There are exceptions, but typically the internal resistance of a
cell/battery reaches a minimum at 100% SOC. Basically, a fully charged,
completely balanced, battery pack is indeed "stiffer".
The notable exception is LiFePO4. The internal resistance of these
particular cells is a very weak function of SOC. They stay "stiff" and
at close to the same voltage for much of their discharge. Only at very
low SOC does the internal resistance rise significantly. The same is
true for very high SOC.
The typical pack in an OEM EV is Li-Ion metal oxide of some sort.
The internal resistance is a strong function of SOC (as is the cell
voltage,) and the resistance is at a minimum at 100% SOC, so the pack is
the most "stiff" when at 100% SOC and completely balanced.
On 5/22/2017 5:10 AM, Willie via EV wrote:
On 05/22/2017 07:47 AM, paul dove via EV wrote:
Still sounds like a hold over from lead batteries. I can't imagine
the voltage of an unbalanced pack being more than 10 volts loss in
voltage. This would not be noticeable.
I agree, it seems inappropriate to talk about "stiffness" from
balancing a lithium pack. Though, I think Bruce was quoting someone
else. Maybe Bill Dube will chime in here, but if all cells are well
top balanced and charged to maximum voltage, that amount above
"normal" voltage represents a tiny amount of energy and is gone in the
first few seconds of discharge. Of course, lithium packs will sag
when approaching discharge rating limits. But there is very little sag
associated with SOC.
Which brings to mind the old "top balance"/"bottom balance" arguments.
Is anyone out there still having good luck with bottom balancing?
Bruce mentioned "guessometer" in a Tesla context. It is my
understanding that Tesla arrives at an estimated range by applying a
constant, linear scale factor to the estimated remaining energy. For
normal use, that scale factor is about 285 wh/m. The driver is
properly left to make allowances for wind, speed, elevation, weather,
etc. The imiev does apply some guessing based on past data. They do
a fair job of such guessing. Leaf, OTHO, does such a terrible job
that it is nearly useless. I remember my wife calling me on her way
home from picking up the new Leaf: "This thing has lost 15 miles of
range in the first 5 miles traveled. At this rate, I won't be able to
get home. Are you prepared to come get me?" We, of course, learned
to mostly ignore the really crappy instrumentation.
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