Though, how will this setup works will depend on what kind of failure
happens.
One possible configuration would be with Big Cable cross connects at one
end of modules, and little cross connects between the 12 cells of the
module:
A) Single Cell Opens - in either module - All pack current comes from
one module It runs down faster, though in the process of doing so, the
other cross connected cells will end up trying to charge the ones from
the discharging module, and all of the small connects will end up fried.
B) Single Cell Shorts - in BMS connected module, will set off alarm.
Good. In other module - once again cascading melted cross connect wires.
Without Big Cable cross connects - i.e Strings are only connected at the
ends. (other than the little cross connects)
A2) Single Cell Opens - all current flows through other string. As
string voltages diverge, little wires start frying. If the open cell is
on the BMS side, it will get detected once the relevant cross connect
fries. If on the non-BMS side, we're back to cascading melted cross
connect wires.
B2) Single Cell Shorts - BMS side detects it quickly. Non BMS side -
cascading melted wires.
Thinking about the more-likely scenarios - different modules at
different temperatures resulting in small differences. I think that it
still runs the chance of melting the cross connects during heavy
acceleration. Note that each "cell" in this case is a 200AH unit which
is actually 20p.
On 10/9/2022 1:09 PM, (-Phil-) wrote:
Well, it depends on how closely matched the packs are. If they are
good quality and closely matched, then you don't really need the cell
interconnects between the 2 strings to be all that stiff. Consider
that most BMS balancing loads are 100ma or less, it's unlikely to have
much more than that flowing between the 2 strings unless there is an
actual failure. I've used silicone-insulated 20awg wire for this on a
pack before. In the event of a short, the 20awg will fuse and the
silicone will not burn and allow the conductors to melt through. If
you have an outright failure then this could happen, but otherwise the
resistance of the 20awg interconnects will limit the cross-flow
current during high pack draw, but allow them to still cross-balance.
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