BMS vs Charge controller settings

nate_syd

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I'm wondering if my charge controller & BMS are arguing...
Setup: I've got 8xCATL300AH in a 24v pack, temps are 5deg-40deg all year round, i'm happy to run them very conservatively for longevity. Victron Smart 100/50 for the charge controller

BMS settings as per SteveS' work (https://diysolarforum.com/resources/general-lifepo4-lfp-voltage-to-soc-charts-tables-12-24-48v.109/), which i'm basing everything around & want to stick to those. So i configured my BMS - which hits overvolt & flicks the Charge Controller from bulk into absorbtion. But i'm not sure if i need to change my Charge Controller lower than default - so that it is "in sync" and both the BMS & charge controller decide to go to absorption & stay there around the same voltage.

I'm thinking the devices need some alignment - if the absorption is pushing the cells/pack to overvolt then its just flicking on/off instead of holding it at the right level & dropping current. So my Charge Controller absorption voltage needs to be *just* below the voltage which triggers a cell or pack overvolt? That way it'll stay in absorption & push the most current without racing past the overvolt & resetting.
Make-a-sense?

By setting the CC absorption below the triggering overvolt i can see solid amps going into the battery pack without triggering overvolt, stopping & starting again.. Before it was triggering overvolt, pausing, then starting again & triggering overvolt... pausing... over & over again.


BMS Settings
  • Cell overvolt: 3.45v
  • Overvolt release: 3.35v
  • Pack overvolt: 27.6
  • Overvolt release: 26.8v
I typically see the pack hit 27.2v when 1 cell triggers overvolt & then it settles.

Charge Controller Settings (Victron Smart Solar)
  • Absorption 27.1v
  • Float 26.8v
 

time2roll

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Having the pack over volt at 27.6 and the cell over volt at 3.45 is redundant. (8x 3.45 = 27.6) You will never hit 27.6 unless all cells are perfectly in balance and then BMS will cut for cell over volt anyway. I will suggest increasing the cell over volt to 3.50 or 3.55 and actually let the BMS hit 27.6 if all are in balance.

Yes set the solar at .1 to .5 below the BMS pack overvolt. I would set the release values closer to the over volt values to have less time with the battery disconnected.

The goal is to not utilize the BMS cut off parameters unless there is some issue to resolve.

Others my give different numbers but I think you get the gist.
 

nate_syd

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Having the pack over volt at 27.6 and the cell over volt at 3.45 is redundant. (8x 3.45 = 27.6) You will never hit 27.6 unless all cells are perfectly in balance and then BMS will cut for cell over volt anyway. I will suggest increasing the cell over volt to 3.50 or 3.55 and actually let the BMS hit 27.6 if all are in balance.
yep understand - the BMS automatically calculates the pack overvolt when you set the cell. Just including for easy maths & to show the 27.6 vs 27.2/1 behaviour more clearly.
 

MisterSandals

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So i configured my BMS - which hits overvolt & flicks the Charge Controller from bulk into absorbtion.
I have never heard of anything like this. I think you are misunderstanding what your BMS is supposed to do.

The BMS is NOT supposed to be involved with your day to day charging or with your day to day load management. Its a safety switch that turns off the battery when one of the safety settings (cell over voltage, pack over voltage, pack under voltage, over current...) are violated.

You need to set you SCC to charge the way you want it and that should be within the bounds of your BMS safety settings.
BMS Settings
  • Cell overvolt: 3.45v
  • Overvolt release: 3.35v
  • Pack overvolt: 27.6
  • Overvolt release: 26.8v
This is nonsense. These settings should be the safely boundaries of your cells/battery, even if on a conservative level. Cell over voltage of 3.6V would be a very conservative BMS setting. Similarly cell low voltage around 2.5V or more conservatively around 2.8V.

These BMS settings will interfere with how your SCC wants to work and is designed to work.

Your SCC settings should be in line with these types of settings.
 

Bud Martin

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Post #1: "I typically see the pack hit 27.2v when 1 cell triggers overvolt & then it settles."
Did you perform top balance (parallel charging) the batteries to around 3.6V first when you first get the cell. How high did that 1 cell get up to?
 

nate_syd

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Messages
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Post #1: "I typically see the pack hit 27.2v when 1 cell triggers overvolt & then it settles."
Did you perform top balance (parallel charging) the batteries to around 3.6V first when you first get the cell. How high did that 1 cell get up to?
yeah did all that, they're A- cells & there's 1 cell is clearly lower capacity than the others, hence the difference between the total & individual. But as a % - its only a few % of full charge so i'm not fussed. Considering adding a 20Ah cell in parallel with that one to even it out, probably wont bother. Its that it'll just be working a bit harder than the others, but nothing too crazy.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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okay first things first, reset the BMS to its proper settings. I dunno what BMS you are using but here are my BMS settings, your's will likely be similar or have equivalent settings. Below that is my Midnite Classic Charge profile which makes use of FLOAT (Constant Voltage variable current) to allow the battery packs to top off and level/balance out nice & politely.

Note I use the Charger BMS8T and a QNBBM-8S Active Balancer on each pack. Currently 4 packs, 2x280AH & 2x175AH in the bank are using this and with the 5th 280AH going in as soon as my final parts arrive my 1190AH system will be finalized. I do not get HVD's because this is not overly aggressive and I am typically in Float by noon +/- during the summer, so that provides a few hours of float for nicely levelled up batteries, while the SCC provides enough amperage to run the inverter & usual draw during the day without calling on the batteries for their stored power.

Setting
Default
My settings
NOTES
Over charge P Voltage​
3.65​
3.65​
Over charge R Voltage​
3.55​
3.55​
Over charge Current​
50​
86A / 150A​
0.5C rate for 174AH / 280AH​
Over Discharge P Voltage​
3.00​
2.65​
CUTOFF Trigger​
Over Discharge R Voltage​
2.00​
2.75​
Release @ this V.​
Over Discharge Current​
300​
175A / 250A​
1.0C Rate for 174AH / 280AH​
Low SOC cutoff​
20%​
0%​
forces cutoff @ % (faulty)​
High Temp cutoff​
50C​
70C​
Diff of Batt Temp​
10​
15C​
Diff of cell Voltage​
30mv​
200mv​
Unmatched cells drift,​
Temp Unit​
C​
C​
Key Beeper​
ON​
ON​
LCD Backlight​
10​
10min​
Cut off Delay Time​
10​
10S​
Current Calibration​
-SET-​
Temp Alarm​
ON​
ON​
Cell Empty Voltage​
2.50​
2.50​
Cell Full Voltage​
4.20?​
3.65​
Default Setting​
Enable​
Balance Parameter​
-SET- (OFF)*​
Passive is ON charge only, start at 3.40V, 30mv diff.​
Battery Capacity AH​
1​
174 | 280​
Label Value of cells​
Battery Power WH​
1000​
4554 | 7168​
FORMULA (NominalVolts * #ofCells * RatedAH) (3.2*8*280=7168)​
Low Temp cut off in Charge​
2C​
2C​
Low Temp cut off in Discharge​
-10​
-10C​

"Midnite Classic-200 Charge Controller settings"
All equipment MUST BE Voltage Corrected & Calibrated (VERY IMPORTANT) see link in my signature on how to do it.
Divide Values X2 for 12V. Multiply X2 for 48V.
Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 (2.750vpc)
Max Volts: 28.7 (3.5875vpc)
Rebulk Voltage: 27.7 (3.4625vpc)
End Amps: 14A (*1)

(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A.
NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

** Coulumbic Efficiency for LFP is 99%

Hope it helps, Good Luck.
Steve
 

nate_syd

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Messages
89
I have never heard of anything like this. I think you are misunderstanding what your BMS is supposed to do.

The BMS is NOT supposed to be involved with your day to day charging or with your day to day load management. Its a safety switch that turns off the battery when one of the safety settings (cell over voltage, pack over voltage, pack under voltage, over current...) are violated.

You need to set you SCC to charge the way you want it and that should be within the bounds of your BMS safety settings.

yep - this is the confirmation i was after.
I was using the BMS to control the whole approach to charging, instead of the Charge Controller (lol - i realize how dumb that sounds after writing it!)
Will revert the BMS & adjust the controller as required.

FYI -
Controller: Victron SmartSolar 100/50
BMS: Overkill 8s 100A
 

MisterSandals

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yep - this is the confirmation i was after.
I was using the BMS to control the whole approach to charging, instead of the Charge Controller (lol - i realize how dumb that sounds after writing it!)
Will revert the BMS & adjust the controller as required.

FYI -
Controller: Victron SmartSolar 100/50
BMS: Overkill 8s 100A
I have a Victron 100/30 and Overkill 120A 4S.
Let me know if you want help with settings.

sounds like you are on right track.
 

PeterBC

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Nov 4, 2020
Messages
43
Finally have a few cycles of our 24 x Lishen 8S3P 272AH (816 AH x 24V) pack. Controlled by Electrodacus. On a barge in France. Shore Power and a Generator are initial charge sources. (we hide from the sun so solar not great investment). Unlike inline BMS's that cut charging or load, the Electrodacus signals the inverter (Quattro 5000) to stop charging or inverting (and also signals a BatteryProtect to stop direct dc loads on low voltage).

You DO want the BMS to control charging. NOT the inverter. General advice is to set absorbtion and float voltages high enough to ensure bulk charging all the time. But I'm interested in discussions about tapering the charge for the final amounts.

The quattro was programmed for Victron default Lithium settings.. 28.4 V Absorbtion (avg 3.55/cell), Float at 27.0 V. (3.375 avg).
Below is charging over about 9 hrs from empty. Note the roll off for last 20mins before BMS signaled disconnect (at 3.55v).
I believe that rolloff was induced because I have 8m cable run from Batteries to Quattro so at 95amps there is a sufficient voltage drop across the cable that as the pack approaches 28V or so the voltage is insufficient to maintain bulk charge. I should increase the cable size (or double it for a 5m section thats not doubled). But as others have mentioned it might be more polite to slow the charge near the end anyway. Acheiving that requires fine balance by setting charger absorbtion voltage for your fixed battery internal impedance and cable impedance.

Several post suggests ideal profile is bulk till 60% SOC, then taper with a constant voltage till current hits a low level (or 3.55v). If tapering is linear then time to charge the remaining 40% would be double the initial 40% (eg approx 6 hrs at 100 Amps, then 8 hrs at 100amps reducing to 0 , so averaging 50 amps). Of course inducing a taper from 60% SOC is extermely delicate if all you have is the absorption voltage, and unknown but generally fixed imedance of your charging cables etc and battery. Experimentation I gather would resolve.

So are the benefits of tapering the last 40% worth the longer charge time ? In my case if thats on a generator I will gladly trade a bit of life expectancy of the pack (as long as not drastic) for reduced run time on the generator. Life span on the genny is also a key factor and expense, not to mention fuel, and also noise /fumes in the boat and around for neighbours. (lets hope no fumes get into the boat or neighbours), but you do get a whiff occasionally.

Also leaving with the default victron profile with a fixed float of 27v after 1hr, created some odd profiles.. See next pic below

fullcharge.jpg

This is what happened when it tried to charge again well after Victron went to float mode. Note the amps dropped slowly from 21 A down to 0.4 amp, and stayed there for a LONG time. I suspect it might have gone on for a very long time at that 0.44A charge rate (into 816AH cells effectively !). 3rd pic shows pack and cell volts at that time. Some have said its possible to get 'low voltage overcharging' if running low amps like that for a long time. The cells were effectively full here at 100%SOC but not near 3.55 v cell cutoff.

I dont see any benefit in deliberately floating with very low current at high SOC like this?

I plan to certainly up the float volts on the Victron to 28.4v or more ..
I'm in 2 minds if I should try to set the absorbtion volts to deliberately induce tapering charge from 60 or 80% or so to slow it a bit near the 100% cutoff.

Nerds might notice the different shapes of the tapering current. One is concave vrs convex (for lack of the correct names of those shaped curves).
I think at high amps we see the effect of a rising pack volts, AND reducing voltage drop across the cable. (for a 'faster' roll off).
At low amps we see more just the pack voltage difference to the float charge voltage with neglible voltage drop.
If I reduced my cable impedance I suspect any high amp tapering would be more the later effect. To start that at a 60 or 80% SOC I'd have to lower the absorption voltage a deal from 28.4 I suspect.

12hrs.jpgcells.jpg
 

PeterBC

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Nov 4, 2020
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I've now set the absorption and float volts to 29v, decided trying to deliberately taper charging using absorption v is too tricky. Also remember I'm charging at just 0.12C, not 1C like many of the tapering recommendations assume.
 

Yurtdweller

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Nov 23, 2020
Messages
103
I'm trying to decide on a new controller, and I'm just not happy with Renogy anymore. I've outgrown my 40 amp, and in between midnight/outback, and victron. I've just built my first LiFePo4 pack, although I been running off a smaller pack that I bought, for about a year. I just grabbed an Ampinvt Cc, but it is not programmable. Thanks for posting all the straight numbers. This will aid my decision, I think.
 
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