Battery testing

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jlgh
Posts: 11
Joined: Fri Dec 18, 2009 10:37 pm
Location: Midlands

Battery testing

Postby jlgh » Thu Nov 01, 2012 9:29 pm

I am working on a conversion (a 2007 Civic) and have been doing some automated testing on 8 cell packs. The first two test series used Headway 38120S cells, and I am in the process of upgrading the rig (more amps) to repeat the test with 8 of the new CALB 40 cells.

The main objective of the testing was to establish whether or not these cells would drift apart in voltage and state of charge after multiple charge/discharge cycles as Lead -Acid cells in a pack do. Here is a typical results for cycle 500 in the first test series:
Image
Cells 6 failed about 70 cycles later, terminating the first test series, and has been omitted from this chart. Anecdotally, Headway cells do seem to suffer from cell failures

There are videos and data available as follows:
A video introduction to the testing (mostly for a mixed audience): http://www.youtube.com/watch?v=U7-WHUTLbHY
Describing the test rig:http://www.youtube.com/watch?v=23kubKKcsiU
Results from test series 1 (two parts): http://youtu.be/KuVIfSimXQc and http://youtu.be/X9W80CS7q6A
A summary of the test data as zipped Excel files is available here: http://tovey-books.co.uk/attachments/File/Test_1_Summary_Data.zip
The raw data (warts and all): http://tovey-books.co.uk/attachments/File/Test_1_Raw_Data.zip (this is a set of CSV files with one row every 30 seconds for a couple of months or so: quite extensive).

The smoking gun: the cells were bottom balanced on assembly and were cycled with no subsequent cell balancing (automated or manual). The minimum cell voltage variance (the difference between the highest cell and the lowest) on each cycle did not change:
Image
Cell 6 was omitted from this graph because this was the cell that subsequently failed

My conclusion is that for these cells (and therefore probably for most LiFePO4 cells), no cell drift occurs. If that is the case then any kind of automated shunting or balancing is unlikely to improve the situation unless you have unbalanced parasitic loads of the kind that you might get from voltage monitors or other instrumentation.

You are most welcome to re-use, re-purpose or re-publish the data but I would ask two things: firstly please don't change or amend the data for re-publishing unless you make what you have done abundantly clear, and explain the reasons why. Secondly an acknowledgement and a link back to http://tovey-books.co.uk would be nice (but not essential)

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Re: Battery testing

Postby GregsGarage » Fri Nov 02, 2012 7:21 pm

Nice work, thanks for making the info available. It would be interesting to look at the data from cell 6, just to see what a failing cell looks like. I'll download the raw data to have a look at in my spare time. :shock:

Greg.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

jlgh
Posts: 11
Joined: Fri Dec 18, 2009 10:37 pm
Location: Midlands

Re: Battery testing

Postby jlgh » Fri Nov 02, 2012 9:07 pm

Thanks Greg - a pleasure.

I think what happened was that the cell capacity dropped to a level that it couldn't live with the 70 DoD I was using, so it dropped off the voltage knee whilst the others were still fine.

Nice looking job on the Matiz. Are you happy with the HiPower cells?

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Re: Battery testing

Postby GregsGarage » Sat Nov 03, 2012 8:56 am

The Matiz has been on the road for 18 months now and has covered over 8,000 miles in that time. No noticeable degradation in the cells so far. These are the same cells that Peter Perkins has in his first Insight conversion done 4 years ago. Since I purchased my cells hipower have discontinued these particular cells, their replacements are supposed to handle higher C rates. The advantage to hipower is that they have a u.k. distributor, David Copperwaith (I assume he still deals with hipower).

I do seem to have a problem with cell group 18 (the cells are grouped as 3x 40ah cells in parallel). I suspect that 1 of the 3 cells is just along for the ride. It's been this way from the start, I didn't do any testing of the cells before fitting them. The effect of the weak cell is that I don't have the range I should, but I've still got plenty for my daily driving. My plan is to swap that cell for a spare that I bought with the pack.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


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