diy solar

diy solar

Possible JBD BMS faulty? Advice please

ES1

New Member
Joined
Nov 20, 2020
Messages
14
I have eight 270amp lifepo4s wired in series and parallel giving 540Ah at 12V. I have set my 120Amp JBD BMS to 80% giving me 432Ah. I have charged the batteries to 432 Ah, disconnected ALL devices and left overnight to rest. I do NOT have an inverter installed as I do not use any heavy accessories. I have disconnected the solar panels. I checked the batteries 24 hours later and they still displayed 432Ah, all well so far. The batteries had a balance of .003, and still do.
I then noticed that in the system it was showing, with a red triangle, an incorrect low temp setting. I reset the temp as the JBD app instructs but then noticed that the 432Ah was now suddenly showing just 302Ah a SUDDEN loss of over 100Ah. I am unsure of what to try next as I have been having this problem for about 4 months when pressing Reset Capacity, however this is the first time I have charged to 432Ah and it has given the biggest SUDDEN drop.
I am considering splitting the battery, make 2 12v 270Amp top balance, and then recharge. I do have a spare 60amp BMS which I can also try as well.
Could this be a faulty BMS controller? your thoughts would be appreciated.
 
Which 80% of your battery are you using, top, middle, bottom or other?
What is your bulk charge voltage?
What is your float voltage?
I wonder if the temperature sensor might be a red herring.
How are your doing bandwidth management for 80%?
Sorry if the questions overlap a bit but its all part of the process.
 
From your post I understand that your configuration is 2p4s, confirm?
 
I am using the Lifepo4 Lite offered by JBD BMS App which I assume from the settings is the top!
Configuration as you have assumed correctly is 2p4s. I have that feeling that by resetting temp it could have reset everything so giving the massive drop in Ah. I do not have bulk or float charged at moment but when at 432Ah a single cell was 3.334 with a 0.003 variation. I will have further info concerning previous and current voltage available a little later however.
Many thanks
 
I am using the Lifepo4 Lite offered by JBD BMS App which I assume from the settings is the top!
Configuration as you have assumed correctly is 2p4s. I have that feeling that by resetting temp it could have reset everything so giving the massive drop in Ah. I do not have bulk or float charged at moment but when at 432Ah a single cell was 3.334 with a 0.003 variation. I will have further info concerning previous and current voltage available a little later however.
Many thanks
I don't think that profile does what you think it does.
Also trying to manage cycle bandwidth by percent is not really feasible unless you have a BMS that can talk to your charge sources and loads.

For lifepo4 you want...
high pack disconnect to be 14.6 volts
high cell disconnect to be 3.65 volts
low pack disconnect to be 10.0 volts
low cell disconnect to be 2.5 volts

Your charge profile should be
bulk/absorb=14.2 volts
float=13.4 volts if you are going to use float

The jbd bms only has ~150 milli amps of balance capacity.
It will likely struggle to maintain the top balance of a pack with effectively 540 amp hour cells.
You need to charge into the high knee on a regular basis to maintain the top balance.
You also need to charge into the high knee for the capacity to reset.
IMO its a necesary stressor.

To optimize your battery longevity you should stop discharging your battery at the point where the weak cell stars to nosedive.
You need to watch your pack discharge at a representative load to determine where the weak cell starts into the low knee.
As a rough starting point start with 12.0 volts or 3.0 volts per cell.
Discharging into the low knee is an avoidable stressor.
 
Last edited:
The battery Pack is 540Ah and NOT 432Ah. Error #1 do NOT lie to the BMS, give it the proper values.
Your Pack Values are: 12V/540AH/6912Wh

When cells are paralleled like this, the Pair "Ghost" each other and appear as One Cell to the BMS. If one cell is weaker than the other the pair is affected "as a set". Runner or Lazy Cells can cause Serious havoc in such a situation and be Very Difficult to track down / troubleshoot. IF people are Hell Bent on paralleling such large cells we always recommend using properly Matched & Batched cells (just like EV's) because mismatches = problems.

If using Grade B, Bulk or just "Voltage & IR Matched" bulk cells, paralleling cells is NOT recommended.
PLEASE BE AWARE: Proper Matching & Batching is a long & costly process that ensures that the cells have the same Resistance (IR) whether at 2.8V or 3.0, 3.2, 3.4 Volts per cell. The Vast Majority of Vendors DO NOT OFFER THIS ! They will Voltage & Internal Resistance Test with a handheld device like a YR-1035+ Battery Tester, this is done at the storage Voltage & Ambient temps, without ANY cycling or validation... Those vid's you see vendors showing voltage & IR are the obvious example. Unmatched cells are best used in Simple Straight Series Configurations.

How to determine YOUR 100% and YOUR 0% !
LFP Gross Voltage Curve is 2.500-3.650
LFP Net Working Voltage Curve is 3.000-3.400, with 3.200 Vpv (Volts per cell) being Nominal.
The NET AH available to you is all within the 3.000-3.400. With Grade-A Matched that is usually extendible by 0.50 in both directions without issues.
Grade-A will test out around 280AH Gross (between 2.5-3.65) and the NET AH from 3.000-3.400 would be 280-272AH.
Grade B/Bulk would test out at no more than 272AH GROSS and provide typically 10-12% Less from the working range.
Be aware that from 2.500-2.900 and 3.450-3.650 only represents roughly 5-7% of total gross capacity , this is due to the very flat voltage curve of LFP.
Remember also that LFP always settles Post Charge and this is normal & expected behaviours, on average within 1 hour of Full Charge, it should settle by approximate 1 Volt IF Fully Charged, if not fully charged it could drop a bit more.

Most people wish to observe the 15-20% reserve to max lifecycles and use 80% of total (gross) capacity.
Here is a profile that would work for you. Terms vary a bit between equipment brands.
Absorb: 13.75 for 15 minutes (3.4375vpc) (some call this boost)
Equalize: OFF
Float 13.7V (3.425vpc)
MIn Volts: 11.0 (2.750vpc)
Max Volts: 14.35 (3.5875vpc)
Rebulk Voltage: 12.0 (3.000vpc)
End Amps: 27AA (*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 NB2: Paralleled cells make this difficult.

Coulumbic Efficiency / Battery Status Meter Efficiency for LFP = 99%.
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 3.400-3.420. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.
* Do Not forget to adjust for Voltage Offsets between Actual Voltage @ Battery Terminal & Solar Controller & Inverter.

Good Luck, hope it helps.
 
Steve many thanks for your response, it's very enlightening, especially your second and third paragraphs concerning Ghosting and Grade B cells. Would you therefor recommend splitting my cells into 2 separate battery packs each having its own BMS,s, and then join into parrelel after the BMS,s. If so would you recommend having 2 similar size BMS,s.
The Battery Pack size as you rightly state is 540Ah, however the 432Ah figure is 80% of 540Ah, which the JBD App gives you when choosing Lifepo4 Light, quick setup. I shall now ignore this quick setup, and use your Absorb and Float profile, looks ideal.
Again many thanks for your advise
 
As I said, unless the cells are Properly Matched & Batched keep them simple.
KISS Applied is always best ! Keep It Sweet & Simple.
I would indeed suggest 2 separate packs with identical BMS' connected to a Common DC Bus in Parallel. Ideally both complete batteries connected to common (+) & (-) BusBars with equal length wires from the batteries to bus bars. This also allows the two batteries to share & divide both the Load & Charge while also providing fail-over fault tolerance should one pack cutoff for a reason, the other keeps going.

Always use the same gauge of wire from Battery Terminal to common Bus and from common bus to SCC/Inverter.
DO Keep the DC wires close together (zippy ties or other but not "tight" just snug). This reduces Elecritical Noise and is very important. It is DC !

Most common installations are Battery Packs connected to Common DC Bus, then to a Battery ON/OFF Switch like a BlueSea 9000 I would suggest the E or M Series due to the higher amp handling capacity.
REF LINK: https://www.bluesea.com/products/category/11/Manual_Battery_Switches

Do remember to FUSE each Battery Pack ! Fusing should account for possible surge pulls as well. 250A is typically MAX as it's not good to pull more from a bank in any case, regardless of Voltage.

270AH Cells can output 270A for One Hour (1 C-Rate) and they can take 135A Charge Rate for Two Hours (0.5 C-Rate). These are the MAX Limits for ESS Grade LFP.

Appreciate also that a 12V System pulling 250A from a battery bank produces 3000W (Uncorrected for inverter loss efficiency) and that is the "edge" in a sense... If you need to pull more amps then the next step is to up the system Voltage.
12V@250A = 3000W, 24V@250A=6000W and so on. The 3000W÷120VAC = 25A (uncorrected)

*Uncorrected: Inverters have efficiency losses, The lower-end models can be as low as 82% Efficient while Tier-1 High-end models can be up to 95% Efficient, and also if the Inverter is Low Frequency or High Frequency which also affects efficiency & stand-by power consumption.
NOTE that the Cheaper 12V "Car Inverters" are usually pretty low grade when it comes to efficiency, as they are only intended for "Part-Time Limited Use".

Hope it helps, Good Luck
 
Steve many thanks for your response, it's very enlightening, especially your second and third paragraphs concerning Ghosting and Grade B cells. Would you therefor recommend splitting my cells into 2 separate battery packs each having its own BMS,s, and then join into parrelel after the BMS,s. If so would you recommend having 2 similar size BMS,s.
The Battery Pack size as you rightly state is 540Ah, however the 432Ah figure is 80% of 540Ah, which the JBD App gives you when choosing Lifepo4 Light, quick setup. I shall now ignore this quick setup, and use your Absorb and Float profile, looks ideal.
Again many thanks for your advise
I have a grade B group of 280 cells. I want to find the weak ones and the strong ones. I have been running these at normal voltages. The bms app says battery capacity is like 46 amp hours. 16 cells at 280 is 280 at 52v. I think the bms app needs to be reset but it's not resetting. I dont know if the app is bad the bms or my cells. Using a jbd s20
 
Back
Top