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.
Anyway, measure the voltage at one current, and then again at another to get internal resistance = (V1 - V0) / (I1 - I0). The battery makers often spec it at 1000 Hz, so your measurement might be slightly off of spec, but since you are matching by the same methodology that'll be OK. Note that a 0 (zero) current for one of the measurements is not OK, as you can get a high float voltage (often called "surface charge" for lead acid) that'll give an artificially high internal resistance and will vary more than using two nonzero currents.
Anyway, measure the voltage at one current, and then again at another to get internal resistance = (V1 - V0) / (I1 - I0). The battery makers often spec it at 1000 Hz, so your measurement might be slightly off of spec, but since you are matching by the same methodology that'll be OK. Note that a 0 (zero) current for one of the measurements is not OK, as you can get a high float voltage (often called "surface charge" for lead acid) that'll give an artificially high internal resistance and will vary more than using two nonzero currents.
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.
TESTING;
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.
ASSEMBLING;
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.
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,
Jim
