Charge!

Since I have battery packs now I need a way to charge them. So I got the charger mounted tonight:

I chose this location because I wanted the weight up front to help with weight distribution. It is also next to the battery terminals at the charger making the cable run short. I also wanted to place it on the passenger side to offset some of my weight as I will be driving it to work sans passenger most of the times.

I went with the Manzanita Micro PFC20. I could have gone with the cheaper Chinese made Elcon but I wanted a made in the USA quality unit and the capability of adjusting the current draw on the fly. We have seen great improvements in controllers (i.e. Evnetics) but this charger really seems right out of the 90's. For my relatively small pack 20amp max is plenty though if I got that PFC60 I could recharge from my morning commute in less then 30 minutes. :) I have something special in mind for the charge port but more on that later....

Summary of A123 module design

I have documenting my miata build in the "All Conversions and Builds" section and on my blog at:

http://electricmiata.blogspot.com/

I thought my pack build up might be worth a summary for others to build.

I went through at least a dozen design iterations and each time tried to simplify it and make it as DIY friendly as possible.  I think the result is a pack that is about as light as you could possibly make it and you can still run max power without any additional life or performance hit.  For my car I will only be driving in nice weather so cell cooling was my only concern.  But it you wanted to put it in a insulated enclosure you could also heat the cells by circulating warm air.

The basic design end up being really just a DIY version of the A123 modules. 

The cells are stacked together with cooling fin sandwiched between every other cells.  They are restrained by end places made of .120" aluminum sheet and 1" aluminum angle.  The assembly is clamped together by 1/4" steel all thread.

For the electrical connections the tabs were bent and sandwiched together with 3 cells connected in parallel. 

I punch holes in the cells using a simple block I made that fit into a 3 hole punch.

The holes went very quickly but unfortunately removing the glue from the barcode stickers did not.  You might have been able to leave it but adhesive is typically very non-conductive.  To reduce the current density at the connections I interleaved the tabs. 

The pack was a little harder to assemble but the result was three times as much surface area for the current to flow through. The hardware was just zinc plated 1/4 bolts, washers and locking nuts. 

The BMS will connect to the extra threads on top of the nuts.

DIY A123 modules ready to go

Now that I am back stateside I took the afternoon to build the final version of my modules.

 I made up some quick templates to crank out new cooling fins that look a lot more professional then Rev 0.

...I also made up new end plates and end bars...

...and replaced the end cables with .25" all thread...

 So in short I remade everything but the cells from before. :)  The result is a module design that I am finally happy with and can easily made with pretty basic hand tools cheap and quickly.  I'd say once the cells were ready (tested, destickered and 2 hole punched) I took me about 4-8 hours to assemble 250 cells into modules.

I dry fit the modules in the back of the Miata so I can start on the mounting brackets.

3 modules get installed here where the fuel tank used to be and a 4th one get's installed up front where the radiator used to hang.

Next up is the mounting brackets for the modules and installing the BMS.  Assuming I don't have to suddenly  leave the continent for work again those items shouldn't take too long.

Now I have power!

Well I finally got a few hours to assemble my 250 A123 cells into modules.  Once I got the system down it went pretty quickly.  I have the cells assembled into 4 modules.  3 identical ones made of 3p18s that will go behind the seats where the gas tank used to go.  The other cells get mounted in a single module of 3p29s that goes up front where the radiator used to be.  I went back and forth on how I was assembling the modules but the final design just uses bolts nuts and washers do make it very DIY friendly.  If people are interested I can post specific details on the tab connections.  

Once I had all the tab connections I needed a way to mount them.  I made up some quick end plates with some L brackets so I can connect cables to clamp them together.

For cables I actually had a bunch of left over flight control cables.  I simply cut them down and spliced them together.  I originally going to use some eye bolts but happened to have these cables. 

I also cut up some  .008 aluminum into 8" squares to act as cooling fins.  I slid them in between every other cell. 

 This greatly increases the heat transfer from the cells.  Individually the cells will be just fine but when you stack them together you decrease the cooling surface area by several orders of magnitude.  To solve this aluminum uses the large surface area from the sides and conducts the heat away to the fins.  The fins stick out 1" from either end of the cell for convection to wick the heat away.  Keeping the cells cool is critical to maintain cell life.  This is a fair weather car so I won't be driving it in the cold but this also could be used for heating if you needed.

This pack still needs some adjustment but I wanted to get it made before I left town for a few weeks.

Cell Socks

Short update: The battery assembly is moving forward slowly with a few set-backs.  The main setback was I changed my mind on the tab connections.  I assembled part of a back with rivets which worked pretty well but reconsidered as I wanted to be able to replace cells relatively easily.  I have tested all of them but these gray market cells...

One other thing while assembling I brushed a tab with a rivet and got a bit of an arc. No damage done but with 250 of these cells the chances are I would do it again. To prevent that I made up some covers (which my wife called cell socks).  It's just some fairly dense packing foam that I cut a slit in so it would slid over a cell tab.

Meanwhile it's back to peeling stickers and assembling packs.  Though I did recently discover EVTV so I have been watching it on YouTube and it has been *interesting* watching the shows.  The show is kind of like a Thirsty Traveler stays home and tinkers in his garage.  It's entertaining and makes this more tedious work go by faster. :)

Pack assembly

Right now I am grouping the cells together.  I was able to balance the capacities within ±0.15ah which is probably withing the uncertainty of my testing.

The most time consuming thing is peeling off all the stickers and getting all the pressure sensitive adhesive off.  PSA is not known for it's high conductivity.  :) So I get to sit in front of the TV with rag and some rubbing alcohol.

For making my connections I was able to modify a 3 hole punch with a simple MDF block to make all the holes in the tabs.

Works well but I might put a better spring in the punches because they often get stuck and I have to wiggle the cell to get it back out.

While I was away UPS left some presents

While I was away working in Spain for 2 weeks the UPS driver left a few boxes on my doorstep.  So after unpacking the bags it was time to unpack the new toys:

The first is the EMW EV Android Dashboard.

http://www.emotorwerks.com/emw3/product/ev-dashboard-by-emw-basic-edition/

It measures the current flowing from the pack to the controller in order to estimate the state of charge (SOC) of the battery cells.  Unlike other batter chemistries you cannot estimate SOC from the voltage.  Lithium has a very flat voltage curve from 10-90%.  Great for performance but it's hard to know how many electrons you still have in the tank.  This uses a hall effect sensor to measure the current flow.  It is designed to work with a android device so I was able to get a good deal on a new Toshiba 10.1 thrive tablet.   It has bluetooth to communicate with the current sensor and a GPS chip for navigation and a nice bright screen. It's not the best tablet in general because it's a little bulky and not the most robust case.  But none of that matters for me as I am mounting it in the center dash.

One other cool feature is that there is an output on the Bluetooth unit so it will drive the stock fuel gauge too. 

I need to play with it more but it doesn't seem to work in landscape mode. Hopefully this is something that I haven't figured out yet or will get added soon.  The other thing is that I wish the gauges were more adjustable as far as ranges and it would be really nice to add the option for warning lights (for high batt temp, SOC < 10% etc).  It seems like this would be easy to add but I have only used a android device for 15 minutes....

The second item was my BMS system:

I went with Lithiumate Lite, EV Li-Ion Battery Management System (BMS).

http://elithion.com/lithiumate_lite_overview.php

It monitors each cell group (in my case 3 cells in parallel) and make sure that the cell doesn't get over charged or over discharged.  I went back and forth on needing a BMS and I think bottom balancing a pack that has had the capacity matched would be fine.  But this is my first time with lithium and wanted the "insurance" of the BMS.  I knew I at least wanted monitoring of all of my gray market A123 cells and going to a full blown management system wasn't anymore expensive with the Lithiumate Lite.  Also on the bright side, not only are they made in the US, they are made here in Colorado!

To be honest having the EMW dash and the Lithiumate is redundant but I wanted to try out both and see how they compared.