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Chargery BMS Cell Readings Inaccuracy. Should I be concerned?

I started out using ring terminals for the BMS connections, but didn’t like the performance. Terminal connections adds resistance. I have 160A currents and want to keep resistance as low as possible at the buss bars. Without the ring terminal, the connecting bolt head makes contact with the top side of the buss bar. That’s as good as it can get. The BMS wires are soldered to the buss bars.
 
I started out using ring terminals for the BMS connections, but didn’t like the performance. Terminal connections adds resistance. I have 160A currents and want to keep resistance as low as possible at the buss bars. Without the ring terminal, the connecting bolt head makes contact with the top side of the buss bar. That’s as good as it can get. The BMS wires are soldered to the buss bars.
Got me thinking. I cleaned the posts and the bottom of the buss bars, but not the top where the BMS connections make contact. I'll have to give that a try on cells 9 and 16 to see if it makes a difference.
 
I cleaned every point of contact with steel wool.
First, I should note that I am using C110 1/8" X 1" copper bar stock for buss bars. I cleaned buss bars associated with BMS wires 7,8,9 and 10. No difference. Luckily I'm using grub screws with all the loosening and tightening I have been doing.
 
First, I should note that I am using C110 1/8" X 1" copper bar stock for buss bars. I cleaned buss bars associated with BMS wires 7,8,9 and 10. No difference. Luckily I'm using grub screws with all the loosening and tightening I have been doing.

@Tony any luck? I just installed my second Chargery 16T bms and seeing similar issues, where some cells are reading almost 50-120mv off the MM reading. My 1st Chargery is spot on, 0-15mv +-. Not sure if i have a bum unit. Plan on updating the firmware to see if that makes a difference.
 
@Tony any luck? I just installed my second Chargery 16T bms and seeing similar issues, where some cells are reading almost 50-120mv off the MM reading. My 1st Chargery is spot on, 0-15mv +-. Not sure if i have a bum unit. Plan on updating the firmware to see if that makes a difference.
I purchased another BMS16T as I couldn't get Jason to acknowledge it was a BMS problem. Jason said he was giving me a discount but I wound up paying $30 less for just the main unit and the balancing leads. Don't think that was a discount. Here are the readings (8/21 is the new BMS):
CellBMS vs DMM Calibrated 7/31BMS vs DMM Calibrated 8/7BMS vs DMM Calibrated 8/21
1-0.010-0.012-0.010
20.000-0.0010.005
30.0000.0000.009
40.000-0.0010.005
50.0030.0060.007
60.0150.0140.007
7-0.009-0.0050.007
8-0.002-0.0020.000
9-0.022-0.0420.003
100.0320.0550.018
11-0.002-0.0060.012
120.0190.016-0.007
13-0.014-0.0110.011
140.003-0.0010.004
15-0.0010.0030.003
160.0360.028-0.013

The readings of the new BMS are much closer to the DMM. I shared this with Jason and let him know my new BMS is running just as hot as the original unit resulting in balancing not functioning (when it should) most of the time. I have not heard back from him.

I also discovered the one of my problems may be related to a bad crimp on one of the BMS shunt leads that was shipped with the original BMS. During setup of the new BMS the connector fell off the lead. I asked if this might have contributed to some of the my problems such as the SOC catastrophic drop to 0. I haven't heard back about that either.
 
@Tony, the shunt lead end falling off would have been a factor resulting in faulty readings.
I always check wire continuity as a matter of rule first when troubleshooting, I do not assume that all is well until I verify it myself.

BELOW is something that is headed to the Manuals (Version 4.01a) when Jason & I finish our latest exchange of info. I am posting this HERE NOW because I believe it will answer some questions, issues and clarify some misconceptions. THIS IS GENERAL so it may / may not apply to any specific problem.


BMS Balancing Information
Some clarification on the Chargery BMS Balancing system. The primary purpose of the BMS is to protect the battery pack cells and to control the flow of energy according to the limits set in the software. By default, the BMS is shipped with the balancing function turned off, as this is a secondary feature of the BMS. This BMS uses Passive Balancing which is not like Active Balancing, it only bleeds off excess voltage from a cell.

In passive balancing, energy is drawn from the most charged cell and dissipated as heat, usually through resistors.

In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells, usually through capacitor-based, inductor-based or DC-DC converters.


Balancing speed / operation:
The balancer uses 1.2A per cell and is capable of leveling the cells out over time. This is not a fast process as it reduces the voltage from cells to bring them in line with the remaining cells. This process also generates heat within the Main BMS Module as it is dissipating the power from the cells. As temperatures increase the BMS will throttle down the power to prevent overheating, this is seen as the balancing stopping & starting continually when looking at the display.

For large capacity cells with higher cell voltage differences, the balancing is slower and some users find this unacceptable. Chargery has added a Fan Port on the newest Main Module hardware to allow for the installation of another heat sink with a cooling fan which will speed up the balancing process. This is optional and requires some modification for the main module.

Balancing and Cell Status notes:
When cells age, their chemistries alter somewhat as does the Internal Resistance over time with charging and discharging. This is typical and normal behaviour. As cells age and deviate from their original specifications, balancing can maintain the cells but this is limited as balancing cannot repair or "reconstitute" cells. As cells continue to deviate, the time required to maintain balance will increase accordingly, until one or more cells reaches a point where the differential, usually Internal Resistance is too great and eh cells should be replaced. (Note 1)

Cells do degrade over time for various reasons, it is not inconceivable to have one or more cells fail before the others over time. A common indicator of a degrading cell is that it will discharge faster than the other cells within the pack, resulting in an imbalance that may trigger a low volt cutoff.

(Note 1)
In some instances, cells may have become deeply imbalanced by very heavy discharges or high charging rates which imbalance the cells within a pack. In such instances, it may be possible to "Top Balance" the cells to even out the voltages between the cells. (this requires putting all the cells in parallel and allowing the voltage to even out across the cells) This may not work if the Internal Resistance is too varied, then in such a case, a more drastic approach may be applied by doing both a "Bottom Balance followed by a Top Balance" which requires that the battery pack be disassembled and reconfigured.

Reference information on Balancing: https://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancing

The Balancing Function process example:
With the BMS installed and "Balance in Storage and In Charge" is ON, at night, when the battery is idle (storage mode) the BMS will balance the cell voltage (by burning off excess on higher volt cells). Assume that the balance time is 10 hours, the balance current is 1.0A. With a cooling fan installed, the BMS will balance for 10 hours at 1A continuously, this means the higher voltage cell will be discharged 10AH. If all cell's voltage is balanced after 10 hours, it means that the cell capacity difference is 10AH. So, if the battery pack capacity is 100AH, it is a 10% difference, if it is 200Ah, it is a 5% difference. This, in fact, would not be an acceptable capacity difference for a battery pack. If the capacity difference is 1%, or 1AH for 100AH battery pack, the cell voltage will be balanced within an hour, even if the pack cannot be continuously balanced, within 10 hours of operation it should be sufficiently balanced.

Higher capacity differences cause another serious problem, the SOC will drop suddenly. The root cause for this is a High Impedance difference which in turn causes a high capacity difference. The Impedance increasing is caused by natural battery fade and internal micro short circuit, the latter is usually the main reason, it depends on battery manufacturing process control.

SOC drops too fast yet AH / WH appears to be enough:
Some users have found that the SOC drops suddenly or the BMS displays enough Ah and WH but cuts off discharge, then the user thinks the BMS has problems. In fact, this is typical & expected behaviour of high impedance cell discharge. At the start of discharge, the cell voltage will drop to the LV cut offsetting, the BMS will cut off discharge per the setting, then the voltage will recover to the normal level. What is observed, is the AH and WH won't decrease or just a little change because the discharging time was very short, the cell looks as if it still has many AH or WH, but it won't discharge because high impedance causing the rapid voltage drop, triggering the Low Volt disconnect.

Correcting a Faulty Battery Pack:
To improve bad battery pack situations is difficult to do and time consuming. The best method is to find the bad cell by measuring each cells impedance within the pack itself, then checking & verifying the AH reading during discharge and replace the deficient cell with new one. If a particular cell has degraded to say 150AH capacity while the others remain at 200AH the failed cell should become quite evident.

The purpose of a BMS is to:
- Provide battery safety and longevity, a must-have for Li-ion.
- Reveal state-of-function in the form of state-of-charge and state-of-health (capacity)
- Prompt caution and service. This could be high temperature, cell imbalance or calibration.
- Indicate end-of-life when the capacity falls below the user-set target threshold.
Not all BMS offer all these features. The most basic functions are battery protection and showing state-of-charge (SoC).

Source: https://batteryuniversity.com/learn/article/how_to_monitor_a_battery

SEE - PART 2 Followed. Exceeded 1000 character limit.
 
Don't make the BMS do the work it shouldn't do. The BMS of course will cut off for High / Low Cell Voltage, High / Low Temps as those are "safety" features to protect your batteries and are more or less the "fail-safe mechanism" that is required to protect the cells & the battery pack as a whole. The preferred & recommended method for controlling Charge / Discharge is to program your Solar Charge Controller (or other charge sources) to cutoff charging just before the BMS forces cutoff. The Load Discharge should also be programmed to cutoff just before the BMS forced it off.

The Chargery uses 2 N.O. (Normally Open) Relays/Contactors to accomplish connection/disconnection. The signals provided to the 2 relays is 12V at a maximum of 1.5A ea. With a Typical default installation, 2 relays are used for charge & discharge control. With the use of a 2 Channel OptoCoupler, that can be reduced to One Relay/Contactor. Alternatively, one can use a Chargery DCC Solid State Contactor to control powerflow. NOTE, that constant cycling of Electro-Mechanical relays shortens their lifecycles and could possibly lead to failure over the long term and subsequently could damage the BMS and / or battery pack.

Depending on your equipment and your configuration, it is possible to use the Relay Signals via Relays or Coupler's to shutdown an Inverter if it has provision for it, or to cut off solar charging. For Example: there are users who have chosen to have the BMS shutdown their Inverters for Low Volt Disconnect with a relay cutting off the power switch on the Inverter after rewiring the ON/OFF switch. Due to the wide variations in equipment and how such can be accomplished is beyond the scope of this document. It should be pointed out, that doing so, eliminates the BMS' capability of being the "Last Stop" fail-safe and that should be seriously considered and weighed.

USE CASE Examples:
Charging: For a 16S LiFe battery charger, the "charge end voltage" of 16S LiFe the battery is 58.4V, so the maximum charge voltage should be 58.4V or 59V max. The extra 0.6V is to compensate charge wire voltage drop. If charging a 16S LiFe battery with a 17S, 18S even 24S LiFe battery charger, the BMS must be cut off charging when the total battery voltage reaches 58.4V or less, this depends on the OVP setting on the BMS. In this example, the battery is only protected by the BMS, due to a lack of charger protection, so it is not safe. We do not suggest charging any battery with a power supply, based on this reasoning.

Some chargers, such as the Chargery C3060 Charger, the cell end charge voltage can be programmed. The settings on the charger should be less than OVP (Over Volt Protection) setting on the BMS. For LiFe battery, the default setting is 3.65V on charger, the OVP setting on BMS should be 3.70V or better. If the setting is 3.65V on the charger and 3.60V on BMS, this is wrong because won't charge the cells to 3.65 because the BMS will cut off charge at 3.60 and consider voltages above that as OVP.

Charging lithium battery uses CC (Constant Current) and CV (Constant Voltage), first charge at CC, when the battery voltage near end of charge, charging current decrease till of end of charging current (5-10% of CC current), charger then stops charging, this is standard charging method. Without CV charging, when charging stops, the battery would not be fully charged. Do NOT charge lithium batteries with higher voltage chargers, such as charge 16S LiFe battery with 65V charger or power supply. If the Charger is CC charge only, it is for NiMh batteries and NOT suitable for LiFe.

What a BMS can & cannot do:
  • BMS monitors and protects cell/battery from damage & extends cell/battery service life as much as possible.
  • BMS won't cure or improve basic battery conditions, including capacity difference, impedance differences, or internal micro short circuits.
  • Restores cell voltage balance status as fast as possible, but this is not the main function, it is a part of improving battery situations. Passive Balancing only "dissipates" higher voltage from cells with higher voltages within the battery pack.
If all BMS settings are correct and in a normal tolerance scope, no matter how the SOC changes, or when the BMS cuts off or not, all depends on battery cell conditions within the pack. Unfortunately too many users ignore the battery condition and assume the BMS will cure battery, which it cannot.

One option if you suspect the battery pack is becoming weakened would be to possibly to use a good Active Balancer which is capable of transferring voltage from High Cells to Low Cells. This is very different from Passive Balancing and requires additional hardware. A Good Active Balancer should be able to handle 3-10 Amps, 3A to 6A is fairly typical and can help to recover weakening cells. Some users are successfully using Chargery BMS' with Qnbbm Active Balancers with several videos on Youtube.

Subject Reference Links:
How to Verify Sufficient Battery Capacity

https://batteryuniversity.com/learn/article/bu_504_how_to_verify_sufficient_battery_capacity

Glossary
https://batteryuniversity.com/learn/article/bu_1101_glossary

What causes Lithium-ion to die?

https://batteryuniversity.com/learn/article/what_causes_lithium_ion_to_die
 
I purchased another BMS16T as I couldn't get Jason to acknowledge it was a BMS problem. Jason said he was giving me a discount but I wound up paying $30 less for just the main unit and the balancing leads. Don't think that was a discount. Here are the readings (8/21 is the new BMS):
CellBMS vs DMM Calibrated 7/31BMS vs DMM Calibrated 8/7BMS vs DMM Calibrated 8/21
1-0.010-0.012-0.010
20.000-0.0010.005
30.0000.0000.009
40.000-0.0010.005
50.0030.0060.007
60.0150.0140.007
7-0.009-0.0050.007
8-0.002-0.0020.000
9-0.022-0.0420.003
100.0320.0550.018
11-0.002-0.0060.012
120.0190.016-0.007
13-0.014-0.0110.011
140.003-0.0010.004
15-0.0010.0030.003
160.0360.028-0.013

The readings of the new BMS are much closer to the DMM. I shared this with Jason and let him know my new BMS is running just as hot as the original unit resulting in balancing not functioning (when it should) most of the time. I have not heard back from him.

I also discovered the one of my problems may be related to a bad crimp on one of the BMS shunt leads that was shipped with the original BMS. During setup of the new BMS the connector fell off the lead. I asked if this might have contributed to some of the my problems such as the SOC catastrophic drop to 0. I haven't heard back about that either.

Thanks for the update. Bummer that the solution was to buy another device.

I wonder if the head unit (LCD screen) does any of the computational tasks? Would your cell values be different if you switched around the LCD screens?
 
If you haven't updated the old one to the current release, I would do that and see if it makes any difference. The new one may already be at the new release.
When you power up the BMS, the screen that flashes briefly shows the firmware release of the display .... I had to take a picture of it because I couldn't read that fast. The release of the BMS itself is at the bottom of the parameter list.

Make sure you follow the outlined process and do the reset defaults after the update.
 
If you haven't updated the old one to the current release, I would do that and see if it makes any difference. The new one may already be at the new release.
When you power up the BMS, the screen that flashes briefly shows the firmware release of the display .... I had to take a picture of it because I couldn't read that fast. The release of the BMS itself is at the bottom of the parameter list.

Make sure you follow the outlined process and do the reset defaults after the update.
No dice, the one unit is still 100-150mv off on a couple cells. I performed the update on both units and it went successfully and reset the units. The other unit is still performing fine with the same cell reading accuracy, 0-15mv + or - difference but this other one seems to be junk. I even switched the units and confirmed the wiring harness is working correctly. The bum unit was showing the 100-150mv delta even on the other battery pack that was really only 20-25mv.

I wonder if I can update the wire resistance values of those cells to force a correct voltage reading...? Not sure if it is possible with the new firmware. Thoughts..?
 
No dice, the one unit is still 100-150mv off on a couple cells. I performed the update on both units and it went successfully and reset the units. The other unit is still performing fine with the same cell reading accuracy, 0-15mv + or - difference but this other one seems to be junk. I even switched the units and confirmed the wiring harness is working correctly. The bum unit was showing the 100-150mv delta even on the other battery pack that was really only 20-25mv.

I wonder if I can update the wire resistance values of those cells to force a correct voltage reading...? Not sure if it is possible with the new firmware. Thoughts..?

Just my opinion, but I don't think the wire resistance is going to make much difference to the voltage readings .... unless the BMS is trying to balance. Maybe turn balancing completely off on the bad one and see if that makes any difference.

There is no way to compensate that I am aware of.
 
Just my opinion, but I don't think the wire resistance is going to make much difference to the voltage readings .... unless the BMS is trying to balance. Maybe turn balancing completely off on the bad one and see if that makes any difference.

There is no way to compensate that I am aware of.

I don't use the balance function of the Chargery given my pack size, 560ah. The passive balance can overheat the unit, plus the cells are in balance already based on the cell matching I did prior to building the pack. I am more concerned with the false voltage reading when it comes to high/low disconnect functionality.

Yeah..after having taken another quick look it appears the resistance screen is just a display view of cell values vs. being able to manage them. oh well.
 
It needs to be sent back and recalibrated. I have seen this issue on the 16T too many times. Never a problem with the 8T

Maybe they are testing / calibrating with an external supply .... and need to do it with a battery connected instead?
 
I'm having a similar issue, which I originally assumed to be related to bad crimps. Since then, I removed my terminals, checked the resistance of each bare wire, then re-crimped and checked resistance along each wire again. The wires all measured similarly, and the crimped-on connections didn't change the resistance at all (although my multimeter may or may not be sufficiently precise to detect meaningful differences).

I'm set up in a 2p4s configuration with wires attached as described in the manual. I'm powered from the battery, not an external supply. My multimeter reads all of the cells at 3.34V +/- 1mv (it's not calibrated, so I'm not concerned with any absolute differences, only relative differences). The BMS records between-cell variance that moves around between 8mv - 14mv.

Is this enough to warrant concern? What other information should I gather to determine whether the BMS needs to be replaced?

1604435213041.png
 
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@solars
I wouldn't worry too much about it at that small of a number. I assume you have balancing off it will affect the measurements if it is bancing
Thanks for the input! I do have balancing off, although the main reason that I care is because it'd force the balancer to bleed off energy needlessly.
 
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