High current (low voltage) power source for cheap! - Page 1
Forum section: electronics / projects

author: Leicester
author_rank: Newbie
timestamp: February 08, 2018, 05:00:30 pm
content: Hi all!I am currently working on a programmable electric load to discharge Li-Ion batteries and testing power sources. The project slowly comes together quite nicely, managed to dissipate a max of 10A from a 24V source with only two MOSFETS and a fourth of the planed cooling capabilities. Anyway, problem is I can't find any higher current source to proceed my testingOne cheap solution off course would be to take some 18650ies I have laying around in parallel, easy to reach high currents that way, but I don't quite like the idea and it's also impossible to do long-term testing that way. Does anybody have a simple and cheap (<- cheap it must be!) idea how I could build a high current (up to 80A would be nice) low voltage (5V-ish) source?I am also fantasizing about the good old microwave oven transformer with a finger-thick second winding, but then I'd need an 80A rectifier...(or are there any cheap ones I did not see?)Anyways, would be cool if anybody would share a thought.I'll most certainly document my finished project in the project section of the forums since this is a private one and not form my professional work.Happy engineering and cheers!/Leicester

author: Nusa
author_rank: Super Contributor
timestamp: February 08, 2018, 05:11:19 pm
content: Do you have a vehicle? Car batteries have plenty of current capability. When the engine is running, you can draw nearly the alternators capacity without draining the battery. Is that long-term enough?

author: C
author_rank: Super Contributor
timestamp: February 08, 2018, 05:51:16 pm
content: You might want to think of all the power waste and the bill for wasted power.There is such a thing as a "BI-Directional buck/boost converter".This would reduce wasted power to converter losses.Your long term supply would then just have to be a little more then converter losses.

author: technix
author_rank: Super Contributor
timestamp: February 08, 2018, 06:43:41 pm
content: If you are okay with low voltage (1.2V or so) you can build one out of a CPU buck converter controller and a lot of MOSFETs and inductors. A high-end overclocked processor runs at 1.4V with 150-200A or more.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2018, 10:43:17 pm
content: Designed and built such a thing forwww.bumblebeebatteries.com, roughly 0-15V, +/-100A (combined output, 2 channels). Maybe a little big for what you're doing. You'd have to ask them if you want to buy one (it's not a standard product, but doesn't hurt to give them some incentive?), though it'd probably be on the expensive side as well (hey, it's test equipment).(And, obvious disclaimer applies!)Tim

author: jbb
author_rank: Super Contributor
timestamp: February 09, 2018, 01:53:27 am
content: It depends on where you are and what your plans are (especially how long you need it for), but you might be able to rent something suitable.Or use a big car battery + FUSE.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 09, 2018, 03:50:05 am
content: Server PSUs are fairly cheap for what they are, if 12V is not too high. If it is, you could parallel a bunch of older ATX PSUs using wire resistance to help balance the load.

author: Psi
author_rank: Super Contributor
timestamp: February 09, 2018, 05:41:56 am
content: I dunno what you consider cheapButprofessional switching power supply 5V 100A 1000W SCN-1000-5http://s.aliexpress.com/yaMfuQzQ

author: ikrase
author_rank: Regular Contributor
timestamp: February 09, 2018, 07:54:12 am
content: If you're OK with moderate-duration bursts of power, try large-sized R/C and drone LiPo batteries. C discharge ratings in excess of 50C are neither uncommon nor exorbitantly expensive.(These batteries have even been used to power small spot welders!)I dunno what kinds of surplus you have access to in Switzerland, but Halted over here in San Jose, USA has some PSUs with UTTERLY HUGE output currents in the range you are looking for, as well as DC/DC bricks that take in rectified and filtered line voltage and put out appropriate DC current, and also some pretty big rectifiers. So checking the surplus world may give you something.

author: max_torque
author_rank: Super Contributor
timestamp: February 09, 2018, 12:45:40 pm
content: cheap, second hand car batteries are the answer! 1 for 12v two in series for 24v, even a pretty knackered one will stick out 200A and fry your DC load instantly if you mess up(or any stray screwdriver or other similar metallic object!)Use a fuse! and you can use a low current dc supply to trickle charge the batteries, fit a suitable power resistor, say 10 ohms (ish) between power supply and battery to prevent the psu from having too much current drawn from it when you load the batteries.

author: Leicester
author_rank: Newbie
timestamp: February 09, 2018, 05:08:11 pm
content: Wow thanks for all the inspirations!Quote from: Nusa on February 08, 2018, 05:11:19 pmThis sounds manly as h*ll! But I think my girlfriend couldn't handle it... the sound and smell of the engine I mean.Quote from: C on February 08, 2018, 05:51:16 pmI don't really care about some kWh, it would only be about 800W for 24h or so. But I have never heard of such bi-directional buck/boost converters before, looks very promising, I'll definitely read up on this, thanks!Quote from: technix on February 08, 2018, 06:43:41 pmI'll take your suggestion into consideration, would be ideal because of the very low voltage, thanks!Quote from: NiHaoMike on February 09, 2018, 03:50:05 amGonna look into that! 12V is a bit on the high side, but this surely could be beneficial having laying around in the future as well. Especially neat idea because I have some friend in the server maintenance business. Thanks for the input!Quote from: Psi on February 09, 2018, 05:41:56 amNot quite as cheap as I need it right now, but still might be worth the investment... Gonna have a look on the second hand market for server parts first as suggested by NiHaoMike, but thanks for the heads-up, thought they were much more expensive.Quote from: ikrase on February 09, 2018, 07:54:12 amWell, I'm not really short on batteries. I have roughly 250 18650 (6A) cells laying around which have to be tested, that's part of the reason I am building the load.Probably gonna have a look on the used server market as several others have suggested as well, thanks!Quote from: max_torque on February 09, 2018, 12:45:40 pmI would need too many car batteries for that, and the voltage is a bit on the high side although manageable. I want to do at least a 12h test, better 24h. At 80A, that would be roughly one big car battery per hour.Thanks everyone for the suggestions, really helped me out and gave me inspiration.I'm most certainly gonna have a look on the surplus server market and/or try to rectify something very low voltage and let it be rippled.... shouldn't bother my MOSFETS right? If I, lets assume transform 240VAC into 1-5VAC, full-bridge rectify it and let my 16 MOSFETs feel it, should be fine right?Cheers and thanks!/Leicester

author: max_torque
author_rank: Super Contributor
timestamp: February 09, 2018, 07:06:27 pm
content: a 12hour test? Really?I'd take a guess that you're going to reach with a few % of peak temperature for the entire device within say 5min (and of course, the semiconductor junction itself (the bit that matters) has such a small thermal inertia, it'll peak in just a few ms!) You also don't actually need to get to a fully stable thermal state. From the asymptotic curve of temperature vs time you can easily estimate the steady state temperature very easily!Generally for power converters i measure a few critical local temps (i prefer small open junction thermocouples bonded to specific locations for LV systems rather than infrared camera etc), test the device at 25dgeC under (sensible) overload conditions, estimate the junction temperature, and as long as that is within the datasheet values you're good to go!At high currents (especially overloads) you are likely to kill the power switches from inductive overshoots in a short time, or from a gross junction overtemp, rather than kill it several hours later from some long term thermal ageing effect.