These are likely modern off-the-shelf LiFePO4 cells. Seriously. The
internal resistance can be extremely low for LiFePO4 cells. 10C charge
rate has been routine for hybrid car use LiFePO4 for 15 years at least.
LiFePO4 cordless tool cells have been capable of this for more than 20
years.
It is just a matter of internal geometry. More/better internal
connections, more/thicker tabs, thicker grids, thinner paste, thinner
separators. It is a balance of paste volume/mass to conductor
volume/mass. You also must be a bit clever in how you connect the whole
thing together on the inside of the cell to minimize the path length.
Low internal resistance makes the pack more efficient, particularly in
high HP applications. Low internal resistance means far less internal
cell heat during charging and while discharging, so thermal management
get much simpler. Aside from giving up a little specific energy, there
are few down sides to lowering internal resistance.
You can water (or refrigerant) cool the conductors. Also, you are going
to have thermal mass on your side, at least at the beginning of the charge.
I am not sure how much fast charge degrades LiFePO4. The cycle life in
well-designed and manufactured LiFePO4 cells in a well-designed pack and
cells can be astounding. Thermal expansion can be a serious issue in a
poorly-designed pack, however.
Bill D.
On 3/22/2025 4:30 PM, Marco Gaxiola via EV wrote:
Back to the main topic; the BYD new pct. release and the ultra fast
charging idea, I have some doubts:
After watching video portions of the new BYD HAN-L electric vehicle being
capable of charging at a Megawatt level and being able to recover up to 250
miles of range in just 5 minutes, I did some brief homework from their
online demo video and wanted to ask your thoughts are;
First following their claims from the video +some minimal math & Ohm's law:
a peak charging power of 1000 Watts (or 1MW) on a 1000 volts nominal
voltage battery pack, would mean that the peak charging current would reach
the 1000 amps.
Since I wasn't able to find all the specs from reliable sources I wanted
first to estimate what type/size of battery pack would be able to stand
such an amount of current/C-rate, so I used their also claimed 10C max.
charging rate and determined it should be at least a 100.2 kWh pack.
Then I determined that, in order to build such battery pack capable of such
claimed performance, this would have to used at least 303 LFP cells (BYD
tech) in series to form a 1000V nominal battery pack and each cell must be
at least 100Ah each for a total of 100.2kWh of energy.
Finally I plotted the charging curve, gathering my data from the EV's
dashboard shown in the video; Instant charge power, state of charge (SoC),
elapsed time and recovered range. Here are my doubts:
(see link below for charts)
- I find the charging curve, instant power Vs SoC lines a little odd.
The gained SoC should be steeper during the 997-1002 kW stage and then slow
down once power drops to 662kW. If you are reducing power by almost half,
shouldn't the rate of SoC increments also be reduced?
- Similar to the added range, the yellow curve should be less steeper
right at the 662kW drop, but instead if continues the same until half of
that power step before the next lower stage.
- In order to safely transfer 1000 Amps from A to B without risks of
melting or fires, in a 'non-liquid' cooled cable system, the cable size
would have to be huge and require at least one DLO-646 wire for each pole
(positive and negative). In this example the cable is rated for ≈815 amps
with a diameter of 1.45 inches each! Can you imagine the size and weight of
the charging cable? If we would go with liquid cooling approach, a typical
CCS1 liquid cooled EVSE cable running at ≈400-500 amps (despite the running
voltage) dissipates around 2.5-3kW of wasted heat, can we imagine the
energy losses on a 1000amp 3-4 meter long charging cable?
Then to add to all this above; these calculations and assumptions aren't
including any thermal management and electrical hardware required to keep
all these cells, high voltage cabling, contractors, etc. properly cooled
and safe from melting from the very high power even the short current
burst. Then we would need to account for a fast battery degradation at such
high 10C rates, right?
Link to a shared google sheet with my data and calculations, please check
it out and comment:
https://docs.google.com/spreadsheets/d/1JjZzukWrvFXYlFQbzC1Mfaj8SKSa08CAIBtJtKhGwkc/edit?usp=sharing
Marco Gaxiola
On Fri, Mar 21, 2025 at 11:07 AM Lee Hart via EV <ev@lists.evdl.org> wrote:
Lawrence Winiarski wrote:
Maybe for vehicles with close enough voltages, all that's necessary is
to connect the packs together to equalize them?
In fact, fast charging has been done this way by EV racers and hobbyists.
The internal resistance of the batteries themselves handles most of the
voltage drop between the packs. It's sometimes called "dump" charging.
If the two packs are nominally the same voltage, the initial peak current
is huge; but it drops exponentially over time until the two equalize at the
same voltage and state of charge. It can take a long time to reach this
point, so it's only useful to get a relatively small amount of charge in
quickly.
But if you want the receiving pack to end at a higher state of charge, the
sending pack needs to have a slightly higher nominal voltage.
Dump charging works; but only if everything is right. With no fuses or
circuit breakers, things can go very wrong if there is a failure!
Yeah, the pack swap has lots of prob if you aren't swapping your own
pack...
I agree. The problems aren't with the hardware; they are with people.
When the same person or business owns both the EV and its packs, battery
swapping works. Thousand-pound lift truck batteries are swapped in just a
few minutes every shift. Or your Ebike battery runs down, so you pull it
and pop in a fresh one in second.
Given human nature, I suspect that battery swapping only works for a
rented or leased EV. Then the same entity owns both the vehicle and battery.
Lee
--
Excellence does not require perfection. -- Henry James
But it *does* require attention to detail! -- Lee Hart
--
Lee A. Hart https://www.sunrise-ev.com
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