DIYrich
Solar Wizard
I would do one cycle to 58.0v. Your batteries seem to need power and a reset. The BMS will protect the battery. After that, do whatever Steve suggests.
Where is my power going?
- Thread starter New England Rhinoceros
- Start date
DIYrich
Solar Wizard
I also suggest adding a shunt, like victron smart shunt, so you can get more detail on the battery.
Also, ask shop solar if the bms has an app.
Also, ask shop solar if the bms has an app.
Yes 3400mV is 3.4V. And the fact that they are all equal that high on the charge curve is good, that means they are pretty well balanced. I kind of doubt your system is anywhere near "failure" you just need to 1) get them fully charged and I believe 2) you still need to figure out why you are using more power than you expect? Refrigerator? 12V power supply on the RV? For context I run a similar RV setup and on a cold night (25F) I can use about 6KWH over a ~20 hour period that is cloudy in the evening and dark at night - just on a little bit of TV time and continuous cycling of the heater fan and with the refrigerator running from 120v (not propane).
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Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
A BMS (Battery Management System) is contained within the battery packs. It is the Guardian & Manager simply put.
It ensures that batteries are not Over/Under charged by enabling/disabling those functions.
Some can balance cells Passively or Actively, others may not at all.
They will also prevent cells from charging outside of proper operational temps.
An Advanced BMS with communications ability can interact with Inverters or AIO (All in Oners). of course there are dependencies.
ksmithaz1
Solar / EV Junkie
Never confuse ignorance with stupidity. The former is easily correctable, stupidity would be staying ignorant and not asking questions. Take your time sift thru the reponses, you'll get the enlightenment you need.
The values you spec seem to be within the boundaries of their published specifications, but there are going to be folks here that have their own ideas on what is appropriate for the various chemistries of batteries. 10K FT view, the bottom line is your 'Big Battery' (all of these batteries have something) has a 'BMS' inside that tries to control the individual cells and make sure they charge evenly. You generally have two voltages spec'd, a 'float' voltage and a 'Bulk or charge' voltage. The former is where you just leave it, think 'trickle charger' for you car battery. The latter is a number you can use to make it charge faster, and you will want to hold it there for a time, as the BMS will use the additional voltage to bleed power from one cell to another so the individual cells all have the same state of charge (SOC), ie the voltage level will be the same for each battery.
So what the battery geeks are taking about is optimizing the values and time that work best for the batteries. If the batteries get out of whack, ie with wide variances in voltage, it causes issues because the cells are in series and it becomes difficult to accurately figure out the SOC, then the BMS will shut things down before it should. This gets more complicated when you run two in parallel, as the battery units may also vary slightly, and the overall objective is to minimize the variances across the board.
(I'll defer back to the battery geeks from here)...
DIYrich
Solar Wizard
Does your inverter tell you how much power is being sent to loads?
These are great and I recommend too. But if you are in a pinch and don't have one OP should consider watching the battery discharge rate (amps) and going around messing with various loads: turn everything off and make sure amp draw is limited to inverter standby. then turn on only the TV. then only the refrigerator. etc etc. the amp meters in the BMSs are usually pretty good too.
ksmithaz1
Solar / EV Junkie
How exactly does the BMS kick in and prevent you from overcharging the batteries? The ones i've seen have a relay, and it should be on when the battery is "ON". There are two voltages in the inverter settings because the inverter/charger is supposed to be monitoring the voltage to the battery. It is supposed to attempt to provide the bulk voltage to the batteries. When the voltages reaches that threshold, it is supposed to drop the voltage to the batteries to the float voltage. If you set the float voltage too high you will cook your batteries. If your inverter/charger can communicate with the inverter then it can tell the inverter what voltage it would like to see, but the BMS has no way I am aware of to control the voltage it is passing thru. The nice part about BMS communication with the inverter is that it can actually request an appropriate voltage based on what it is seeing at the cell level.
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Sorry but I am going to be terribly BLUNT on this.
Far too many battery pack sellers will use the ALLOWABLE VOLTAGE range in their specs which is terribly wrong... Even with perfectly matched cells, stuff WILL get awry past the working curve. This is just the way it is.
Nominal Cell Voltage is ALWAYS STATED AT 3.200 Volts for Standard LFP. Dead Centre of the Working Range which is 3.000-3.400 YES that Very Flat voltage curve. IF Nominal Voltage was stated as 3.075 that would put it dead centre of 2.500-3.650.
EVE, CATL, LISHEN and all the others producing Standard LFP, test & validate the cells to deliver their stated AH from the WORKING RANGE and not the allowable.
Confusion Source: quite often, "shifty" vendors flogging great market cells will say they are rating the cells from testing BUT from 3.650 down to 2.50. Customer get's them, charged to 3.450 and tests to 3.000 and get's "maybe" 80% of spec'd AH. It's happened a LOT and a lot of folks here have hit that... Again why some of us tell folks to get A-Grade Matched & Batched with Factory Reports only.
Finally, we have people here using every conceivable "assembled retail battery" out there pretty much (for this class of gear). A great many have just followed the "label" charging to 3.65 etc etc... They hit problems, we troubleshoot, they finally get to reasonable, proper conservative charging profiles and POOFDAH the Problems go away... time & time & time again... BTW: it gets exhausting !
There are links in my signature & several TECH threads I've written over the years for folks here... Have at it and Good Luck.
ksmithaz1
Solar / EV Junkie
The BMS has a mosfet that disconnects the battery pack from the bus voltage for over current protection, over voltage protection, and when it determines charge is complete. Seplos for example will charge to 100% SOC and then disconnect the pack from the bus until SOC falls to 96% when it reconnects again. Left with no load it will constantly cycle between 100% and 96% disconnecting and reconnecting. That is just the sequence of operations for the BMS, they aren't all the same. See the graph I posted earlier as an example. The SOC was falling coming into the left side of the second graph, when it got to exactly 96% it reconnected pack to supply using the mosfet.
and I'm guessing when you mention "relay that should be on when the battery is on" you mean the breaker/switch that you manually turn "on"?
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Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Some BMS' use Relays/Contactors to control power flow, generally these are for higher than 300A constant use applications. The general ESS BMS' we use are FET Based without relays.
It is NOT the job of the BMS to do anything else other than protect & balance cells inside a battery pack. Balancing if it has that capability, most SmartBMS' have at least passive balancing (burns off hi cell voltage - wastes it), Active Balancing is a step further and actually transfers power from Hi cells to Lo cells without loss.
Charging Profiles are managed by the Solar Charge Controller, Inverter/CHARGER or AIO (All-In-One) which combines the lot.
It is NOT the job of the BMS to do anything else other than protect & balance cells inside a battery pack. Balancing if it has that capability, most SmartBMS' have at least passive balancing (burns off hi cell voltage - wastes it), Active Balancing is a step further and actually transfers power from Hi cells to Lo cells without loss.
Charging Profiles are managed by the Solar Charge Controller, Inverter/CHARGER or AIO (All-In-One) which combines the lot.
zanydroid
Solar Wizard
I've heard (I vaguely remember from CC_Dexter) that BMS will have separate protection for charging and discharging situations. How does that work? Is the FET configured in "wire" or "ideal diode", with extra inputs to the FET driver/controller to allow flow in different directions depending on the pack voltage?
IE if pack is between low and high voltage cutoff, FET behaves like a wire.
Above high voltage: ideal diode allows power to flow out (IE enforces lower voltage outside the battery than the battery current voltage)
Below low voltage: ideal diode allows power to flow in (IE enforces higher voltage outside the battery than the battery current voltage)
In this case, what happens if the battery is above high voltage cutoff, but there is a load that is too big for the charger? What (if anything) causes the BMS to turn on the FET to allow the battery to share load with the charger but not charge the battery?
(apologies I haven't looked at a schematic (though I doubt this will show up in one since it's largely logic in the controller not the circuit itself), this stuff has just been living rent free in my head)
IE if pack is between low and high voltage cutoff, FET behaves like a wire.
Above high voltage: ideal diode allows power to flow out (IE enforces lower voltage outside the battery than the battery current voltage)
Below low voltage: ideal diode allows power to flow in (IE enforces higher voltage outside the battery than the battery current voltage)
In this case, what happens if the battery is above high voltage cutoff, but there is a load that is too big for the charger? What (if anything) causes the BMS to turn on the FET to allow the battery to share load with the charger but not charge the battery?
(apologies I haven't looked at a schematic (though I doubt this will show up in one since it's largely logic in the controller not the circuit itself), this stuff has just been living rent free in my head)
DIYrich
Solar Wizard
3.65v is the max for a lifepo4 cell. For a 16 cell pack, that is 58.4v.
ksmithaz1
Solar / EV Junkie
I think this is key... Unfortuntely, any time you start working with equipment that is not designed from the outset to work together you may have to tweak things. The published specs for this unit specifies a range, normally one would probably look to be in the middle, which would seem to line up with the recommendations here for the most part. I think there is a teensie bit of black magic in futzing with BMS's and batteries. It's like baking, I use the same ingredients and oven temps as my Grandma, but her biscuits are always way better
!You really need sensor data / logs for this stuff. If you are going to cobble together a system, and you have an issue, you really need data points for an accurate analysis. Without a tool to monitor the BMS on each pack, and some sort of logging of not only SOC and voltage, but the actual load and usage it's kind of tough. There might be some vampire load on the system like some failing device sucking down electricity causing the problem. I like to start at the end and work my way back. How much power am I actually using at the AC side of the picture... I have 100% charge, on 18KWH of batteries now according to the inverter or other sensor device I output 15KWH and the batteries were done. Nice, sounds about right figuring a 7% loss and some aggressive labeling of battery capacity! If you shouldn't be using that much power, well, find the load. Now if you use 5KWH and the batteries were done, OKAY, split the battery pair, charge one up to 100% repeat the test, try and isolate the fault. Chip away until you find out why
The graph above demonstrates what a reasonable charging cycle should look like, I'll leave the hard number to chemistry discussions for you guys, but I'd be sure I was solving the actual problem before I got too happy fiddling around with a bunch of charge settings. Be kind of annoying to find out there was a space heater kicking on somewhere tapping the system and sucking it dry.
I'd also look into firmware updates and changelogs for the batteries if there are any. "Big Battery" seems to be a reputable firm thus far, at least they have been around for a bit. I would hope they would not send you too far out in the weeds for manual settings.
There are other resources but to quote a good post on this forum here is a good explanation of separate charge and discharge fet control https://diysolarforum.com/threads/bms-mosfets-explained.29318/.
the discharge FETs remains connected when the charge FETS disconnect due to charge completion.
DIYrich
Solar Wizard
I would guess the bms is checking the voltage on the inverter side of the relay. If voltage is lower than the max, it closes the relay to allow power to flow out. The interesting case is if the battery then pulls up the voltage above the max.
Maybe the bms is smart enough to bleed down power to the max. Even when relay is open. Like it bleeds charge when balancing.
zanydroid
Solar Wizard
The thread quoted in #66 I think covers it beyond most mortals need to know.
(I don't know if the same techniques would work for relay-based BMS. There's a modest amount FET-only behavior in the linked article, and of course transient effects are important to worry about, when the system is switching between different modes. That is somewhat addressed in the linked article. For instance, compared to my guess at the implementation the switching between modes is more gradual)
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
The thread is off on a Walkabout in the outback.
The OP will now only get further confused trying to absorb things outside of the Original Post.
What's the banner at the top of the page say ?
Zany, wanna learn about Relay BMS, search for Chargery - days + of reading there.
The OP will now only get further confused trying to absorb things outside of the Original Post.
What's the banner at the top of the page say ?
Zany, wanna learn about Relay BMS, search for Chargery - days + of reading there.
ksmithaz1
Solar / EV Junkie
No I mean the solid state relay or physical contactor on the board where the battery cables and bus cables connect. I've disconnected my batteries from the inverter and I've never seen an 'open' state, from either brand as long as the breakers/power switches were on on my rackmount units, I guess it could, but I would think that might have interesting latency effects if your setup was cranking out and then a cloud popped over and it needed the batteries all of a sudden to supplement. That behavior is inconsistent with the overcharge warnings of the documentation on my units as well. Otherwise it sounds reasonable if it could flip back and forth fast enough.
ksmithaz1
Solar / EV Junkie
Actually what the OP wanted boiled down to why he did not seem to be getting the expected output from his inverter based on the theoretical quantity of power in the batteries he had. He could only run his big screen and a light bulb for a couple hours. I defer to my earlier post mainly verify the problem is with the batteries before you fiddle with them. If true then isolate the batteries one at a time to see if you can narrow it down. If not find what's sucking down the juice.zanydroid
Solar Wizard
As discussed above, as long as the battery is above low voltage cutoff the BMS will be designed to avoid hard cutting like that, and the logical/physical concept of charge vs discharge paths (even if they're shared, in which case it's a mostly logical but still physical thing) ensures that you can pull energy off the battery even if BMS wants to block charging.
Zwy
Emperor Of Solar
I already knew the answer to the question I asked, hence the
You are using the BMS communication for charging control, that doesn't apply to anyone charging LFP with a USER setting in the charger. Huge difference.
Now if you quit using BMS communication, will you use 58.8V?
It's a rhetorical question of course.
Hardergamer1
New Member
Steve_S, You say it's the "shifty vendors flogging great market cells" but it is the manufacturer "BigBattery" that states the 58.8V max, 55.6v-58v charging, and even gives a 10-year warranty on them with their settings... But, I mostly agree with you that charging this high will decrease the cycle life etc, but one problem I have seen especially with cheaper and badly programmed BMS is that they can be set to only balance when they get very close to max cut-off voltage, often just 3-4%.
For instance, recently I was asked to replace a BMS on an ebike that wasn't balancing the cells properly, the cells were Samsung INR-25R 4.2v cells and the BMS would only start balancing at 4.18v @10mv by the pull-down method only when charging, and this is why giving a blanket statement on all Lithium batteries and BMS's can be problematic, which is why I would recommend to charge them to at least their minimum manufacturer recommended voltage of 55.6v (3.5375v) or they simply may never activate there balancing mode.
Also, I may just add, that if they are being charged too fast by the generator or big solar, then the BMS may have very little time to balance the battery packs, which is why it can help to bring the battery up to the point of balancing and then charge at a much lower amperage giving the BMS/battery time to absorb and properly balance the pack before cells hit cut off and stop all charging.
https://bigbattery.com/wp-content/uploads/2023/08/Spec-Sheet-48V-MMTH-Plus-ShopSolar.pdf
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zanydroid
Solar Wizard
Based on the BigBattery BatteryEVO drama thread I linked earlier (which touches on some funky stuff going on with their business lately), I wouldn't trust Big Battery this far.
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