[RE-wrenches] Automatic Dark Start of Depleted Lithium Batteries
Bill Battagin
frenergy at psln.com
Sun Feb 23 10:47:03 PST 2025
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 at 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
> <steve at 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 at 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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