Constructing my first battery pack questions?

[Fuzzy Head]

Hello everybody

This is sort of a continuation of this thread which was about me trying to find out what I needed and needed to know to make a battery pack:
http://endless-sphere.com/forums/viewtopic.php?f=14&t=47329

I am building a 36v lifepo4 battery pack and I have bought these parts and am in the process of constructing it and making sure all is ok:
http://eclipsebikes.com/headway-lifepo4-38120s-10ah-p-1011.html
http://eclipsebikes.com/lifepo4-432v-cell-30a60a-p-1041.html
http://eclipsebikes.com/lifepo4-battery-charger-p-1037.html
I got 13 cells so I could have 1 as a backup and for tests.

Everything arrived just over a week ago and I charged all the cells using a home made linear power supply one cell at a time (power supply had a cv of 3.60 and cc of about 1A) All charged very quickly to 3.60v implying they were fully charged recently. I left the cells in the original box for about a week and just put it all together.

I know with these lifepo4 cells it is critical that they not be overvolted, which is my concern here. I have started setting everything up and meticulously made sure everything was wired up correctly. Once done I decided to check my charger so I plugged it into the assembled battery pack and began checking the voltages of cells a few seconds later to make sure all was okay. I quickly noticed that the first cell I checked was about 3.8-3.9v. I unplugged the charger and checked all the voltages. They were all within normal apart from the 2 on the positive end. which were about 3.7v at this point. I touched the 2 bleed resistors for those 2 cells and they were slightly warm and within minutes both overvolted cells were down to exactly 3.6v the rest were at about 3.5v.

I suspect it may have been because I chose to charge the cells while they were already fully charged but am to inexperienced with these to be sure. Was this likely the case?

Was any damage done to my cells?
What are likely causes?
How would you advise I proceed?

Thank you

 

[999zip999]

I found that 3.55v is pretty full and going up to 3.65v. is very quick. And after 3.65v. goes up at a faster rate. 3.8v. is safe but they live below 3.6v. As is just fluff and I don't go there on Lifepo4 for just a small gain. You have a bms that came with the kit ? As it needs to high enough to trigger the bms.

 

[Fuzzy Head]

I ordered the BMS in the link in my first post.

I did notice that in the info it takes the BMS between 0.5 and 2 seconds to respond to an overcharge. If it is the case that these cells were the strongest cells and they reach a high voltage quickly past 3.65v could it just be that the BMS did not respond quickly enough to prevent the cells from getting so high with the 4 amp charger?

Should I try discharging the pack with say car headlamps in series for 15 minutes then try charging again?

 

[dnmun]

3.9V is normal. that is where the BMS suts off charging and it will drain charge off through the shunt resistors until the voltage drops to 3.8v where it start charging again.

someone here (in Camas, Wa) is selling the 38120 cells for $12 each on CL.

 

[Fuzzy Head]

So all is well, that is good. Thanks

I have looked into buying cells from quite a few places but the problem here in the UK is availability. Buying from overseas is always more expensive because of delivery costs. The seller I used had really low delivery costs and I worked it out to be slightly cheaper.

I will now try discharging the cells and recharge them. to see how things go. From what I have read a few light discharges are a good idea with new cells.

One issue I have found with connecting these cells up is how to connect the main power wires and the 11 wires from the bms to the batteries. Currently I have just got the wire between the washers of the connecting screws but I would like a better connection. What do you all do?