Building a small A123 Battery Pack

[jerrysimon]

This post is part of a set of posts following my project to "E" my Brompton :

A. Wheel Building - Tongxin in Brompton wheel

B. Building a small A123 Battery Pack

C. E-Brompton First Ride/Tweaks

D. Taking a Tongxin motor apart

You may also like to check out daniel.weck's post here which covers the simlar use of Tongxin & Bafang motors and inpired me to give this a go.

A couple of related links as I was building

Throttle Controls

Connectors


Ok as promised here is my post recording my attempt to build a 6s1p (6 series 1 parellel) A123 battery pack. I will then use two of these in series to make up a 36v pack

Again a couple of points to make before I post :

1. *** WARNING/DISCLAIMER *** This post follows my progress. It is a personal project to see if I can make good use of this small light pack. These packs can carry and discharge high currents, so I in no way condone or recommend anyone to build them even after reading these posts. If you do its your choice and you are completely responsible. Not sure if this is the correct legal jargon but you get my drift RIGHT!

2. I am not an electronics expert but have built and used a lot of different battery packs of various types in my other hobby which is RC model planes and helis. So again any experts or anyone if you like, chip in with suggestions, comments & encouragement as you want.

3. Again I am not the first to do this its been done before a lot in the RC Model and Ebike environment. Freedom E also use something simular in their lightweight packs.

The requirement for me was to come up with a light small battery pack for my E-Brompton. A123 cells have been around since about 2006 when they first appeared in DeWalt drill power packs. Intially people stripped these down to scavage the 10 cells out of them. I won't go into great detail but will give you a few links to look at.

These mostly came from Daniel in some email exchanges but I have read many before myself :


Dissecting DeWalt 36V Packs (A123 Systems)

Youtube Video Dissassemble Dewalt 36V battery

Dewalt A123 BMS battery interface solution

Endless-sphere.com • View topic - Dewalt A123 BMS battery interface solution

Youtube Video - Soldering Cells

Note I am not a fan of soldering directly on to the cells, but some do.

The care and feeding of a123-based packs...


My choice was to either to get hold of a couple of DeWalt packs for disassembly, buy second hand used cells from Dewalt packs or buy new cells. As I wanted to test these packs the latter would not give me a clear idea of whether they work or not given that some cells would have been used so I have decided to buy new cells. The cheapest way to do this is to get them from Ebay from either HK or China. If you keep numbers down you can buy them cheaper without incurring import duty. Alternatively you can buy new ready made up 6s1p packs from here or packs made up from ex DeWalt cells from here

Again I did not want to go down this route as firstly new ones are expensive approx £70/6s1p pack and again the used ones could give me spurious results. Also at least if a cell fails in my packs I can take them apart easily knowing how they are built and replace it.


Ok so lets look at these cells. This is a sample of three that arrived yestday from Hong Kong

I am quite pleased with these and will be ordering 12 more for my two packs of 6s1p. They came well packaged and protected from shorting on the welded tabs.

So why get them with welded tabs ? Well as I said if you solder to the battery facing themselfs you can risk damaging the cells. For a tiny bit extra they will spot weld solder tabs on for you.

They are larger than I expected. I am use to using Sub C cells for my RC models here is a comparison of the two. The Sub C cell is a 3000 mAh NiMh cell. In fact I had thought of using NiMh cells in 3 packs of 12v connected in series. The problem is they are heavier and I would probably have to charge all three packs seperately. They do however go up to I think 4600mAh now so it certainly worth thinking about.

Ok finally lets mesure the voltage of the cell

Hmmm they are suppose to have a nominal voltage of 3.3v. The Sub C cells are 1.2v so you can see why we need less A123 cells to make up the equivalent sized pack. However these have a lower nominal capacity of 2.3Ah. I am going to build two 6s so nominal voltage of 19.8V * 2 packs is 39.6V. Some people just use the power drill packs as they are but these only have 10 cells in them so nominal voltage of 33V. The cells should not exceed 3.6V when fully charged. So we have Nominal: 3.3V, Max.: 3.6V, Min.: 2.0V.

I am pretty sure 12 cells are going to be needed what are other peoples thoughts ?

Regards

Jerry












Working on post........

 

[jerrysimon]

Ok lets see how much they weigh

So 12 of these is going to be arround 900g maybe a little more after I add shrink wrap, solder, balancing taps.

The big question seems to be do you have to balance these cells on each charge. Its certainly not difficult and not that expensive to buy a charger that will balance 6s cells. I have purchased a Turnigy that will do just that

Turnigy Charger

I got mine here In fact I think I got it off ebay for a little less.

These type of balancing chargers are used a lot in the RC world but probably not that well known by your average joe. For ebikes batteries tend to have all the intelligence built into the battery known as a BMS circuit. In the RC world the batteries are dumb and all the intelligence is built into the charger. As you see it comes with various leads etc. The other pain with these types of chargers is they need a 12v power supply. Certainly most of the cheaper ones do, very few plug into mains. Here is how I get round that with a converted PC power supply.

Thats another one of my chargers but it only does NiCads, NiMH and PB batteries. That is one of my heli batteries there with it too.
As your probably begining to think this is all going to be a hassle to charge/balance each time and your right! But as explained there are some who believe these A123 cells don't need balacing every time, at least not in this application. In that case we can put both packs in series and then use a simple domestic 36 volt charger.

Something like this which are readily available off ebay.

This certainly seems to be the setup that Freedom E are using if you check out Caroline's post. I think this charger uses something called float charging, comment anyone ?


ok I am going to post a few more pics and will then update later

 

[jerrysimon]

To make the battery parts I needed a few other parts including battery connectors, balance taps and shrink tubing. You need 6s balance taps which basically connect to each battery and allow its voltage to be monitored by the charger and the charge adjusted so that each cell can be brought to the same voltage i.e. balanced.

The connectors for the taps vary a little depending on what charger you have and I found it quite difficult finding them.

In the end I found a uk supplier of these 6-Cell JST-XH Pig-tails here

Pig tail tap connector and Deans Ultra plugs

Initially I plan to use Deans Ultra plugs for the batteries themselfs. These are well known and well used in the RC world community and are capable of carrying high currents. In the ebike environment they are not really known at all and I am only using them initially because I have lots and all my charging equipment uses them. I may swap them out later as they are not really that easy to connect and disconnect and sometimes they can spark a little.

You can also get the Deans Ultra plugs from the same place here

Balance tap into charger

The above shows the balance connector inserted into the correct plug of the charger which as I said will balance up to 6s. As shown there are other connectors which are used depending on how many cells you are balancing.

I have just ordered another 12 cells (I want a few spare) so this post probably won't have much of an update until they arrive in a week or so.

 

[jerrysimon]

******** Holding post 3 *************

 

[barrycoll]

good luck Jerry...as I am watching with interest regarding my future with the EWC Nano B.

all the best Barry C.

 

[daniel.weck]

Thanks a lot for your detailed photos and links Jerry ! This will come handy in the near future. Regards, Daniel

 

[daniel.weck]

Hi Jerry, normally there are vent holes on one end of A123 cylindrical cells, are yours covered by the tabs ? Cheers, Dan

 

[emissions-free]

Good Stuff Jerry,

I'll have to get myself some of these RC bits. I've had a brief look into them and they look really interesting.

It's great that your goig to the effort of showing your progress for othyer to follow or at least understand a bit more. Top man

Cheers
Paul

 

[jerrysimon]

Hi Jerry, normally there are vent holes on one end of A123 cylindrical cells, are yours covered by the tabs ? Cheers, Dan

Yes they are covered but as they are spot welded round the edge and not over the vent hole they should be fine. The welded tabs are exactly the same as those that are attached to the DeWalt packs.

Regards

Jerry

 

[jerrysimon]

Charger turned up. Real cheap off ebay same as the one shown in the Freedom E post.

Front

Back

No sign of batteries yet. Hopefully next week, though its getting near to christmas.

Regards

Jerry

 

[jerrysimon]

Ok look what turned up this morning

They got stuck in Hong Kong with the Christmas post backed up but they are here now. The supplier seems very reliable and they come well packaged and he keeps up good comms even if in my case, they were a little delayed.

 

[daniel.weck]

Ooohh, looks good !
I'm thinking of getting A123 or Konion cells from Germany, see here for info:

http://www.pedelecs.co.uk/forum/electric-bicycles/5197-public-service-german-supplier-dewalt-a123-bosh-fatpacks-powertool-batteries.html

...still thinking about it.

EDIT: more info here http://www.pedelecs.co.uk/forum/electric-bicycles/5031-brompton-crystalite-209-motor-4.html#post66117

++Dan

 

[jerrysimon]

So we start by deciding how we want to arrange them. I considered two parallel rows of three each side and three rows of two in a triangle shape. In the end I went for the novice builders side by side, with each rotated positive next to negative. This is probably the easiest to do and as I plan to have two packs back to back, it seemed the easiest to me.

I first cut off the unwanted welded tabs and then soldered the required ones together. The tabs mean you don't have to solder directly onto the battery ends though some people do this. My soldering skills are not that good so this again is by far the easiest and prevents too much heat being applied to the cells themself.

I then used some thin CA to glue the cells together.

Next I added the balance taps. Red to positive and the other six wires to the cell connections and the final negative.

Ok here it is made up on the scales with all the associated parts i.e. balance tap and deans connector but still need to be shrink wrapped.

I have yet to tidy up the wires and tape them down and then cover it will some 100mm heat shrink tubing (I need to get some damn).
So two of these packs are going to give me an all up weight of under 1kg.

Ok lets stick it on the Turnigy charger/balancer

It took three minutes to balance them at 3.60V each. I think they are almost at full charge anyway. I am not quite sure as I wanted to balance them first, before I fully charged them.

Checking on my cheap volt meter I have 21.4V. The Turnigy is probably more accurate so as it balanced them all at 3.60V that is 21.6V. So two packs will give me 43.2V at full charge.

Tomorrow I will build the second pack and a series lead to connect them together.

Regards

Jerry

 

[daniel.weck]

That's exiting ! Thank you very much for sharing !

 

[Herb]

Jerry,

Where did you buy the cells from ?

I know nothing about Radio Control magic

and would be interested.

Herb

 

[jerrysimon]

The cells came from ebay

12 PCS New A123 2300mah 26650 Lifepo4 Battery Cell - eBay (item 190353671259 end time Dec-28-09 23:17:47 PST)

The cells I have purchased were brand new (with soldered tabs spot welded on) unused, as I wanted to test them out from new and not complicate this by using seconds.

However at the moment there is a guy selling s/h DeWalt batteries that Dan linked to here. He is also selling Bosh packs at quite a good price. I think the Bosh are almost new whereas the DeWalt's are used and may contain dead sells. *** EDIT just saw Dans post above which collided with mine lol

I am thinking of getting a few of his DeWalt packs as I would then strip them down.

Regards

Jerry

 

[averhamdave]

Just checked accross on the Endless Sphere forum on the topic and see there is quite a dispute going about specs on these cells!

Continue to follow the topic with interest Jerry and appreciate the step by step appraoch you are taking us through - and the pics. Thanks

As you only have a battery pack (using 2 of these 6S packs) of some 3 to 4ah. How far will that take you do you think? Also what is the C rating of the assembled pack do you think?

Is it true that opinion is divided on the need for balance charging? It's obviously a pain to do this as you have recognised! However given the cost today of these Bantam clone chargers from China then it wouldn't be the end of the world to possess and run two or even 3 of them concurrently.

Thanks so far

 

[jerrysimon]

or are Lion voltages different to Lipo voltages?

Also you only have a battery pack (using 2 of these 6S packs) of some 3 to 4ah. How far will that take you do you think? Also what is the C rating of the assembled pack do you think?
Thanks so far

Glad you like the thread.

Yes these are different to Lithium Polymer cells. They are known as LiFePO4 and have a nominal voltage of 3.3v and charged voltage of 3.6v. They can be charged to a higher voltage but its not recommended you do this often. It definately seems they do not need balancing every charge like Lithium Polymer do to maintain their use/life, at least as I have read anyway. They also have a safer chemistry and are not at risk of combustion like lithium polymer are, if damaged/punctured. I have read they take a constant discharge of 35C (69A)and bursts of up to 100C! (138A). They can also be charged very fast at 4C (10A) though I only plan to charge them at less than 0.8C maybe faster if I am using the Turnigy. I doubt I will pull more than 12A from them, so I am hoping in this application they will have a very long life. We shall see!

Their charge cycled is 1000, some say very much more.

It will only be a 2.3Ah pack yes, but I could double up two packs for 4.6Ah. Again I have been told that they can be paralled up without the need of diodes to balance the voltage/charge. Two packs will still be under 2kg in this format. As I live in Cambridge which is pretty flat, I am hoping to get between 8-10 miles out of a single pack. We will see. My commute is 10 miles return so I could keep a second charger at work if need be.

Its all an experiment really and of course I am having lots of fun doing it

Regards

Jerry

 

[NRG]

Nice work Jerry, keep it up! How are you planning to monitor the LVC of the pack? The lipos I use don't need balancing every charge, they have proved to be incredibly consistent between charges and I'm very confident in their performance and at the power density they are very hard to beat but then there are the risks involved with them! Look forward to your progress.

 

[jerrysimon]

Nice work Jerry, keep it up! How are you planning to monitor the LVC of the pack?

Thanks. The power density of the Lipos is very attractive you are right and they are good value. I suspect for ebike usuage, being much gentler than the RC environment, is the main reason why yours are remaining in balance so well. What do you charge yours with ?

In terms of LVC the controller cuts off at 31.5v which equates to 2.6v/cell which is within tolerance.

I have read that the first 20 charges are crucial for these cells and not to discharge them below 33% during that period. Just to be safe I shall go out intially on some short rides ensuring that the pack does not go down below 50%

Regards

Jerry