[RE-wrenches] Strange battery overvoltage issue (Midnite AIO / Lithionics Battery)

[Zeke Yewdall]

I see on the Lithionics spec sheet it says a charged voltage of 57.6 volts
 This seems very high for a 16 cell LFP battery.  I am used to using 55 to
56 volts, maybe 56.5 volts at most on a 16 cell LFP.  I suspect that the
57.6 is the protection voltage of the BMS, not the appropriate absorb
voltage setting.  I have seen this error a lot on lithium battery spec
sheets, where they give the overvoltage protection level, which is not an
appropriate absorb setting for typical solar equipment.

Not everyone will agree with me, but the way I set up absorb voltages when
doing open loop lithium battery systems is to NEVER let the BMS do
anything.  if the BMS shuts off charging, that means that I already
failed... the charge controller or inverter should have stopped charging
before the BMS feels the need to protect the cells.  Same with low voltage
disconnect... if the BMS shuts off due to low voltage, I already failed --
I should have shut the load off before the BMS felt the need to turn of
discharge to protect the cells.  The BMS is like an airbag...only to react
in cases of emergency when the rest of the system doesn't work properly.

Open loop equipment (all of the traditional outback/magnum/schneider, etc
stuff, plus any AIO's in open loop setup) is designed to always have a
battery in the system to stabilize the DC bus voltage.  Without the
stabilizing effect of the battery, voltage spikes can occur.  I find that
many charge controllers, especially the midnite classic and victron ones,
can actually keep the system fairly stable without batteries, but charging
from inverters is more questionable, and especially charging from any DC
generators can cause severe spikes if the battery disconnects. I have fried
equipment when a lithium battery disconnected from a large kohler DC
generator and the generator couldn't react fast enough to keep it from
spiking to 75 volts or more.  This is why you never want the BMS to
disconnect... which means setting absorb at a level where the BMS never
feels threatened.  Lithium battery manufactures giving the voltage at which
the BMS disconnects as the "charge to" voltage does not help the
situation.  That may work when charging it with a dedicated lithium battery
charger, without anything else connected to the battery while it's
charging.  But in a functioning solar system with charging from multiple
sources and loads all occurring at the same time, we need to be smarter
than the battery, IMO.

Zeke
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