Where is my power going?

[smr.ruby]

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".

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"?

 

[Steve_S]

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.

 

[zanydroid]

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)

 

[DIYrich]

3.65v is the max for a lifepo4 cell. For a 16 cell pack, that is 58.4v.

 

[ksmithaz1]

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.

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.

 

[smr.ruby]

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)

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]

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?

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]

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.

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]

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.

 

[ksmithaz1]

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"?

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]

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.

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]

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.

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]

I don't DIY/fiddle with my batteries or inverter settings. I let the battery BMS to inverter communications handle it. Thus far it seems to provide expected results. Based on my polling data I have seen as much as 57v in the last 30 days...(purple)

View attachment 185567

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]

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.

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

 

[zanydroid]

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...

https://bigbattery.com/wp-content/uploads/2023/08/Spec-Sheet-48V-MMTH-Plus-ShopSolar.pdf

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.

 

[Hardergamer1]

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.

Then this tells us a lot about their business model, and we and the OP could do with knowing the real specifications of their products, especially their BMS and cells...

 

[Zwy]

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.

Those are not LFP cells, those are LiIon.

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

 

[ksmithaz1]

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.

Perhaps, but to the point, of course not, because that is not the specification provided by the manufacturer of my batteries. EG4 spec is 56.2 for the Bulk charge +-0.2v and 54v float charge +-0.2v so if I was so inclined to not use BMS communication, I would make sure any manual setting is in that range. The Ruixu specs 56/54.6, so I would set the 2nd one to those values. In other words I would probably RTFM and use numbers they told me, unless had a compelling reason not to.

The whole charge portion of thread was started because of a suggestion that maybe the bulk charging level was set too low, which apparently it is not, since the vendor had suggested 58.0 (58.8 was listed as the maximum, 58.0 was listed as the end of the bulk charge range). Someone suggested that will blow up the battery, I merely looked up the manufacturers data sheet for the batteries, and noted that the numbers he was told to use fell within that spec.

Now the vendor could be totally wrong and clueless about the product they engineered, so far be it for me to dispute anyone here who has done actual research with the exact same cells and hardware that is in the Big Battery units. OTOH I did not see anyone with Big Battery batteries, commenting so I would weight any comments accordingly. A lot of the folks here have done a lot of work with a variety of BMS's and cells, so they have formed their opinions based on that research, I would never intimate they should be ignored, I'm simply noting what the manufacturer says.

Frankly I think it's crazy not to make sure all your stuff speaks the same language before you buy it. Based on responses to my earlier queries it shouldn't matter how hot you try to charge it the BMS will stop it. I'll stick with BMS comm, but I'd also lean towords what the guys who engineered it say, though that could indeed be sub-optimal. I notice with the BMS comm the voltage actually drops well below the listed 54v float voltages at 100% charge, and EG4 extends the warranty with EG4-EG4, so I'm guessing they are looking out for their own interests with the magic charge rates I see. No pony in this one, just noting what the builder sez

 

[Steve_S]

If you have 6 Packs in One Ban, you can ONLY charge at one voltage & a set AMP value... You can't pick which pack get's what. Using the common normal voltage parameters is the only option.

 

[Hardergamer1]

Those are not LFP cells, those are LiIon.

Which part do you mean? " Those are not LFP cells, those are LiIon" Which must mean everything I said isn't LEP, so the seller of the BigBattery Lifepo4 battery is not LFP/Lifepo4? comments like that are how the OP and others will lose track of this thread, and clearly shows you may post a lot but have little understanding of what you are talking about,

And If you are talking about the Samsung 25R cells, yes it is not Lifepo4 but ether Lithium chemistries can do their balancing in the same way, the only reason I used the 25R cell + BMS for an example was it was resent in my mind, but I like plenty of others have had the same findings with Lifepo4 BMS balancing just the same, and as I build and often repair battery packs using both chemistries you get to see it all first hand.