Hi,
I apologize if this information has already been posted. If not I hope it will help the Lithium Conversions (didn't see it on those threads).
NiMN chargers either use a hard cutoff voltage or a rise in heat to terminate charge. LiFePO and I believe other Lithium Chemistries terminate charge at the cutoff voltage by gradually reducing current (float charge).
The chargers you have will use a rise in pack temperature for charge termination.
Thermal runaway is probably not the main reason Toyota only charges to 80% but I think the remaining information is accurate.
http://www.aminorjourney.com/velma-the-plug-in-prius/
NiMH batteries are a bit tough to charge; They continue to accept a charge even when they've reached full. This energy can't be stored and is turned into heat instead -Not good when you have a whole pack of them in the back of your car! To prevent fire in the original Prius, Toyota only charge the batteries to 80% of their total capacity, since it's this last 20% of charge which can lead to overheating (or thermal runaway as it is better known).
I'd been advised by Norm and other PHEV pioneers that a final charged voltage of 238V would be the safe limit that I could charge my pack to. I would need a charger which would cut power immediately when reaching 238V and not allow any kind of float charge as used with Lead Acid batteries.
http://endless-sphere.com/forums/viewtopic.php?f=14&t=2498
One of the advantages of using a123-based packs is that you have a lot of flexibilty in how you organize packs, both from a total voltage point-of-view, and in the capacity you need. What really drives configuration, more than anything is how the packs will be charged and balanced. If packs are made in multiples of 4 cells, you can use standard SLA-type chargers. What a123, and other LiFe cells need is to be charged at whatever max rate the cell will handle, until the voltage reaches 3.65V. At that point, the voltage needs to be held there while the current is gradually reduced until it gets down below about 1A. or about 10% of the max rate.
It is not usually practical to have a charger that can do more than 10-20A, but still, this high charge rate ability is unique to a123 cells. The max charge rate for Chinese LiFe cells is rarely above about 0.5-1C. Anyway, it turns out most SLA chargers use the same sort of constant current/constant voltage (CC/CV) charging profile that is best for a123 cells. For a 12V SLA, a typical SLA charger will peak out at around 14.5.V. The cutoff voltages for 24V, 36V and 48V SLA chargers are about 29V, 44V and 58V, respectively. The optimum 3.65V/cell CC/CV cutoff voltage for a a 4-cell a123 pack is 14.6V, and for 8-cell, 12-cell and 16-cell packs, the equivalent voltages would be 29.2V, 43.8V and 58.4V, respectively.
What ia standard SLA charger will not do, however, is balance the cells in the pack. SLA-type cells will self balance, at the end of the charge cycle. a123 cells, and most other Lithium-based batteries, need to be "manually" balanced.
