, with the pos being thinner, rated for 5A I think (I'll also fuse the pack at it's main terminals)All of my cells came from ex-dyson vacuum battery packs, so they worked far harder in their past life, I think around 16 Amps per cell for the duration of their operation, those packs get very hot, making around 300 watts from just 6x 18650. It's like battery cell murder !

author: carpetpaul2129
author_rank: Contributor
timestamp: November 27, 2022, 10:48:12 am
content: so a better design/type of resistor, even with the same resistance value can handle greater power ? with these cement resistor being the cheaper and less capable end of the resistor types ?

author: ledtester
author_rank: Super Contributor
timestamp: November 27, 2022, 03:03:41 pm
content: Quote from: carpetpaul2129 on November 27, 2022, 10:48:12 amHere are ways to increase the amount of power you can safely dissipate through a resistor:- use multiple resistors in parallel- mount the resistors on heatsinks- add active cooling (like a fan)This forum post:https://www.eevblog.com/forum/beginners/dummy-load-for-bench-power-supply-resistors/msg3786482/#msg3786482shows pics of a simple dummy load that uses "power resistors" mounted on heatsinks and cooled by fans. The post says it "stays coolish at 7V and 15A" which is about 100 watts.

author: TimFox
author_rank: Super Contributor
timestamp: November 27, 2022, 03:16:14 pm
content: Quote from: carpetpaul2129 on November 27, 2022, 10:48:12 amResistors are manufactured in different models that vary all over the map in rated power dissipation.Naturally, high-power resistors are physically larger than lower-power units."Cement" resistors are at the cheap end of the range for high-power wirewound resistors, but each model has a specified maximum power.Where it gets tricky is that the manufacturer may specify the maximum power under different conditions for different models.Leaded resistors are usually specified at a given ambient (air) temperature, but metal-cased resistors are specified at a given case temperature, since they are intended to be screwed down to a metal heat sink.As a rule of thumb, it is a good idea to keep the power dissipation in a leaded resistor below 1/2 its rated power.Legitimate manufacturers will post a data sheet online with full specifications.At 1/8 ohm, Mouser has 10 W and 25 W models in their search engine.At 0.120 ohm, they have 1, 2, 3, 5, 7 W, jumping to 200, 300 W, and 994 W (interesting).Here is the manufacturer's data for a 300 W resistor:https://www.ohmite.com/assets/docs/res_280.pdf

author: carpetpaul2129
author_rank: Contributor
timestamp: November 27, 2022, 03:29:47 pm
content: very interesting indeed ! much reading and learning to be doneI wish I'd studied this subject at school, I'd have at least a head start then