I thought I’d share a real-world example of SOC drift on the upper end when a lithium battery is operated for long periods with shallow cycling and little to no absorption time, and how a full bottom–top recalibration resolves it.

System behaviour (last ±30 days):

Battery mostly cycled between ~80–100% SOC

No regular sustained absorption phase

Loads relatively light during the day

Result: gradual SOC drift upward, visible as the “compressed” upper band on the graph

This is typical behaviour for coulomb counting:

Small measurement errors accumulate

Without a true reference point (bottom or top), SOC slowly loses alignment with actual capacity

Corrective action taken:

Disabled MPPTs and grid charging

Set ESS minimum SOC to 0%

Discharged the battery under a controlled resistive load (~0.15C)

Allowed the BMS to reach low cut-off (~2% SOC) → bottom calibration

Left battery at rest

Re-enabled MPPTs and charged with no AC load

Allowed battery to reach 100% SOC with low tail current top calibration

Result (visible on the graph):

Clear SOC reset at the bottom (≈2%)

Clean reset again at 100%

SOC once again tracks voltage and real capacity correctly

Cell balance after charge: within ~10 mV

Temperatures uniform

Takeaway:

Lithium batteries do not need frequent deep cycling but occasional full bottom–top calibration is essential when: Cycling is shallow, absorption is rarely reached,SOC starts behaving “optimistically”

This isn’t a battery fault — it’s normal coulomb counter behaviour and easy to correct with a controlled recalibration cycle.

Graph attached shows the effect clearly.

Edited December 29, 2025Dec 29 by TaliaB
Spelling and grammar

There is also another factor to be taken in account: the internal losses. As I observed on my LEOCH 48100TB packs the BMS counts and discounts Ah as metered to calculate SOC but neglects internal losses. The result is that when approaching full charge the BMS displays 100% SOC while the pack has not yet reached full charge voltage and is still charging. The difference between the displayed SOC and the true charge is the internal loss. If the battery is not fully charged as may happen on cloudy days the error of internal losses accumulate.

Note that my system manages charging only on the base of pack voltage.

50 minutes ago, Beat said:

There is also another factor to be taken in account: the internal losses. As I observed on my LEOCH 48100TB packs the BMS counts and discounts Ah as metered to calculate SOC but neglects internal losses. The result is that when approaching full charge the BMS displays 100% SOC while the pack has not yet reached full charge voltage and is still charging. The difference between the displayed SOC and the true charge is the internal loss. If the battery is not fully charged as may happen on cloudy days the error of internal losses accumulate.

Note that my system manages charging only on the base of pack voltage.

Good point that’s an important additional contributor and it aligns well with what is visible on the graph. What you are describing is essentially the non-accounting of charge inefficiency (internal losses) in voltage-based or simplistic coulomb-count SOC estimation. The BMS integrates Ah in/out, but unless it explicitly models efficiency, all charging Ah are treated as stored energy, even though some portion is lost as heat and internal resistance. As a result: SOC can reach 100% before the pack has actually reached its true full-charge voltage. The battery may still be accepting current, but the SOC display is already saturated. On days where the pack never reaches a proper absorption stage, the error accumulates cycle after cycle.

This is especially visible in systems where: Charging control is primarily voltage-based. Absorption time is short or absent (cloudy days, shallow cycling) loads remain connected during charge.

In practice, this reinforces the same conclusion. Without a true reference point (either a confirmed bottom cut-off or a sustained top-of-charge condition with low tail current), SOC will inevitably drift. Occasional full recalibration (bottom and/or top) is not a fault condition it is a requirement of coulomb counting, particularly when internal losses are not explicitly catered for.

So your observation complements the original point well. SOC drift is not only about shallow cycling — it’s also about unnaccounted internal losses when the battery is never allowed to complete a true charge.

Well spotted.👍

Was wondering about the pacebms and how its calculates SOH% of the battery.

Is this just running the battery down to low voltage cell cutoff then charging back to full pack voltage and checking the AH charge vs AH configured ?

Was messing around with a unbalance battery and wondered what would happen if the low cell hit the low voltage cutoff would it drop the SOH when it recharge and OVP kicked in setting the battery to 100% before it was balance misleading that the battery has less SOH vs when its perfectly balanced and fully charged?

If that make sense

Also does the it now use the less SOH number as the new total? for % SOC now. Lets say a 100ah battery now is at 50SOH does it use the 50AH as 100% full?

Edited December 30, 2025Dec 30 by Stefan Cornelissen

1 hour ago, Stefan Cornelissen said:

Was wondering about the pacebms and how its calculates SOH% of the battery.

Is this just running the battery down to low voltage cell cutoff then charging back to full pack voltage and checking the AH charge vs AH configured ?

Was messing around with a unbalance battery and wondered what would happen if the low cell hit the low voltage cutoff would it drop the SOH when it recharge and OVP kicked in setting the battery to 100% before it was balance misleading that the battery has less SOH vs when its perfectly balanced and fully charged?

If that make sense

Also does the it now use the less SOH number as the new total? for % SOC now. Lets say a 100ah battery now is at 50SOH does it use the 50AH as 100% full?

On my batteries with a PACE BMS the SOH is calculated only when you do a deep discharge and hit UVP on any cell.

By example, let's say it is a 100AH original capacity, and you hit UVP on a cell when the battery SOC is 5%, then the SOH will be set at 95% (being the 100% original capacity less then 95% you used until UVP).

Just note that this is firmware based, and since PACE supplies hardware to a number of different battery manufacturers with different firmware needs the logic might well be different on different batteries.

I had a few videos of this since I tested it a while back with a battery that had a bad cell to show the manufacturer what was happening for the warranty claim.

I completely ignore SOC on my batteries (with the exception of cell balancing) and use voltage alone without BMS communication.

I use SA to taper down charge current as voltage increases to ensure sufficient absorption time. Have set the PACE BMS to trigger 100% SOC when pack voltage hits 55.2v AND the charge current drops below 0.5 Amp in order to make sure cell balancing is triggered (once charge current stops cell balancing only continues if the 100% SOC was triggered).

I try and do a deep discharge (sub 2% SOC) at least once every quarter to check battery health.

4 hours ago, Stefan Cornelissen said:

Was wondering about the pacebms and how its calculates SOH% of the battery.

PACE SOH estimation assumes balanced cells and completed charge/discharge end points. Imbalance can temporarily depress SOH readings. SOH usually recovers slowly once cells are balanced, full charge (with tail current) is achieved and a few normal cycles are observed.

A reduced SOH reading in an imbalanced pack is often a measurement artefact, not actual degradation especially if it improves after proper balancing and full charge cycles. A weak / low cell will hit LVC early. Discharge stops even though energy remains in the rest of the pack the BMS sees less Ah delivered and that can temporarily bias SOH downwards.

On the charge side:

If a high cell reaches OVP early charging stops before the pack is truly full and usable Ah appears reduced.

If this happens repeatedly without giving the balancer time to do its job, the BMS can indeed: Infer reduced usable capacity, slowly adjust SOH downward. This does not mean the cells have degraded only that the usable window is constrained by imbalance.

The example i posted was on PACE bms's FW E-Tower 10kwh( 2 units in parallel) I installed the 5kva Multiplus with the 10kwh e tower in mid 2022 and the SOH is still 100%. The balancing delta between the 2 units is 9mv at 100% Soc. So always ensure proper cell balancing.

In ESS systems the bms's logic is not overcompicated as there is no need for it.

The project i am working on in La Réunion employs HV 192 cell lfp battery modules in series/parallel configurations and make use off high end REC bms's with SOH logic that consists of Cycle count, DoD weighting, calendar ageing, temperature exposure and Kalman filtering that are part and parcel of the SOH model. The REC bms's ensure cell balancing of ≤1mv.

PS look at my profile picture most beautiful island La Réunion 😀

Edited December 30, 2025Dec 30 by TaliaB
Added last sentence

12 hours ago, TaliaB said:

If a high cell reaches OVP early charging stops before the pack is truly full and usable Ah appears reduced.

That comes close to what I am experiencing with my latest acquired LEOCH 48100-S. They are different from the earlier 48100-TB and have different BMS. Look at the screenshot below.

This brand new pack does it every time it reaches full charge. It goes on cell over voltage protect. While cell 11 has unexplained low voltage the difference to the other cells triggers balancing of them. This situation prevails then for quit some time until the balancing currents have lowered the cell voltages. I think this is not sustainable and noticed Averge for a guarantee claim. Up to now I have no answer. No other pack behaves like this. Note also the 98% SOH display that Averge could not explain. What do you think of that?

Edited December 31, 2025Dec 31 by Beat

1 hour ago, Beat said:

That comes close to what I am experiencing with my latest acquired LEOCH 48100-S. They are different from the earlier 48100-TB and have different BMS. Look at the screenshot below.

This brand new pack does it every time it reaches full charge. It goes on cell over voltage protect. While cell 11 has unexplained low voltage the difference to the other cells triggers balancing of them. This situation prevails then for quit some time until the balancing currents have lowered the cell voltages. I think this is not sustainable and noticed Averge for a guarantee claim. Up to now I have no answer. No other pack behaves like this. Note also the 98% SOH display that Averge could not explain. What do you think of that?

I hope you get a satisfying answer from Verge. This is quite sad. Over the holidays I have had time to do some tests on a Gitar 50Ah and found at a delta of 200mV between highest and lowest cell the BMS would just cut out the battery and no charging would take place.

The error code indicates to contact the supplier. A pity it will not charge to get this lower cell back up and reset the BMS. When I manually charge this cell again the battery works fine. The same cell also goes in OVP during charging. Luckily it is an old battery.

Without comms it's difficult to check if the balancing takes place.

Edited December 31, 2025Dec 31 by Scorp007

2 hours ago, Beat said:

Note also the 98% SOH display that Averge could not explain. What do you think of that?

This is caused by the 214mV delta. You need to stay on Averge for a replacement as this battery module will deteriorate as the cell matching is not good at all. Different internal resistance and most probably diffrent cell capacity.

1 hour ago, TaliaB said:

This is caused by the 214mV delta. You need to stay on Averge for a replacement as this battery module will deteriorate as the cell matching is not good at all. Different internal resistance and most probably diffrent cell capacity.

The troubling fact is that this cell 11 behaves normally in all other conditions, voltage in line with the others.

6 hours ago, Beat said:

The troubling fact is that this cell 11 behaves normally in all other conditions, voltage in line with the others.

At any stage of the charge discharge cycle you should not see the delta apparant in your bms snapshot. Both cells 11 and 13 is problematic the bms is doing damage controll instead off managing the cell balancing. It is clear that cell matching was not done correctly in this new battery module. My advice LEOCH need to replace the problematic cells or even better replacement. This is a snapshot of a 314ah Eve diy battery managed by a JK bms after more than 4 years of service. Properly matched cells with 4mv delta.

On 2025/12/31 at 2:21 PM, TaliaB said:

This is caused by the 214mV delta. You need to stay on Averge for a replacement as this battery module will deteriorate as the cell matching is not good at all. Different internal resistance and most probably diffrent cell capacity.

Below the response from Averge:

Good morning,

The cells are balancing and within 400mV at full or empty which is normal.

The battery will only be replaced under warranty if the capacity is less than the spec: being

Less than 80% SOH (or usable capacity) before 5 years or less than 70% before 10 years.

8 hours ago, Beat said:

Below the response from Averge:

Good morning,

The cells are balancing and within 400mV at full or empty which is normal.

The battery will only be replaced under warranty if the capacity is less than the spec: being

Less than 80% SOH (or usable capacity) before 5 years or less than 70% before 10 years.

It's a shame from a longer time seller to give you this reply. Recently I found Gitar BMS would cut on a delta of around 230mV even if far from a low voltage level by the lowest cell.

Cannot say what your next move should be. 😢

On 2026/01/19 at 7:36 PM, Scorp007 said:

Cannot say what your next move should be.

I had some email exchange with Averge. I wrote: Yes, the BMS is functioning correctly, set aside the weird SOH readings. However the cells 11 and 13 are not functioning correctly. When approaching full charge cell 11 dives to significant lower voltage while even with very low charging current of a few A cell 13 raises its voltage over the cell OVP threshold. That should not happen as long as the packs terminal voltage remains within specified bulk charge voltage. These two cells are sick. A small system with only this one pack could not operate correctly.

And further: The issue of this pack is the abnormal cell over voltage of cell 13 at full charge that triggers cell OVP and disconnects the pack from the 48V bus. This is a dysfunction and indicates a health problem inside the cell. It is beyond being unbalanced. It happens every time the pack reaches full charge.

Averges response:

We want to assure you that this is a normal protective function, not an indication of a problem.

The OVP briefly pauses charging to allow for safe cell balancing, a routine process in lithium batteries. This prevents any cell from exceeding strict safety limits. The definitive test of battery health is its performance, which in your case remains uncompromised:

• It provides its full rated capacity.

• All communication features (display, inverter integration) work perfectly.

• It recharges fully after an OVP event.

 If functional capacity were affected, we would share your concern. However, since the system continues to perform exactly as specified, the OVP is simply doing its job to ensure long-term safety and reliability. This behaviour is expected and within normal operational parameters for lithium technology.

I am helpless.

17 minutes ago, Beat said:

 

I am helpless.

I can understand your frustration with them. I had a friend getting over 1yr old lead acids with problem and the manufacturing date was more than a Yr before they were delivered. Their response was plain and simple it does not matter as we did recharge them every 3 months so they are as good as any new battery.

9 hours ago, Beat said:

I had some email exchange with Averge. I wrote: Yes, the BMS is functioning correctly, set aside the weird SOH readings. However the cells 11 and 13 are not functioning correctly. When approaching full charge cell 11 dives to significant lower voltage while even with very low charging current of a few A cell 13 raises its voltage over the cell OVP threshold. That should not happen as long as the packs terminal voltage remains within specified bulk charge voltage. These two cells are sick. A small system with only this one pack could not operate correctly.

And further: The issue of this pack is the abnormal cell over voltage of cell 13 at full charge that triggers cell OVP and disconnects the pack from the 48V bus. This is a dysfunction and indicates a health problem inside the cell. It is beyond being unbalanced. It happens every time the pack reaches full charge.

Averges response:

We want to assure you that this is a normal protective function, not an indication of a problem.

The OVP briefly pauses charging to allow for safe cell balancing, a routine process in lithium batteries. This prevents any cell from exceeding strict safety limits. The definitive test of battery health is its performance, which in your case remains uncompromised:

• It provides its full rated capacity.

• All communication features (display, inverter integration) work perfectly.

• It recharges fully after an OVP event.

 If functional capacity were affected, we would share your concern. However, since the system continues to perform exactly as specified, the OVP is simply doing its job to ensure long-term safety and reliability. This behaviour is expected and within normal operational parameters for lithium technology.

I am helpless.

Revert with this response to Averge.

"This is not a routine OVP or balancing event. The pack repeatedly isolate itself at full charge, require a manual BMS reset to restore operation, and exhibit high post reset current all clear signs of a cell level issue triggering constant OVP, not normal behaviour. Capacity alone does not make this normal. Please advise on corrective action for the defective cell"

9 hours ago, Beat said:

I am helpless

Insist on a RMA process this unit is still under warranty not so?

@Beat Did you purchase the battery module directly from Averge Technologies in Clayville or from a different supplier.

Edited January 28Jan 28 by TaliaB

47 minutes ago, TaliaB said:

@Beat Did you purchase the battery module directly from Averge Technologies in Clayville or from a different supplier.

Thank you for your advises.

Yes I purchased all 6 packs directly from Averge. This last one purchased about a year ago is still fully under guarantee..

1 hour ago, TaliaB said:

The pack repeatedly isolate itself at full charge, require a manual BMS reset to restore operation,

As mentioned this happens every time the pack reaches full charge. I cannot reset the BMS remotely but wait until it releases OVP by the cell voltage falling below OVP release at 3.38V. As the pack is disconnected there is no discharge current other than the little balancing current, it takes hours to release.