This is hosted on the EVBMS group on groups.yahoo.com so you may need to register with that group to retrieve the file. I just read through it and it's a very interesting presentation on the battery pack market and some technical details of battery management systems.
http://groups.yahoo.com/group/EVBMS/files/Building_Battery_webinar_3-4-09.pdf
They're estimating the market in 2010 to be e.g. $40B for electric bicycles, $200B for hybrid EV's, etc ... In 2010? Uh? This is big business (maybe).
They spend some time focusing on large format cells from Electrovaya versus the small format 18650's like is used in the Tesla. Their claim is lots of small cells leads to thermal problems, battery management problems, etc and large format cells don't have this problem.
There is a most interesting chart of the alphabet soup of different lithium battery chemistries. From LiCoO2 through Na3V2(PO4)2F3 etc.
Causes of Cell Imbalance
Thermal Gradients
PackImbalance
Poor Cell Capacity Matching
Impedance Variations
Heat –Self discharge doubles
for each 10º C rise
Non-Uniform Thermal Stress
Non-Uniform Electrical
Loading of Pack
Chemical Efficiency Variations
High discharge rates
High-Cell Count Imbalance
Temperature Becomes a
Greater Factor:
Gradients Are Larger
Physical Cell Arrangement Can
Influence Temperature
High Rate Charge/Discharge
Mik, with that slide it reminds me of your experimenting with the Vectux's pack management system.
The last half of the presentation goes over a cell balancing chip from Texas Instruments. It's interesting seeing details of how cell balancing works. But I'm unsure how practical that is for large format packs since it says it can only bleed 200ma. It also goes over both capacitive and inductive redistribution methods.
it depends how large you define large format as.
ive seen 700ma balancers work effectively on 200AH cells.
ive even heard of the same ones used on 260AH cells effectively.
LiFePO4 just doesnt go out of balance that fast.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
What I was thinking is ... the capacity of the balancer to bleed off charge determines how quickly the cells can be balanced. The slide deck discussed a .2A capacity so it would take 5 hrs to bleed off 1AH of capacity.
I suppose if the cells are not very far out of balance that it doesn't take a high capacity balancer to balance them effectively.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki