I want to charge at maximum current to 80%, and then shut off. My
thoughts on this subject are listed as text below, or you can follow
this link to see the images of my charging profiles:
http://www.summet.com/blog/2016/01/03/default-charging-profile-charge-to-80-capacity-quickly/
I welcome any comments about issues or tricks I may have overlooked.
Thanks,
Jay
Because I have a relatively short commute, and rarely anticipate needing
my full 100% pack capacity, I have chosen to charge my truck to an 80%
SOC on a daily basis to maximize battery life. One nice thing about the
80% level is that most batteries can be charged at a relatively high
rate of speed up to 80%, and then you need to slow down the charging a
bit to prevent them from overheating. (This is why DC Quick Chargers
will quickly bring an empty battery up to 80%, but then slow down quite
a bit after that.)
My first attempt at programming an 80% charging profile was very simple,
just set MaxV to 128.5 and set the TermC (termination current) to 2
amps. This works well, it gets the pack voltage up to 128.5 volts and
holds it there until the battery stops accepting much current. The only
issue is that it is wasting time, because for a good amount of the
charging period the current flowing into the battery is less than the
maximum 30 amps (4.0 kW) that the chargers can produce. The charging
curve looks like this:
http://www.summet.com/blog/wp-content/uploads/2015/09/charging_2amp_cutoff.png
In an effort to speed up my charging, I set the MaxV to a higher amount
( 128.9 volts) so that at the 128 volt level the battery pack would
still be accepting a higher current, and then set my TermC to the
highest level my EVCC allows (10 amps). This results in a charging curve
that looks like the following:
http://www.summet.com/blog/wp-content/uploads/2016/01/charging_10amp_cutoff.png
As you can see, I spend more time delivering 30 amps before the current
draw from the pack starts to drop off, and the curve reaches the cutoff
point much quicker. My EVCC currently limits the TermC parameter to 10
amps or less, probably because they have not anticipated somebody trying
to charge only to 80% as quickly as possible.
If I could set the TermC parameter to 20 or 25 amps it would allow me to
set the MaxV up higher (131?). The goal would be to find a voltage
setting such that the pack would be drawing 20 or 25 amps right when it
hit the 128.5 volt level. So my curve would basically be flat, full on
charging at 30 amps up until the very end when it would start to taper
off and the charging would end at 25 or 20 amps.
I’m not worried about setting the MaxV higher than the actual voltage
I’m attempting to hit, as it is still much lower than my pack’s actual
max voltage, and if something were to change with the pack chemistry
making it miss the TermC cutoff, the MaxV would still stop the pack from
reaching 100% charged. (and the termination timeout would eventually hit.)
This is the end of my main thought, but you can continue reading for…..
Extra Info about my pack, charging to the 80% level, etc…
The 48 modules from my Nissan Leaf battery pack (LMO/LNO chemistry) are
arranged in a series of sixteen sets of 3 parallel cells. This gives me
a 180Ah battery with an absolute maximum voltage of 134.4 (4.2 volts per
cell). In actual use the 4.2 VPC level should never be reached, if you
charge the cells to 4.1 volts per cell they are around 99% charged. As
the extra 0.1 volt difference between 4.1 and 4.2 doesn’t really buy you
much extra capacity, most people use 4.1 volts per cell (131.2 volts)
as a safe “full” or 100% capacity on the Leaf Cells. My Mini-BMS units
will start to shunt voltage at 4.1 VPC, and raise an over-voltage alarm
at 4.2 VPC.
However, if you charge the cells to 4.0-4.01 VPC (128-128.5 volts) that
corresponds to about 80% of their maximum possible capacity. To maximize
cell and battery pack life, you want to minimize the time the cells are
fully charged or fully discharged. If you can keep the cells between a
10%-80% state of charge (SOC) it will maximize their battery life.
Keeping them above 10% SOC is easy…don’t drive until they are empty.
Keeping them at or below 80% SOC is also easy, simply turn off your
charger when they are only 80% charged. This is why the early Nissan
Leafs had the option to only charge the battery to 80%. (Removed in the
newer Leafs due to EPA regulations on how maximum range is calculated.)
If you want the extra 20% of range that charging to 100% SOC would give
you, it is better to charge to 100% and then immediately discharge the
pack by driving it (as opposed to leaving it sitting at 100% charge all
night). This is one reason why Nissan Leaf’s have a charge timer, so
that you can tell the charger to finish just before you start your daily
drive. (The other reason is so that you can tell it to start charging
only after the low cost nighttime electricity rates start, if you are on
a time of use metering rate plan.)
My 2nd charging profile will be a 4.1 VPC full 100% capacity charge, for
when I feel the need to really go long distances. (or to get the pack
ready for a top balancing equalization charge, my future 3rd charging
profile.)
Jay
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