Battery module design for 11kWh lipo pack.

[x88x]

Ok, starting on actually designing the modules (intro here) for a ~11kWh lipo pack I am building for my electric motorcycle conversion. Pack will be 25S and at least 24P (as wide as I can make it, basically), constructed from a crapton of 5S 5Ah Turnigy lipo packs. These packs will be formed into 5 modules, each module being a load of packs in parallel. These 5 modules will then be connected in series to complete the pack. Each module will contain one of methods' LVC/HVC boards, managing the cells in the module.

I'll see what I can put together in sketchup tomorrow, but for the moment a text description will have to do.

Basically what I'm envisioning is 3 rows of packs, with a copper bus bar down each side, and one down between each border. Like so:

Each pack will be connected to the adjacent bus bars with ring terminals, and the leads shortened as much as possible to minimize the resistance between the pack and the bar. I'm currently thinking 3/16x3/4" copper bar for the bus bars. This should give me a conductor cross-section area of between 3/0 and 4/0, so I'm thinking that should be fine. Currently thinking 2/0 for the inter-module links.

For the shell, I'm thinking some sort of high density plastic, held together with bolts. I ordered some 0.5" HDPE and some M6 socket flat head bolts to experiment with, so we'll see how that goes. In the mean time though, I wanted to put together a cheaper, lighter, easier to modify mock-up to get an idea for what I'm looking at. Enter foam-board.

I found an arts and crafts store near me today and went to pick some up. Found some nice 0.5" stuff for not unreasonably much, so picked up a few sheets of that to work with. Based on my measurements, I came up with this:
(turned out a little short, thus the little bit tacked on the end)

Pack in box:
(yes, I know it looks too wide; box is designed for 5S, that's a 6S)

I'm hoping to be able to cut a bit off the top; gonna pick up some aluminum bar tomorrow the see what I can work out.

Comments/concerns/recommendations/etc are welcome and appreciated.

 

[nechaus]

How about using carbon fiber insted of metals for the battery box?

 

[x88x]

How about using carbon fiber insted of metals for the battery box?

You mean instead of the plastic? Mainly because CF is hella expensive and much harder to work with.

 

[Kin]

Curious, why such thick plastic? Is there a particular extent of damage that you're designing for? You could also consider a fiberglass matt or fiberglass cloth and epoxy container.


So what's your process of designing this module? You have made some insane lipo packs before. Are you just following those busbar systems?

edit: Where I said you have made some insane lipo packs before I was confusing you with jozzer. http://endless-sphere.com/forums/viewtopic.php?f=34&t=38171&hilit=miata. You definitively want to read about his process of weeding out the bad cells.

 

[x88x]

Where I said you have made some insane lipo packs before I was confusing you with jozzer.

lol, I was wondering about that.

The reason for the thick plastic is because I want it to be structurally rigid, durable, easy to assemble, simple, and fairly cheap. I also want it to be able to survive skidding across asphalt at 70mph without spilling batteries everywhere...you know, just in case.

 

[x88x]

Hahaha, wow, ok, so I got the sheet of HPDE today...yeah....so, you may have a point about the thickness. This stuff is heavy ...also appears quite strong and durable...methinks I'm gonna grab a 1/8" sheet; I think that'll be more in line with what I'm looking for.

EDIT:
Whoops, meant 1/4"...I think 1/8" would be too thin.

 

[jonescg]

I plan on using 8-10 mm acetal (Delrin) for my battery box. It's probably a bit too heavy, but it's definitely skid proof

 

[x88x]

I plan on using 8-10 mm acetal (Delrin) for my battery box. It's probably a bit too heavy, but it's definitely skid proof

lol, yup. This 1/2" stuff (~12.7mm) is strong enough I seriously think I could make a scooter or something out of it. Just one splate for the bed and you're done. ...hmmm, might try that...

 

[x88x]

I finally got my credit card yesterday, so I was able to start ordering parts in earnest.

So I have some good news and some bad news with regards to the batteries...

Good news is I placed several very large orders on Hobbyking for batteries.

Bad news is I wasn't able to place as many orders as I wanted to. They ran out of packs...and they don't have any in stock at their Hong Kong warehouse either...so if anybody wanted to buy 5Ah 5S Turnigy 20C packs, you're out of luck.

Ah well, I was able to get 57 packs (was trying to get 150), so I should be able to get a ~30 mile pack until the get more in stock. And these are pretty popular packs, so hopefully that won't be very long.

 

[grindz145]

but it's definitely skid proof

Thanks for that datapoint

 

[x88x]

Came home last night to a nice big package waiting for me.

What could it be?

All in their own little boxes.

And out. That pile will represent one module once I am able to order all that I need.

I have to say, while they're not quite as compact as my 6S Zippys (the 6S Zippys are only ~1/8-3/16" thicker than these 5S Turnigys), they do feel better constructed. Little things, mainly. Thin foam padding on the ends instead of just fiber-tape, nicer wire hookups (especially on the balance leads), etc.

Ok, now it's time to get working.

 

[x88x]

Been a while since I've been able to update on this; got stuck working 5 days straight of 12 hour shifts....not fun...

So, updates.. Got my Powerlab in; that thing is serious nice...even if it wasn't obvious that it needed to be running off a battery to discharge more than 100W. -_^

I also got the rest of the batteries (for now). 5kWh of yummy, yummy, lipo.

Now the fun job of testing all of them. I charge them up, balancing, and then switch Powerlab profiles and run through two cycles of 40A (8C) discharge, 10A (2C) charge. All told it takes about 80 minutes each. Well, three down, 52 to go...

My test setup:

 

[x88x]

Well, it's been a while, but I finally have some exciting updates!

I finally finished testing all the battery packs for phase 1 (ie, smaller pack while I wait for them to re-stock the US warehouse)! And a beautiful sight it is too.

I ended up being 3 packs short, so I did break down and order 6 from the Hong Kong warehouse...shipping came to almost half what I paid in shipping for each of the 27-cell shipments from the US warehouse! O_O

So, now that I finally know how many I have to work with in this pack, I started working on the battery box. I tweaked my initial design a bit after experimenting with the plastic-only construction...was just too hard to make and not quite strong enough for my liking. So I I went with 1/16"x3/4" aluminum angle for the corners. Bolts go through the alu and thread into the plastic. Seems to be holding together nicely...I do need to get myself a real M4 tap though; the 8-32 I'm using is close, but not quite there. Good enough for a prototype though.

I've already found some points I need to address...needs to be about an inch taller to start... Oh well, the materials are relatively cheap, and this is why we make prototypes, neh? Does look pretty sweet though, if I do say so myself.

 

[x88x]

Well, it's been quite a while since I updated this. Life and work been getting in the way.. I do have some updates though.

I ended up tweaking the module enclosure a bit in order to decrease complexity and increase strength. I'm using closed-end blind rivets on most of the connections instead of screws, and only using screws to hold the top on.

I also finalized my design for the bus bars and parallel boards.

Bus bar (3/4x1/8" C110 copper bar), cut and prepped for drilling:

Main hookup hole drilled (this will take a 2/0 AWG terminal, bolted on with a 3/8" bolt):

I forgot to take a shot of one fully drilled, tapped, and sanded, but there will be plenty more for me to get those.

Now, for the parallel boards that will let me only need to feed one nice 7-conductor, shielded, wire from each module to the control module with Meth's high/low cut-off boards.

Pile-o-parts:

And much soldering later, all 5 done:

Module fully wired (except for the hookups). I think I'm going to shorten the positive leads on the batteries...in case you hadn't guessed, this was not how I originally planned to do the hookups...much more compact though.

And closed up, with mock-up cables (don't worry, they're not far enough into the box to touch the bars):

This is one of the small modules. I will have one more like this,and one big module that will basically be three of these in series. I have some 1.5x1/8" C110 copper bar on order for the two series connection bus bars.

...speaking of that bigger module...

So, back to the shop to make more bus bars!

 

[skeetab5780]

30amp crimp terminal !! noooo

 

[x88x]

30amp crimp terminal !! noooo

Do you have reason to think that'll be a problem or is this just another 'crimp vs solder' thing? They're 3M 10AWG, 1/4" ID, 9/16" OD, 1mm thick, nickel plated copper, nylon insulated ring terminals, properly crimped. I can't find a current rating for them from 3M, but similar ones from other manufacturers range from 37-48A. On top of which, while they may see as much as 55A very briefly (as in, maybe 2-3s max) in this smaller pack, I would be shocked if I will be pulling pulling more than ~9A through them continuously. I likely won't be pushing it hard until I have the full pack anyways, at which point that drops to a max of 27.5A burst, 4.5A continuous, so I really am not the least bit concerned.

 

[Farfle]

30amp crimp terminal !! noooo

Do you have reason to think that'll be a problem or is this just another 'crimp vs solder' thing? They're 3M 10AWG, 1/4" ID, 9/16" OD, 1mm thick, nickel plated copper, nylon insulated ring terminals, properly crimped. I can't find a current rating for them from 3M, but similar ones from other manufacturers range from 37-48A. On top of which, while they may see as much as 55A very briefly (as in, maybe 2-3s max) in this smaller pack, I would be shocked if I will be pulling pulling more than ~9A through them continuously. I likely won't be pushing it hard until I have the full pack anyways, at which point that drops to a max of 27.5A burst, 4.5A continuous, so I really am not the least bit concerned.

As long as they are crimped ok, with that many in paralell, you should have zero trouble. Might throw some clear varnish over the bolted connections though.

 

[x88x]

As long as they are crimped ok, with that many in paralell, you should have zero trouble. Might throw some clear varnish over the bolted connections though.

Yeah, I've been trying to think of some way of insulating the whole bar assembly. Once the 2/0 cable is bolted to it though, it will be pretty much locked down in place, but it would be nice to have a bit of insurance..you know, just in case I'm a dumbass and drop something in there...

Something else I neglected to mention re current carrying, it helps that they're all basically heatsinked to several ounces of solid copper and subsequently a good bit of 2/0 welding cable.

Oh, and yes, they are crimped properly. Since I knew I was going to have to do some 200 crimps for the full pack, I went ahead and got the proper tool for the job.

 

[grindz145]

If it can go wrong it will. Just on my little 3kwh ebike pack I ended up with a bunch of failures I couldn't have imagined would occur.Vibration sucks...

Capton tape can be a bit messy, but not as messy as some alternatives. It might not hurt to cover the bus bar in it afterward.

Conformal coating would be a nice way to prevent corrosion, and enough of it might even prevent some accidental contact. Though I'm not really sure how well it will stick to the buss bar.

I gotta say that battery box looks really sexy! I'm taking notes honestly! I made a similar battery tray for my ebike using similar construction, but that battery box is really good looking.

 

[x88x]

I gotta say that battery box looks really sexy! I'm taking notes honestly! I made a similar battery tray for my ebike using similar construction, but that battery box is really good looking.

Thanks! My goal was simplicity and reduction of parts. Comparing what I ended up with to other large hobby lipo packs, I think I've made some pretty good headway in that direction.

 

[Frank]

Bump.

Just wondering how it's going - also, will your busbars be attached to the box at all?

 

[x88x]

It's still going...just keep getting side-tracked by other things...you know; life gets in the way... I ended up consolidating into two boxes, one with 20 packs, the other with 30, and consequently changing some of the series hookups. I'll try and get another update up this week.

 

[K-ray]

Hey X,
Nice set-up, I've done similar and like your cleanliness
Was thinking pipe insulation from the hardware store Would work well
for the buss protection. Comes in 6ft lengths of various inside dimensions...
Cut to length, slit down the center and slip on. Electrical tape wrapping the
ends and center will keep it in place. Should make for a light, shock and sound
absorbing solution thats cheap and easy to replace if needed :wink:
Cheers to all and Happy New Year!
K

 

[circuit]

I see a potential problem with this pack: no fuses. If one cell gets shorted internally, a very high inrush current will go from other parallel cells and will cause a fire. To avoid this, fuses must be used at each pack and each balancing wire.