[RE-wrenches] Automatic Dark Start of Depleted Lithium Batteries

Sat Feb 22 13:06:17 PST 2025

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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