Postby Grumpy-b » Fri Nov 23, 2012 10:36 am
You have to sort out the good from the bad cells and get them replaced. If you are charging as a string, with a BMS then this will almost certainly cause the charge to shut down when the first cells start to get into their high voltage area, ie above the nominal cell voltage . Perhaps 4.2 or whatever your particular cells need.
If the cells are unbalanced then the low power cells will sag, and their ability to pass current will impact on the whole packs ability to pass current.
I would suggest thatyou bottom balance the Pack. In Lifepo4 cells their is very little stored power in the cells once they reach 2.7 to 2.5v (make dependant) so taking each cell, not the pack, down will ensure thatthey are basically empty. As you then recharge the whole pack they will charge together all having the same current passed through them. If they are similar they should come up together. If there are weak cells they will hit their high voltage earlier, but at least the whole pack will have a similar charge. There is little point in an unbalanced pack with weak cells to trying to charge the low cells manually up to full voltage, as even if you bring them up, the charge in the poor cells that will have reached high voltage earleir will be less and so will limit the current that can pass through the whole pack.
With temp, Thunderskys say dont charge below 0c, the latest CALBs can go way below that. One option is to do a very low current (say 5 amps) charge for say and hour initially to in effect warm up the cells, before doing a normal charge of much higher current.
Some commercial systems have inbuilt heating, this makes high current charging immediately more of an option. Some have been done electrically with heated sheets or wires some with heated water systems. Since most of our batteries are under the cars or subject to the air movement under the car, one thought would be to have a wraparound skirt to the car so stopping cooling airflow and insulating during charge. You could even hve a separate fan or greenhouse heater to keep things toasty. Or for my smart which has cells mostly under the floor, I have considered a cheap inflatable raft, slip it under inflate and hey presto an insulated layer.
Overall I think the big issue here is the cell imbalance, and possibly the initial cell volatge. WIth a set of original Nicads the freshly charged voltage would have been over 130v, which the system can readily handle. So it would help to have your nominal pack voltage nearer to that than 120. Evan in his 106 conversion had a higher pack volatge and has had few problems.
It may be possible to make changes to the controller to lower the eco light voltage , but then if your cells are of low capacity they wont be able to deliver the required higher current. So its not going to help.
Grumpy-b
then you would never have cold batteries. Seriously, this is all related to the temperature of the pack. The colder it gets the more it sags. Shame there is no way of telling the 106 controller to lower the min voltage or something.