DIY Electric Car Forums banner
1 - 2 of 2 Posts

1,199 Posts
Discussion Starter · #1 · (Edited)
Our Headway batteries are scheduled to arrive on January 4th So I'm trying to get ready to test and assemble.

I have been picking up tidbits of information here and there in various threads that I think may be important when I start building my pack. I want to get the best start possible.

The pack will consist of 50 3 cell buddy packs. I have read that the internal resistance and capacity should be balanced within each buddy set and then between each buddy pack.

Since I am ignorant of many basic things in this field, so ignorant I often don't even know how to ask the right question. I'm hoping you all will help me understand and learn.

How is the internal resistance of a battery measured? A sensitive or special VOM? If so how would I recognize the righr equippment? If it is not equippment I assume ther is a test sequence and math. If it is, could someone outline the procedure or recommend some reading.

Figuring capacity; I seen this mentioned in conjunction with other information, but I still don't clearly understsnd it. Could somebody(s) go over it again.

The testing I already plan on,
1.) Charging each cell to a specified voltage say 3.5 volts. We will use a Cell Log8M and buzzer to let me know when 3.6 volts is reached.
2.) I will then let the cell normalize and record that voltage. (Should I record the time it takes to normalize?)
3.) Next I will apply about a load of about 150 amps with 4 big fixed resistors for a specified time (30 seconds) while recording the battery voltage drop and temperature rise. The same resistors will be used for each test.
4.) Record the recovery voltage. (Should I record the time)

The question I have here is what percent tolerance is acceptible? I'll have 5 spares from this batch and I have 5 more from an earlier production batch so I might be able to swap out a few.

What I believe I understand is, when assembling a 3 cell pack I should look to match internal resistance and capacity as closly as possible while maintaining a close match of the averages measurements between all 50 3 cell packs.

Doing the sorting and matching once I get the data is going to be a pain, but I think I can do an excel sheet to sort that.

Of course the other possibility is to throw them all into a box and reach in and blindly and grab three.:D:eek::eek::p

Since we are going to beat the crap out of these cells I want to get things as balanced as possible to start off. I think we have a pretty good charging monitoring system figured, so if we get off to a good start we can maybe get a few years out of the pack.

Thanks in advance for any help,

1,199 Posts
Discussion Starter · #3 ·
I just got some Headways for testing. They are little works of art with the blue covers and orange holders -- If they test to be as good as they look they'll be awesome batteries. The bus bars won't fit with the 3 ring holders, though, apparently they are designed for the 4 ring holders.

Thanks for the information, Just what I was looking for.

I do love this forum, no matter what you need to know about EV, someone has an answer.

The Headways are really nice arn't they, not only compact power but like you say pretty too.

We have five 38120P cells we got for testing, Darin was actually doing the testing because he has the huge resistors in a frame that we are using as a load. We charged one battery up with a 3.6 volt DC to DC converter and let it normalize in the garage (about 25 degrees farenheit). The battery normalized at about 3.4+a bit volts at that temp. The load unit we have is around 150 amps. (veries with temp). At that temperature that little 8 ah sucker put out 140+ or so amps for 2 minutes without dropping below 2.2 volts. We then let the battery sit for 5 minutes. The batterey recovered to 3.3 volts. We thn checked the temperature of the battery, it had gone from about 25 degrees to around 80 degrees. So with the battery at about 80 degrees and the volage recovered to 3.3 volts we hit it again the voltage dropped only to 2.55 volts and the current wnt up to 165 amps and it held for another 2 minutes.


After sitting for another 5 minutes the battery had recoverd to 3.28 volts.


We plan on using this for competition, should do wonders as long as we can keep the battery cool enough. Since a single Hook in i tractor pull seldom lasts more then 20 seconds at this level of competition I think we are ok.

Those nice little orange blocks do put the batteries close together, for track racing I would think you would want to make racks like CroDriver or JackBauer have done to allow better cooling. there, are also ways to attach fins to the cells to help as well.

Sorry to run on and on and on ....... again.

Have a great New Year.
1 - 2 of 2 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.