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

[jay]

One advantage of the blue ion if I have it right is they had a charge circuit built in using diodes.  Which meant it could charge even though it was not discharging.  Pretty trick design.  

I do think the Lithium folks need to have a much better way to deal with someone who has black start situation besides having to remover the cover ( its thats even possible ).  Yes would be more cost, but what would it cost to have a small 48v 5 amp charger built in with an access port on the outside.  Or a port that you connect 48v nominal to and push a button which has a diode in it, so you can’t discharge only charge from that port. 

We can talk about it should never happen but like has been discussed there, the arc fault shuts down the CC, people are gone for a vacation or whatever come back to a dead system. There are lots of reasons that shit can happen that don’t include people being negligent or stupid with their system. yeah yeah stupid is probably the majority but still.  
It needs to be easier more reliable to have the system in failsafe mode and to get it out of that mode.  

I get it there is no perfect design, but we need some improvement because with more lithium batteries we are seeing this more and more.  For us old timers, the big advantage of lead was that if the inverter turned off, just getting the generator started would get it all back to working.  

I like what Jason is talking about in regards to some sort of communication that when the inverter turns on from PV or generator it will force start the batteries so they can charge.  And yes it could also be programmed so it won’t pull from the batteries until some given %. 
More tech needs more tech. 

Jay



> On Feb 22, 2025, at 10:02 AM, 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 <mailto: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 <mailto: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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