On 6/14/2013 9:22 PM, Peter Gabrielsson wrote:
While I agree that resistance is important you are unfortunately not really
correct that it causes imbalance due to energy loss in series strings.
The situation is not that simple. Imbalance results from many factors,
of which cell resistance is just one. Others include temperature, age,
coulombmetric efficiency, charge/discharge rate, self-discharge leakage
current, etc. There are also external factors, like a BMS or other
things connected across individual cells, or parts of the pack.
Cells with a higher internal resistance heat up more due to charge and
discharge current. The temperature differences cause the other factors
to change, creating imbalances between cells.
On a single charge/discharge cycle, the change caused by this effect is
quite small. But the errors are cumulative. Without some means to
correct them, they tend to get larger over time.
As an analogy, imagine a bunch of identical buckets. Every day, you put
exactly 1 gallon in each one, and then draw out exactly one gallon. If
they all started out "balanced" with exactly the same amount of water,
they should stay balanced forever, right? But they don't. One may have a
pinhole leak. Another tends to run hotter, so more water evaporates from
it. If these errors don't get caught and corrected, there will
eventually be big differences in the water level in each one.
Lead-acids are usually slightly overcharged on purpose. Like filling the
buckets until each starts to overflow out the top. It's messy and
wasteful, but it does insure that every bucket is full on every charge
cycle. This prevents errors from accumulating.
With lithium, you don't want to ever let the "bucket" run empty, or
overflow. You can't see the "water level" inside, so all you can do is
watch the voltage as a way to guess at its state of charge.
As Don Blazer said, while the resistance of an individual LiFePO4 cell
can be quite low, there can also be large differences between cells.
He's seen 2:1 on the CALBs. I've seen 4:1 or more (on Thunderskys).
--
The greatest pleasure in life is to create something that wasn't there
before. -- Roy Spence
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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