Headway Pack Builders thread

[dak664]

If a cell fails to short the low-gauge balancing wires will short like a fuse. This fixes the immediate neighbors issue when a cell shorts. In my pack I am using a a 0.75” x 0.6” x 0.020” copper tab in between each cell that is connected in series. In a 5p group these copper tabs have a relatively low-gauge balancing wire that connects them.

I like the small balance wires but rather than daisy-chaining them I'd suggest bringing them all up to a top common rail. You might then get useful diagnostic voltage readings trackable to a unique connection; a few milliohms extra could be revealed as a few millivolt difference when charging at 10 amps or so. You'd be lucky not to have any iffy connections, I had several out of my 33 read more than 10x the ~0.5 milliohm resistance until I tightened them some more (measuring from screwhead to screwhead on strapped cells at a 2 amp charging current). You could connect to that common rail with polyfuses too, or a 5 pin header to allow monitoring of an individual cell. To see how useful it might be, I'd make a protype cell and measure the voltages while charging and discharging.

 

[oatnet]

Hi Adam!

Thanks for the kudos! It sounds like you have put a lot of thought on this, which is especially important in a Car conversion, you can't hop off it like an ebike, voltages and currents are much higher, and the investment is more significant. I would pay special attention to safety systems for your controls too. There is a lot of disinformation out there, question everything you read until you puzzle it out for yourself.

I would challenge a few of your engineer's assumptions (even though I too am merely a network engineer )

I have heard first-hand reports of nickel chemistry cells (nimh/NiCd) reversing (what I assume you mean by shorting) but I have never heard reports of LiFe reversing. It has been my experience that they simply go flat and puff up, I have only been worried about the mechanical aspects of the expansion. Do your engineers have specIfic experience with LifePO4?

There has been a lot of experience parallelling lithium cells across the ev spectrum from ebikes to the Tesla, I am not aware of any issues with "cross currents", I don't recognize the issue.

For both issues I'd point to the Killacycles pack construction, which parallels a whole mess if a123 cells together by spot-welding the terminals to a copper strap and discharges them at phenomenal rates.

As dnum points out, using a full copper strap also acts a structural element that prevents the cells from twisting relative to each other. OTOH I use a 'stick' of 4 cells joined by set screws, to drive the contactors on my vw bus, Dort of an electric key. I have been carrying it around unsupported for 7-8 months now, with no sign of loosening. I wouldn't use it as a pry bar or make it much longer, but the caps are not as fragile as one might think.

However, if you want a parallel fused design, I applaud your use of flat tabs to maintain maximum contact patch, but I'd recommend a fuse material over the 16gauge wire. From my experience, copper wire is going to be a heating element that melts the cell casing before it degenerates into a plasma fireball. I don't know if you've considered this, but make sure your solution is shielded at the negative terminal of the cell where it crosses the positive lip at the end of the cell casing.


-JD

 

[oatnet]

I like the small balance wires but rather than daisy-chaining them I'd suggest bringing them all up to a top common rail. You might then get useful diagnostic voltage readings trackable to a unique connection; a few milliohms extra could be revealed as a few millivolt difference when charging at 10 amps or so.

Once they are connected at the common rail, wouldn't they all show the same voltage? If I parallel 9 good cells and 1 dead cell and measure each one screw to screw, I read the same voltage on all 10.

-JD

 

[dnmun]

oatnet, your multicell series is surrounded by a steel floor on one side and the wood strips on each side and the foam underneath as i recall so they really cannot get any twisting on them, imo.

i also have changed my mind about how fragile the end caps are too since i finally have the cells to look at, that epoxy is strong and the positive cap looks like it is spot welded in the ring around the perimeter, so really strong comapred to what i had imagined.

my apologies to everyone i lectured in self righteous ignorance. they really are pretty strong, imo.

it would be nice to find a source for premade copper connector straps though, 12mm wide, screw holes=38.2mm on center, 6.5mm holes, strap thick enuff to carry 100A

 

[dak664]

Once they are connected at the common rail, wouldn't they all show the same voltage? If I parallel 9 good cells and 1 dead cell and measure each one screw to screw, I read the same voltage on all 10.

Maybe not while charging at an amp per cell. Rail to rail will read the charger voltage, screw to rail will have the contact E=IR drops, which will be zero on a dead cell and a millivolt per milliohm on the charging cells. When I said screw-to-screw, I meant across one of my copper straps, not across a cell.

 

[dak664]

it would be nice to find a source for premade copper connector straps though, 12mm wide, screw holes=38.2mm on center, 6.5mm holes, strap thick enuff to carry 100A

16 ounce copper flashing is nominally 22 mils thick, so cross-section is 0.022 square inches per inch of width, about the same as 6 gauge wire which is used for 40-100 amps depending on allowed ohmic heating and losses http://www.powerstream.com/Wire_Size.htm. Thicker than that becomes rigid pretty fast, so I'd go for 25mm wide. Methinks for a short strap the contact resistance will dominate the power loss so the 100 amp rating would be arguable if you go for UL approval

Skin depth would become important above 1KHz for the #6 wire as opposed to 50KHz for the flashing; don't know how much of a practical consideration that would be. Self-inductance ~0.02 microhenry of the 25mm strap would be about half that of the wire, two-thirds that of a 12mm strap. That might translate into less heating of switching FETs, if it isn't swamped by the rest of the wiring.

 

[oatnet]

oatnet, your multicell series is surrounded by a steel floor on one side and the wood strips on each side and the foam underneath as i recall so they really cannot get any twisting on them, imo.

As installed, you are correct. I intended to build a battery box with a shelf just tall enough for each plane of 10 cells plus a layer of Phenolic on the top and bottom. Looking back at Adam's drawing, the planes are stacked vertically instead of horizontally so the cells are totally supported by the endcaps (assuming space has been left between the cells to accomdate a puffed/failed cell), and as you orignally asserted I think that bouncing around in a car would be too much stress on the endcaps in the long run. They are strong, but any joint can suffer fatigue.

Also, because i figured that stopping/starting were the most significant forces on the cells in a car, I oriented the cells running left/right to minimize movement on the endcaps. Adam, your drawing may look iike your cells are oriented front/rear instead, I think this motion may be undesirable.

it would be nice to find a source for premade copper connector straps though, 12mm wide, screw holes=38.2mm on center, 6.5mm holes, strap thick enuff to carry 100A

The straps are easy to make. The 1" copper strap I used from Metalelectric on ebay was cheap and plently of metal to carry 100a in the short distance cell-to-cell. It was really easy to drill with a $100 drill press and a $15 metal bit from Ace hardware cut the holes (with a little 3-in-1 oil on each hole). Stacking up a bunch of straps let me cut a bunch of holes with each drilling. Metal shears let me trim them to fit.

-JD

 

[dnmun]

more work!

i was thinking afterwards that headway already makes just such a jumper, they use it for the interconnects on the 1P packs, and the terminal binder on the 2P packs.

shoulda thought to order some of them when we got all the cells. could shared out a huge box of thousands of them among everybody. bet the cost that way woulda been peanuts. never thought of that, plugs and harnesses, yes, but not the interconnects. and theirs are really thick too.

 

[ahambone]

Look at the bottom of this page:
http://www.batteryspace.com/lifepo438120pmsizecell32v10ah100asurgerate32whwith6mscrewterminal-unapproved.aspx

Thanks, scoot!

--Adam

 

[ahambone]

I have heard first-hand reports of nickel chemistry cells (nimh/NiCd) reversing (what I assume you mean by shorting) but I have never heard reports of LiFe reversing. It has been my experience that they simply go flat and puff up, I have only been worried about the mechanical aspects of the expansion. Do your engineers have specific experience with LifePO4?

This particular EE is designing a commercial BMS for LiFePO4s for another EV conversion. He has experience with LFPs (as in he's touched them and has them on a lab bench) but probably not as much experience discharging them over hundreds of cycles as we've seen here.

There has been a lot of experience parallelling lithium cells across the ev spectrum from ebikes to the Tesla, I am not aware of any issues with "cross currents", I don't recognize the issue. For both issues I'd point to the Killacycles pack construction, which parallels a whole mess if a123 cells together by spot-welding the terminals to a copper strap and discharges them at phenomenal rates.

I agree, I think the cross currents issue is a myth and a non-issue. This is why I've made the balancing wire on my test tab set 16AWG stranded versus 24G solid. I want there to be minimal resistance between the cell ends when the charge and balance so that they both balance smoothly and so that the BMS gets as honest a read on the voltage of the parallel set as possible. I can't take credit for the copper tabs idea - that came from an engineer named Joe from the Houston Home Metal Shop Club. He's a really bright guy and had a lot of positive suggestions to offer.

As for the insulating the negative sides of the cells - I'm using the Headway Condoms (oh wait, this is suposed to be a family show! ) idea. I've got tons of 700x23 bike tubes laying around. I'll be covering at least the negative end caps of my cells with segments of bike tube to prevent short circuits. The 20 mil copper tabs when bolted between the cells are fairly rigid and I don't see them bending near the can of the cell anyway. I'll post a picture of how the setup looks assembled from the first set of tabs I soldered together for testing. You will be able to see in the photo there is a bit of spare room in the V chasm between the cell ends. Copper is so soft that it acts like a bit of a lock-washer on the junctions between the cells. I've played with "sticks" of these cells too and the set screws do a good job holding the cells tightly together both with and without the copper tabs inserted between the cells.

Okay, gotta get back to work now.

Cheers,
--Adam

 

[ahambone]

Allright, putting off work for one more response...

Looking back at Adam's drawing, the planes are stacked vertically instead of horizontally so the cells are totally supported by the endcaps (assuming space has been left between the cells to accommodate a puffed/failed cell), and as you originally asserted I think that bouncing around in a car would be too much stress on the endcaps in the long run. They are strong, but any joint can suffer fatigue.

Also, because i figured that stopping/starting were the most significant forces on the cells in a car, I oriented the cells running left/right to minimize movement on the endcaps. Adam, your drawing may look like your cells are oriented front/rear instead, I think this motion may be undesirable.

JD, you're absolutely right about the front-to back motion. That may be an issue - I'll talk that over with some people and see what we come up with.

As for the stress on the end-caps: It is hard to tell from the drawing but each "card" of vertically stacked cells has a steel back plane to which each "stick" of cells is zip-tied to (or wax rope tied or twist-tied to, the actual mount is yet to be determined). Basically each cell should be supporting its own weight via bracing to the steel back plane for each card.

The steel back plane was suggested because it is rigid, relatively cheap to make, and fire retardant. Of course it requires extra care to ensure that the cell cans are properly isolated from the back plane. For that we are using a combination of the headway condoms and some 3M self-vulcanizing rubber tape that will be place around the cell cans where the zip-tie mounts to the back planes are located to both absorb some shock, protect the blue head-shrink and distribute the load to a wider cross section of can for the cells weight.

Cheers,
--Adam

 

[dak664]

Did a full equalization charge on my 36volt/10AH pack yesterday with monitoring. After initial long equalization at 100ma I had gone 23 cycles of ~290 Wh with the Bionx charger (which is about 80% efficient, so ~230 Wh on the pack) without activating the BMS bypasses. It was getting noticeably sluggish on the final hill, but today it was back to what I remembered the first time

Here's the full data set including overnight and top-up this morning:

 

[dnmun]

your BMS leaves one cell at 3.47V? and another around 3.57?

i never have figured out what you are doing with your balancing, but like travis said, this is the wrong place to be discussing your views about cell balancing. maybe you can start a thread specifically for your setup. nobody knows what tinyAT means either.

so maybe we can keep travis's thread just for pack building?

 

[frodus]

that was the idea.... except all these people keep IGNORING THAT and posting in this thread.

Basically, PLEASE KEEP THIS ON TOPIC. I'm the OP of the thread, it was my intention that this be pack BUILDING only. Discuss BMS elsewhere, as I already asked, several times.

The long and short of it, the chinese BMS's out there are complete crap, and deserve their own individual threads pertaining to all of their "issues".... If its the Fecther/Goodrum BMS, discuss it in their thread. If its custom, create your own thread. If its Headway, put it in that existing thread.... so on so on so on...

 

[frodus]

that was the idea.... except all these people keep IGNORING THAT and posting in this thread.

Basically, PLEASE KEEP THIS ON TOPIC. I'm the OP of the thread, it was my intention that this be pack BUILDING only. Discuss BMS elsewhere, as I already asked, several times.

The long and short of it, the chinese BMS's out there are complete crap, and deserve their own individual threads pertaining to all of their "issues".... If its the Fecther/Goodrum BMS, discuss it in their thread. If its custom, create your own thread. If its Headway, put it in that existing thread.... so on so on so on...

Yeah, what he said!!!!!

 

[frodus]

that was the idea.... except all these people keep IGNORING THAT and posting in this thread.

Basically, PLEASE KEEP THIS ON TOPIC. I'm the OP of the thread, it was my intention that this be pack BUILDING only. Discuss BMS elsewhere, as I already asked, several times.

The long and short of it, the chinese BMS's out there are complete crap, and deserve their own individual threads pertaining to all of their "issues".... If its the Fecther/Goodrum BMS, discuss it in their thread. If its custom, create your own thread. If its Headway, put it in that existing thread.... so on so on so on...

Yeah, what he said!!!!!

I completely agree with Frodus

 

[frodus]

that was the idea.... except all these people keep IGNORING THAT and posting in this thread.

Basically, PLEASE KEEP THIS ON TOPIC. I'm the OP of the thread, it was my intention that this be pack BUILDING only. Discuss BMS elsewhere, as I already asked, several times.

The long and short of it, the chinese BMS's out there are complete crap, and deserve their own individual threads pertaining to all of their "issues".... If its the Fecther/Goodrum BMS, discuss it in their thread. If its custom, create your own thread. If its Headway, put it in that existing thread.... so on so on so on...

Yeah, what he said!!!!!

I completely agree with Frodus

Couldn't have said it better myself!

 

[frodus]

Yeah, what he said!!!!!

I completely agree with Frodus

Couldn't have said it better myself!

Yeah, can we keep this on topic please? this BMS talk is getting annoying.

 

[julesa]

Yeah, what he said!!!!!

I completely agree with Frodus

Couldn't have said it better myself!

Yeah, can we keep this on topic please? this BMS talk is getting annoying.

Thanks for keeping the signal-to-noise ratio high, Frodus. :lol:

 

[frodus]

Yeah, can we keep this on topic please? this BMS talk is getting annoying.

Thanks for keeping the signal-to-noise ratio high, Frodus. :lol:

Not a problem bro!

Now, back on topic!!!!


I got 200 of the cell holders (each one holds 2 cell ends) and 100 4-hole bussbars.... should be able to use 4 bussbars per 6-pack plus 1 bussbar at each node (except the pos and neg).... for a total of 128 + 30 = 158 bussbars. I'd need 1 holder for each cell, so 192 cell holders. Should start to look something like this (this is my friend Richard's bike pack):

here's another one of a smaller sub pack:

 

[frodus]

Here are some pictures of the sub pack I built tonight. I think the cell building blocks are basically the same as the orange ones. They don't crack when you put them in, they crack when you're moving the batteries around inside. They're very snug on one end.

To help with this, I cut back some of the heat shrink tubing on the cell like this:

and then inserted into the building blocks like this:

I used the Buss material from headway:

and put together a 6-cell sub-pack:

Only 186 more blocks to go.....

I also got a Vicor 48V power supply and 48V to 3.7V DC-DC converter so I can charge all the cells to a 3.65V....before I assemble the pack.

Higher resolution pictures are in my album here:
http://www.evfr.net/coppermine/thumbnails.php?album=35

Enjoy!

 

[Jay64]

Do you guys have those building blocks and bus bars for sale yet?

 

[frodus]

yes, they're just not on the website yet. Just got the shipment in like fri/mon and haven't updated the product offerings.

 

[ahambone]

I can confirm that those red cell blocks work great, as compared to the older more orange blocks. I ordered a set from battery-space (since EVComponents didn't have them listed yet). I don't have to remove the head shrink for mine to fit but it is a bit snug. I'm definitely happy with the way they work. Now I just need to find some all-thread that's small enough to fit into the holes that are in them so I can sandwich mine together. I think this may beat out the card-and-slot design I was thinking of earlier because it's so darn easy.

Best Regards,
--Adam

 

[frodus]

yeah, these things are great! its not TOO snug, I just wanted to try removing the heat shrink a little and they fit perfectly without being too tight. I may or may not do them all this way.... still deciding.