1000W Discharger Project

[magudaman]

A CBA schematic is available somewhere on Candlepower Forums. Probably a hand reverse engineered version. There is a lot of CBA experience over there. The hotwire guys run some pretty amazing power levels in their flashlights, and various lithium battery packs. Not quite ebike levels, but more than you'd probably expect. My hotwire regulator is good up to about 15 amps and 40 volts and they wanted higher voltage upgrades for it.

Thanks Alan I'll have to try to hunt that down.


So talking with the guys on all about circuits forum I got some answers to my problems. It turns out that me adding capacitors was bad. That was slowing down how quickly the op-amp could react to changes in current. This is bad when it starts to go into an over current situation and would blow it self up. So I took out the capacitors I had added. They had me change the resistor from output of the op-amp to the gate to a 300 ohm and the sense resistor to a 10K. The the most critical change was 100pf capacitor that runs between output and the current sense line. For the first time today I was able to get to 60 volts and 1.3 amps with out a bunch of smoke blowing in my face. I did add a bit of a variable to the mix though since I was all out of my original FET I had to use an IRL2910 (CBA Fet) and drop my op-amp voltage to 5V. Though as stable as it seemed I think it will be fine once I go back to 10V and get my 200v fets in. If all goes well I might even be able to do something like 150w per device with the new .3c/w junction to case number, if my big sink can handle it.

Now that this problem is sort of solved the next step is to try my hands at CNC'ing a PCB board. Then I have to make 10 of them. Totally stoked to keep this moving forward! It might even be really cool to end up with a totally modular setup where the user buys 150w blocks that tie into a central setup.

[Alan B]

Great progress!

What are you using for PC layout? I was looking at diptrace and they mentioned a way to convert a layout to a milling g-code. There's a free version of diptrace available. I haven't tried it yet but have heard some good things about it.

[magudaman]

I am using Eagle. It was fairly easy to learn and it looks like most board places like it too if I want to go that direction. Eagle has what is called PCB-Gcode add on that I still have to learn. It doesn't look very user friendly, it command line as far as I can tell. There is also a cool board place that apparently you can get boards for around $1.70 per square inch with no setup or anything: http://dorkbotpdx.org/wiki/pcb_order

[Alan B]

You might check batchpcb.com, they have good pricing and are Eagle friendly. They collect a bunch of boards onto a larger panel and send it out so it takes slightly longer but the quality and pricing is good. Looks like your suggestion has better pricing, don't know much about them though.

Eagle is common but a painful program to learn and use and expensive to do anything commercial with, so I keep looking for other choices.

[magudaman]

You might check batchpcb.com, they have good pricing and are Eagle friendly. They collect a bunch of boards onto a larger panel and send it out so it takes slightly longer but the quality and pricing is good. Looks like your suggestion has better pricing, don't know much about them though.

Eagle is common but a painful program to learn and use and expensive to do anything commercial with, so I keep looking for other choices.

It took me a little while to get it but once I built a circuit all the way through I feel I like I have it down fairly well. I feel like even though I just started with it I could build a simple circuit in about 5 minutes.

So I decided not to go the CNC route and instead tried for the first time making a board from scratch with the Toner Transfer method. Considering its my first board it turned out pretty good. In fact I made all the traces way too small for this method but they all ended up working. This is 2oz board. It was a struggle to solder up correctly since there is no silk screen and everything is extremely tight, I ended up with a bunch of bridges I had to find and clean up.






There was only one mistake that reared itself today and it turns out I had it wrong in my schematic. It was an easy fix on the prototyped board and my next batch it will already be fixed. I also got and attached the new 200v FET to my circuit and by golly it works beautifully!

Here is a little video of it at 100w at around 60V:



So now I should be able to make my 10 boards and order all of the components from digikey.

[bigmoose]

Don't know if you gents know about the ACDC_EL750B Load. Rated 750 watts 150 amps max, 55 volts max. It will do constant current, or constant resistance and is externally programmable. You can parallel them also. They are not in production anymore, but there are some floating around from time to time. I snagged 5 of them during the past year for a project that I finished up around Christmas. If a couple of these would help someone out, let me know.

[CamLight]

Eagle is common but a painful program to learn and use and expensive to do anything commercial with, so I keep looking for other choices.

I've used it for all my commercial stuff for years now (several dozen boards). While it was...umm...interesting, to learn IMO it's a great package for the price. I haven't found any other that gives you that much for the dollar without being just as tough to master.

But, many people do find it exasperating to use so I can understand that you're still looking.

[CamLight]

So now I should be able to make my 10 boards and order all of the components from digikey.

Great news, congrats!

[Alan B]

Comparing prices, diptrace is cheaper than eagle for similar capability, though comparisons are not exact.

Reports are that diptrace is easier to learn compared to eagle, and not a command line program with graphics glued on like eagle but an integrated graphical system. Diptrace supposedly has a better system for pad layout and multiple component placement with array feature.

One problem with eagle for a "for profit" but occasional user the pricing is pretty high. "Light" versions of both programs are under $100 but standard versions jump to $900 for eagle vs $350 for diptrace, and Pro versions soar to $1600 for eagle and $700 for diptrace.

I've installed diptrace to give it a shot. Free version is 300 pins, 2 signal layers, non-profit. Includes schematic, PCB layout, parts libraries and autorouter. G code converter to make pcb on CNC router is free.

http://www.cadsoftusa.com/shop/pricing/

http://www.diptrace.com/order.php?en=1


Nice progress on the discharger.

[CamLight]

Comparing prices, diptrace is cheaper than eagle for similar capability, though comparisons are not exact.

Looking forward to hearing what you think of DipTrace! I've heard some not-so-good things about DipTrace (in addition to Eagle) but it's such a personal choice for so much of it. Trying both programs, all of them actually, is really the best way to go IMHO.

[magudaman]

Got all the parts ordered. Ended up being kind of a pricy project once everything was said and done but I'm having fun with it too. I finished my board design yesterday and made some final modifications today. It should be ready for etching as soon as my parts arrive later this week.

I'm now messing with the layout on my heatsink. I plan on having the buss bar running down the middle and the their will of course be 10 of those resistors too:



I ended up shrinking the board down from my initial size to make it so it will fit. I now only have room for 1 fastener that will hold down the board but with the FET attached too I think it should be ok.



I am of course going to have to beef up the traces with some solder.

I ran my numbers again and should be able to run 300 amps for single cell 3.7 lithium batteries while still keeping a little head room for junction temp. Total dissipation of 1100w.

[darkshirikens]

You know lately ive been thinking of building a single cell and pack charger for myself and i really didn't know where to really start with the single cell I've read around though i seen things about resistors but which one i don't know , but for the whole pack i was just going to use the light bulb trick to drain the pack. im not electronically inclined and reading parts in this thread gave me a slight headache trying to figure out what i could use for what i need so i need help mainly with the single cell discharger or pack or something that can work for both and what im looking for is like .5c through 5c on a headway pack with 12ah cells.

[CamLight]

You know lately ive been thinking of building a single cell and pack charger for myself and i really didn't know where to really start with the single cell I've read around though i seen things about resistors but which one i don't know , but for the whole pack i was just going to use the light bulb trick to drain the pack. im not electronically inclined and reading parts in this thread gave me a slight headache trying to figure out what i could use for what i need so i need help mainly with the single cell discharger or pack or something that can work for both and what im looking for is like .5c through 5c on a headway pack with 12ah cells.

It would probably be best to post this in a new thread. That way many more people can read it, and help you, and any responses won't distract from the ongoing 1,000W discharger thread.

[magudaman]

Thanks John for trying to keep the tread on topic. All the parts have arrived and it has kept me very busy the last couple days! No task has completely failed but nothing has gone smoothly either. The day the parts arrived I started my work on creating the boards. This proved to be a real nightmare. Since I was now trying to do 10x boards at once the board area was much larger than doing a single board like I had test. I did finally get one of my transfers to actually work well enough that I proceeded with the etching. Of course it wasn't until after I do all this that find out there is actually a paper specifically made for this type of transfer, Pulsar Toner Paper. And just to rub a bit of salt into the wound it also available from digikey for a fairly reasonable ~$1.2 a sheet. Apparently it makes crystal clear traces and as soon as it touches water the paper falls right off, no scraping or extending soaking.



The boards turned out pretty nice and all the traces seem to survive and make connections. Now even though I only have a total of hand full of components I still have to solder 36 leads per boards, so 10 boards became fairly tedious pretty fast.

The end product:





The final specs of a single board:

200V Max
30a max
110w Continuous

After that was finally complete it was time to get the heat sink all prepped for the mounting of the 10 boards, 10 current sense resistors, and bus bar. I laid out the part in cad then place them on the heat sink. This was a bit of a challenge since the sink actually already had a good 15 or so holes in it. I then marked all the holes on the sink and started drilling . With in a couple holes I broke a drill off deep in the sink. GRRRR. The bit wasn't the sharpest and had clogged. I then had to move that hole and a couple other holes around to make sure everything would line up. I had originally planned to tap all the 40 or so holes but gave up on that after my 4-40 tap bottomed out and broke off in another hole. Luckily I was able to extract the piece. I decided to go to a #4 screw. After all the holes were drill out some more I added a small chamfer and then sanded the sink up to 1500 grit (overkill). It did really shine up nice even with the million plus holes:

(damn Finger!)

I went ahead and put together the bus bar which is 0.5 x 0.25 and created a fuse holder. I currently only have a couple fuses and will need to order some 300 amp ones for the real test.


Now it was time to assembly line the devices on the heat sink. I laid down the resistor with heat sink grease, put all the wires on the boards and then attached them to the sink. After some testing I found 2 boards were not working. After some testing I found the connector was missing a solder on the one and the other had a small trace bridge. Once I started testing all the boards I did notice their current varied by about 4-5% which I assume is due to the 5% 25w resistor. I should have incorporated a little trim pot on the board to compensate for that.



Now what’s left is soldering up the jumper wires that inter connect all the boards, building a case/fan holder, and integrating the microcontroller . I’ll of course post here once I do some full battery tests with manual control. I’m really excited to be able to do 100 amp tests on a couple cells I have around. I also wanted to test a couple of RC Lipo Packs and see how they hold up.

So if I was to do the process again I would go try to go with a TO-247 package FET so I could use less total devices. The other thing that would greatly simplify the process is getting away from a separate current sense shunt. I should have no problem using a wire based sense resistor though I would give up some resolution. I want to start messing around with surface mount stuff so I don’t have drill the boards but I think silk screens become much more important.

So got all the boards interconnected and have been doing some full discharge test with only one fan. I emptied my A123 to zero volts on accident I pushed that little 12v pack to 140 amps or around 1200 or 1300 watts intermittently. I then did my big headway packs both about 450wh at 50v 20 amps. Everything worked absolutely beautifully. The sink was keeping nice a cool at around 44C to 52C (17C ambient) with the fets at around 60C on the one side and 82c on the other (fan was blowing on one end). With two fans blowing I'll get more balanced air and I'm sure warmer weather shouldn't be a problem even at 1200W. Holds the current perfectly too.

gotta hurry off to work, hopefully I can read over this tonight and figure out what I missed and doesn't make any sense

[CamLight]

The final specs of a single board:

200V Max
30a max
110w Continuous

<snip>

So if I was to do the process again I would go try to go with a TO-247 package FET so I could use less total devices. The other thing that would greatly simplify the process is getting away from a separate current sense shunt. I should have no problem using a wire based sense resistor though I would give up some resolution. I want to start messing around with surface mount stuff so I don’t have drill the boards but I think silk screens become much more important.

So got all the boards interconnected and have been doing some full discharge test with only one fan. I emptied my A123 to zero volts on accident I pushed that little 12v pack to 140 amps or around 1200 or 1300 watts intermittently. I then did my big headway packs both about 450wh at 50v 20 amps. Everything worked absolutely beautifully. The sink was keeping nice a cool at around 44C to 52C (17C ambient) with the fets at around 60C on the one side and 82c on the other (fan was blowing on one end). With two fans blowing I'll get more balanced air and I'm sure warmer weather shouldn't be a problem even at 1200W. Holds the current perfectly too.

gotta hurry off to work, hopefully I can read over this tonight and figure out what I missed and doesn't make any sense

That's great news, congratulations!!!

Be careful when using copper wire as a current sense resistor (if I understood you correctly). Their resistance changes MUCH more as they heat up vs. a current sense resistor. Up to 40-50 times more. That will cause your current to drift a lot as the wire heats up.

If the hot-end FETs are about 82C on the outside, they're "case" temperature (measured in the middle of the plate touching the heat sink) is probably about 90C. Are you still using the AOT410L? If so, with a junction-to-case resistance of (worst case) 0.45C/W, the FET junctions are operating at 90C + (110W*0.45C/W) = 140C if at a 110W power level. That's a great temperature to use as the maximum for long-term reliability (80% of the 175C max).

If you can get all the FETs down to 62C (assume 70C "case" temp), that brings your junction temp down to 70C + (110W*0.45C/W) = 120C....very nice.

Assuming 120C junction, and assuming that you're operating each FET at 110W, you have a system thermal resistance of...
120C - 25C ambient = 95C temp rise
110W/FET x 10 FETs = 1100W total
95C / 1100W = 0.0864C/W system thermal resistance

Those FETs have a 175C max temp rating. That's a 150C allowable temp rise at 25C room temp. For max power, 150C / 0.0864 = 1,736W (if you can get all FETs down to 62C in your above described test).

For high reliability, i.e., not operating at max temp, the allowable temp rise = 140C (80% of 175C) - 25C ambient = 115C. For high-rel power, 115C / 0.0864 = 1,330W.

I would expect that you could start discharges at up to 2,000W without problems as long as the power level dropped (as the pack discharged) to below 1700W in, perhaps, 15 seconds or so.

Nicely done!!!!!

[frodus]

This is a great project and I've been following closely, as well as talking with John (who f*cking rocks BTW). He helped me figure out max heat dissipation for my Darlington Pairs, heatsink and fans which really helped. Just need to get some proto-board and mod the circuit I was using with some of the Mod's John was talking about.

Keep us updated on this, it's almost like we have an open source discharger here.

[kfong]

Nice work Magudaman, is the based off the national dummy load circuit? Or the CBA. I think I might build one.

[magudaman]

Nice work Magudaman, is the based off the national dummy load circuit? Or the CBA. I think I might build one.

Thanks Kfong, well I started with it based on the national dummy load but then was shown it is just really a well known current sink circuit. Once I knew that there are tons of examples of these things all over. It also took some tuning to figure out why my setup was blowing up at over 35V. It turned out the gate capacitance was actually slowing down the speed in which the op amp could react to changes in current. This was probably brought out even more though because I had gone to a fairly low resistance shunt.

That's great news, congratulations!!!

Be careful when using copper wire as a current sense resistor (if I understood you correctly). Their resistance changes MUCH more as they heat up vs. a current sense resistor. Up to 40-50 times more. That will cause your current to drift a lot as the wire heats up.

If the hot-end FETs are about 82C on the outside, they're "case" temperature (measured in the middle of the plate touching the heat sink) is probably about 90C. Are you still using the AOT410L? If so, with a junction-to-case resistance of (worst case) 0.45C/W, the FET junctions are operating at 90C + (110W*0.45C/W) = 140C if at a 110W power level. That's a great temperature to use as the maximum for long-term reliability (80% of the 175C max).

If you can get all the FETs down to 62C (assume 70C "case" temp), that brings your junction temp down to 70C + (110W*0.45C/W) = 120C....very nice.

Assuming 120C junction, and assuming that you're operating each FET at 110W, you have a system thermal resistance of...
120C - 25C ambient = 95C temp rise
110W/FET x 10 FETs = 1100W total
95C / 1100W = 0.0864C/W system thermal resistance

Those FETs have a 175C max temp rating. That's a 150C allowable temp rise at 25C room temp. For max power, 150C / 0.0864 = 1,736W (if you can get all FETs down to 62C in your above described test).

For high reliability, i.e., not operating at max temp, the allowable temp rise = 140C (80% of 175C) - 25C ambient = 115C. For high-rel power, 115C / 0.0864 = 1,330W.

I would expect that you could start discharges at up to 2,000W without problems as long as the power level dropped (as the pack discharged) to below 1700W in, perhaps, 15 seconds or so.

Nicely done!!!!!

Thanks John it was your devices that inspired me and your circuit that got me started on this whole project! Thanks for the new thermal analysis, I'm loving those numbers! I need to get both fan mounted properly and really get a better idea of how it is working. The FET I ended up using is a FairChild FDP61N20
http://www.fairchildsemi.com/ds/FD/FDP61N20.pdf
It rated at .3c/w max!!!

I get those fans mounted properly and get some even heat on the sink then do some more thermal measurements.

[magudaman]

Making progress on the software side now:

[jateureka]

Nice work!

I looked into something similar to this a while back for slightly lower power to test battery capacity but ended up buying a tester for 600W.
Here are the related threads:
500W active load 100V 50A http://endless-sphere.com/forums/viewtopic.php?f=14&t=7446&start=75
Battery capacity tester / active load circuit http://endless-sphere.com/forums/viewtopic.php?f=14&t=17692

[Gregory]

Hi Magudaman, great work on this discharger. Good write up too.

How's the software side coming together?
And do you have any thoughts on your plans for this - selling a kit? Open source? Retail?

Cheers,
Greg

[magudaman]

Hi Magudaman, great work on this discharger. Good write up too.

How's the software side coming together?
And do you have any thoughts on your plans for this - selling a kit? Open source? Retail?

Cheers,
Greg

Thanks Greg for the compliment, I did finish writing the software and have the beast fully working. I got myself pretty tied up in trying to make my board schematic sort of dropped the whole thing. So I recently decided to make the board schematic a little easier and just be the power supply stuff and controller. I put the fan speed controller and filtering on another board, this should make it a bit easier to put together. My original intention wasn't to really sell these guys but I had thought about trying to get some boards made from the design.

I still think it would be really cool to sell 100w units, heat sinks and all at a reasonable price that could be networked together to create 500w or 1000w or really any combination. Then have a control board that is a separate purchase either analog control (cheap unit) or a digital controller (more expensive but has constant watts and voltage cutoff). I still just don't know that it would be cost effective verse something like a CBA with amplifiers. Really I have too many projects going on already.

[HPEV_llc]

So, I blew up my CBA recently and have found it to be rather undersized for sometime now so I figured it was time to build my own serious discharger.

So how did you "blow up" your CBA? Was it operator error or equipment fault?

By the way, great job on your discharger! I'm looking to do charge and discharge testing as well myself and if you could set something up that could use a 1,000 watt charger and/or a 1,000 watt power supply and allow for controlling and data logging 1kW charge or discharge that would be fantastic.

[frodus]

how'd this go?

I'm building my op-amps soon, trying to drive with one pot, filtering heavily on the op-amp input.

Using an off the shelf Voltage/Current controller and Temperature controller to alarm and shut down when setpoints are reached.

[reagle]

I may be a bit late to chime in here, but Dave Jones over at EEVBLOG had a nice tutorial on making constant current loads:
http://www.eevblog.com/2010/08/01/eevbl ... y-testing/