Of course not all clients are monetarily capable or willing to oversize
a Lithium based battery but many can. In my mind this is a simpler
solution. Its a strategy we're using more to avoid that voltage cliff
and reduce the likelihood of even nearing the BMS shutting down with all
of its hassle factor. This higher battery capacity allows a little
higher LBCO voltage again to keep the system away from the dreaded BMS
cutting out. With the availability of Midnite's PowerFlo batteries at a
much more reasonable price, the added cost for an increase in off-grid
reliability may not be out of budget for many systems.
Gotta love the conversation about parasitic loads also, I hope
manufacturers are looking at that.
My $.02
Bill
Feather River Solar Electric
Bill Battagin, owner
4291 Nelson St. (Shipping)
5575 Genesee Rd. (USPS, UPS)
Taylorsville, CA 95983
530-284-7849, 258-1641(cell)
CA. C10 Lic # 874049
Solar Powered since 1982
Home of the Sunny Side Up
On 2/22/2025 1:06 PM, William Bryce via RE-wrenches wrote:
Everyone wants to compare the AIO inverters to the older low
frequency inverters when it comes to the idle power draw. But everyone
now wants all the bells and whistles.
Nothing is free, and if you take an older system then add up the
charge controllers draw, SCP monitoring system, Gateway device, and
then add a battery monitor to the mix you will find the Idle draw is
much higher than what the MFG says for the inverter alone.
Feel free to check the math with a Schneider XW with 3 VH MPPT
controllers, a gateway, a SCP, and lithium batteries running closed
loop. Oh, the Schinder does not have any smart loads like most AIO do.
My 2 cents.
Logan
On Sat, Feb 22, 2025 at 12:03 PM Jason Szumlanski via RE-wrenches
<re-wrenches@lists.re-wrenches.org> wrote:
The voltage cliff is a real issue. Even though LFP batteries can
be pretty deeply discharged without damage, for practical purposes
we need to set the LBCO on the inverter at a relatively high
voltage (or SOC) to avoid the steep part of the cliff altogether,
rendering a good part of the capacity essentially useless. In
situations where there is a readily available charging source at
all times (auto-start generator or grid), there really should be a
way to overcome this game of chicken between the battery protect
mode and inverter LBCO. In the off-grid world, I don't consider
that a bell & whistle, but more of a required feature.
Maybe it's not so much an inverter issue as much as it is a needed
battery feature. Imagine if there were a dry contact on a battery
BMS that told it to stay on regardless of how it was feeling that
day (subject to safety shutdowns, of course). Then you could force
the battery to be alive with 48V DC connected when there is
generator output voltage present, for example. Of course, there
are risks with this simplistic example, like if the
inverter/charger is faulted and cannot charge the battery.
I think the right answer is closed-loop communications that can
tell a BMS in protect mode to wake up because there is a charging
source ready to go. If Midnite could implement this with
AIO/Powerflo, it could be a very powerful selling point. On the
other hand, maybe it's not that important as long as the inverter
reliably reaches LBCO well before the battery goes into protect
mode. That answer could be in closed loop communication logic
where the BMS sends a warning to the inverter that it is about to
shut down, so the inverter can stop inverting on the command of
the battery, but keep the battery connected so a charging source
will charge it.
In other words, maybe it would be better for the battery to be in
command of the inverter's LBCO rather than the inverter's own
fuzzy logic.
The parasitic draw issue does need to be addressed. I went through
some calculations on some typical systems I have in the field. For
example, one system has a 120kWh battery with four Sol-Ark 15Ks. I
think the inverter manufacturers prefer "idle consumption" to the
derogatory parasite comparison, but whatever you call it, let's
assume 360W for four inverters. If the inverter LBCO is set at 12%
and the protect mode is triggered at 2%, that gives us 33 hours
until the battery reaches protect mode in theory. That is a
substantial amount of time to get a charging source on the
battery. But in practice, I have seen many batteries enter protect
mode before a "properly" programmed inverter LBCO engages itself.
That brings up another feature request. How about dropping the
idle consumption of paralleled inverters and just keeping the
primary inverter at full idle?
Side note: I inherited a site where a Lithionics battery BMS is in
control of the 2-wire start for a generator. In theory, this
should work, but in practice, the owner often finds the BMS in
protect mode with the generator not started. I haven't dug too
deeply into this issue yet, but direct BMS control of the
generator is another interesting option. But then you would want
to build in all of the quiet time, charge percentage/voltage
limits, exercise, and other logic that typically an inverter
handles. This is an example of how a BMS is in control of the
charging source, but it would be better if the BMS was telling the
inverter what to do in terms of AGS and LBCO.
Jason Szumlanski
Principal Solar Designer | Florida Solar Design Group
NABCEP Certified Solar Professional (PVIP)
Florida State Certified Solar Contractor CVC56956
Florida Certified Electrical Contractor EC13013208
On Sat, Feb 22, 2025 at 11:17 AM Steve Higgins
<st...@surrette.com> wrote:
Hello all...
The first issue is that inverter/charger parasitic loads have
increased exponentially in the past 20+ years. When the LBCO
cuts out, the inverter may shut off, but it does not remove
itself or any other DC-connected device from the battery.
These devices still draw a parasitic load. In the 1990s, the
Trace SW would pull about .3 to .4 amps of current from the
battery when connected to it. Today, many manufacturers use
cheaper transformers, and the high-frequency inverters draw a
much higher current. Some of these all-in-one inverters draw
1-2 amps of current from battery banks, just connected and not
even turned on.
What's important here is that the battery voltage is already
very low when you trigger an LBCO shutdown (it's not a
disconnect). For a 48-volt system, this is 44 to 47 volts,
depending on where you set the LBCO. When a Lithium battery is
this low, the voltage dropoff is much higher. With a lead
battery, the voltage dropoff is much more linear, but with
Lithium chemistry, this voltage dropoff is more like a cliff.
This is why it's important for many of these Lithium systems
to set the Battery cutouts a bit higher so people have more
time to fix the situation before the BMS shuts down.
Ideally, the customer should be educated not to
over-discharge the bank, which would help. Many of these
customers want turnkey systems that they don't want to think
about but don't want to pay for it or do the work that is
required to maintain it.
Now, if the battery had gone into "Protect" mode and the BMS
had shut down, the battery is outputting very little
votlage... the inverter/charger needs voltage to run. There
used to be a line of inverters in the marine and RV market
that would do what we called "Dead Battery Restart". This
meant there was a parallel circuit in the power supply so that
when you supplied AC to the input, a secondary power supply
bypassed the regular battery power supply and would power up
the inverter and allow the charger to run. Most of the
inverter manufacturers got rid of this circuitry because it
was not cheap, took up space on the boards, and was just
another circuit that could get damaged with generator/shore
power surges. I don't know of an inverter today with this
dead battery restarting circuit.
With this, you need to be very careful. If the customer has
cratered the battery voltage and drawn down the voltage so low
that they have damaged the cells, jumpstarting the battery can
create a charging hazard, and that could cause the cells to
swell internally. If that happens, the battery will get warmer
and warmer under charge, and eventually, you could have a cell
rupture. This can happen quickly with Li-ion, but with LFP,
it's much harder to create this problem. Usually, in LFP,
cells will swell a bit, and the current interrupter on the
individual cell will open up and drop that string, and you
will lose capacity.
Like everything else, it's a race to the bottom on cost; this
affects quality and features... Everyone wants the "Bells &
Whistles," but they don't want to pay for it.
Steve Higgins
On Sat, Feb 22, 2025 at 5:08 AM Jason Szumlanski via
RE-wrenches <re-wrenches@lists.re-wrenches.org> wrote:
I have been thinking a lot recently about the reasons
off-grid systems can shut down, and working on strategies
to prevent these nuisances that require manual intervention.
Ideally, a BMS should never shut down due to low
voltage/SOC because a properly programmed inverter should
reach it's cut off well before the BMS decides it needs to
protect the battery, especially where there is closed loop
communication. But let's say that happens, where the BMS
does make the DC battery output go to zero.
It seems to me like the inverter should be able to start a
generator, and then signal to the BMS that a charging
source is available. But I'm not aware of any system that
actually does this. The inverter should be able to wake up
the battery. I can see this being particularly possible
where one manufacturer is writing the code (I'm thinking
Midnite AIO/Powerflo).
Of course, the inverter would have to have power in order
to do that, so if it's nighttime and there is no PV, the
inverter power would need to come from somewhere. I have
two thoughts. First, someone could manually start the
generator, waking up the inverter, but they would not have
to reset the BMS if the inverter told it to wake up. The
second way would be for the inverter to somehow close the
2-wire start circuit upon inverter shutdown, restoring
power to the inverter automatically.
If those are not options, an external NO relay powered by
the inverter output could be added to the 2-wire start
circuit, perhaps with a time delay to return to the NO
position to allow the generator to remain powered until
the inverter does it's thing and starts charging the
batteries.
Anyway, my question is whether any inverter/battery
combination out there works in a way that the inverter
tells the battery there is a charging source available to
wake up the BMS and reconnect DC power. And if not, why?
Jason Szumlanski
Florida Solar Design Group
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