There are 4 of the same model battery (SHOTO 5.12kWh) attached in parallel all connected to a Kodak inverter.

It is programmed for battery only from 16h00 through to 08h00 the next morning

By 14h00 the following day, all 4 batteries reach 100%

Right now it is 09h30 and the batteries display different state of charge. The charge rate is also different. Strangely enough, battery 3 is the newest, 9 months old

Is there a reason for this, and can I do anything about it?

Are the battery cables all of the same length?

What we found was with more than 2 batteries, the draw would be all over the place between the packs until all packs were connected to busbars using cable of equal length.

Excuse how untidy this is, but this demonstrates how we set these batteries up:

Each positive and negative is the exact same length, connected to thick copper busbars rated for 372A. Thick cable of equal length then runs to the battery fuses and then to the inverter.

We did this because the batteries were drawing at unequal amounts and throwing the SOC completely out. This mostly solved the problem.

Another thing to consider, are your batteries the same age? Different age batteries, or batteries from different batches, can have differing internal resistance, causing the draw to be different.

6 hours ago, chrisc said:

There are 4 of the same model battery (SHOTO 5.12kWh) attached in parallel all connected to a Kodak inverter.

It is programmed for battery only from 16h00 through to 08h00 the next morning

By 14h00 the following day, all 4 batteries reach 100%

Right now it is 09h30 and the batteries display different state of charge. The charge rate is also different. Strangely enough, battery 3 is the newest, 9 months old

Is there a reason for this, and can I do anything about it?

Your question - normally not much can be done if the cable are close to the same length. From your results of charge the newest battery should have the lowest resistance as being never and should charge at a higher rate. Secondly the same battery is at the lowest SOC and is another reason for it to charge faster. As they charge they would top out being fully charged even if the lengths are not the same.

a difference of 156 to 221W between the different batteries is of no concern. Just because you can measure the difference no need to worry. I have different sets where at times one can see values like 1.2A, 1.8A, 7A and 11A at times. The same happens during discharge. Batteries are all different makes and ages can be different by over 4 years I guess. The 7A charging battery I am the 3rd owner. No idea of age or cycles but still working very well. The 11A battery is a 12V drop in Hubble doing very well after 3.5 years.

6 hours ago, chrisc said:

Is there a reason for this, and can I do anything about it?

Yes there is. It's the difference of internal resistance due to manufacturing tolerances. That difference is likely to be way higher than the difference of resistance in unequal lead lengths. I.E. my oldest pack that connects with the longest leads shows consistently highest charge and discharge currents. The only thing you could do about it is to open the pack and check the cell connections. I found one pack with poorly tightened bolts on the cell terminals.

I ran into this recently as well. My word of caution especially if you run your SOC on the inverter to discharge low is that the inverter is most likely reading the avg SOC and this can lead to batteries actually going below the set SOC limit if they aren't fully charged every now and then. This happened to me this winter and I had to send in 2 batteries for repairs that was discharged to low to frequently.

8 hours ago, chrisc said:

There are 4 of the same model battery (SHOTO 5.12kWh) attached in parallel all connected to a Kodak inverter.

It is programmed for battery only from 16h00 through to 08h00 the next morning

By 14h00 the following day, all 4 batteries reach 100%

Right now it is 09h30 and the batteries display different state of charge. The charge rate is also different. Strangely enough, battery 3 is the newest, 9 months old

Is there a reason for this, and can I do anything about it?

Shoto uses a PACE bms if I recall correctly?

Have you checked if the batteries starts discharging at the same time, in other words, after fully charging them to 100% SOC and then using the batteries to supply the load, does each battery start discharging at the same time?

The PACE BMS will close the discharge MOSFETS when a cell triggers the Overvoltage Protect alarm during charging, normally set at 3.65v. The BMS will keep the discharge MOSFETS closed as it is trying to balance the cells even though the inverter might be pulling load from the other batteries.

So you might have a situation where 1 or 2 of the batteries only start supplying load when the others are already down to 90% SOC (by example). The bigger the battery bank the more pronounced this effect can be, since the BMS will only open the MOSFETS when the load warrants it (is big enough), and since you have 4 batteries in the bank this will need to be a substantial load.

This effect is also more pronounced in winter since the batteries have less time to balance the cells.

I had the same issue, eventually solved it by adjusting settings to avoid hitting OVP. The local importer could not solve this, eventually the Chinese manufacturer pointed us to PACE who supplied me with a sollution.

The bsttery cables are within 30mm of each other, I took pains to get that right

In answer to the last post, I noticed that battery 2 starts discharging some time after the other 3. Right now they have been discharging for 4 hours. The SOC of each are: 1 - 90.9%, 2 - 98.3%, 3 - 83.3%, 4 - 92.7%. At 16h00 when the AC went off, they were all 100% and battery 3 dropped by 10% within 90 mins

Is there a way to alter this? The bulk charge is set to 56.3V

The monitoring is done directly from each battery via a LAN cable to a Raspberry Pi

12 hours ago, chrisc said:

There are 4 of the same model battery (SHOTO 5.12kWh) attached in parallel all connected to a Kodak inverter.

It is programmed for battery only from 16h00 through to 08h00 the next morning

By 14h00 the following day, all 4 batteries reach 100%

Right now it is 09h30 and the batteries display different state of charge. The charge rate is also different. Strangely enough, battery 3 is the newest, 9 months old

Is there a reason for this, and can I do anything about it?

Which battery on your graph is the master battery? Master reports SoC to inverter, in most BMS communication protocols (especially with Kodak/Must/Mecer and many Chinese inverters), only the master battery’s SoC and voltage are used to determine when to end bulk and absorption phases. If the master battery is the one with the highest SoC or lowest internal resistance, it will reach absorption voltage first. The inverter sees this as the system being "full" and terminates absorption early, based on CV timeout or SoC. The other batteries, which haven't reached 100% or even full absorption voltage yet, are cut off early. Over time, this leads to increasing SoC drift,poor balancing capacity under-utilization and possibly faster degradation of the lower-charged batteries

24 minutes ago, chrisc said:

The bulk charge is set to 56.3V

That seems to me too high. The manufacturer of my LiFePo Batteries specifies 53V. I never get cell overvoltage alarm.

8 hours ago, Beat said:

That seems to me too high. The manufacturer of my LiFePo Batteries specifies 53V. I never get cell overvoltage alarm.

Remember you have 15 cell and 16cell battery

15 cell is around 53v = 3.53v per cell

16 cell normally around 56-57v to get the same voltage per cell

Edited July 28, 20251 yr by Stefan Cornelissen

The booklet for the SHOTO battery states 56.4 bulk and 54 float

If I reduce the bulk to 56V, then the batteries stop at 98%

Its 08h00 now, and ....

10 hours ago, TaliaB said:

Which battery on your graph is the master battery? Master reports SoC to inverter, in most BMS communication protocols (especially with Kodak/Must/Mecer and many Chinese inverters), only the master battery’s SoC and voltage are used to determine when to end bulk and absorption phases. If the master battery is the one with the highest SoC or lowest internal resistance, it will reach absorption voltage first. The inverter sees this as the system being "full" and terminates absorption early, based on CV timeout or SoC. The other batteries, which haven't reached 100% or even full absorption voltage yet, are cut off early. Over time, this leads to increasing SoC drift,poor balancing capacity under-utilization and possibly faster degradation of the lower-charged batteries

I don't know that there is a master battery

As I said, the data comes from the batteries, not the inverter. Its a Raspbery Pi. There is no BMS communication from the batteries to the Kodak OG7.2 inverter

Edited July 28, 20251 yr by chrisc

Do these cell voltages tell you anything?

At what voltage do the bms reset the SOC on them?

Do you have a screen shot of all 4 batteries as soon as they are at 56v? or well 56.4v?

Battery

1 - 54.6
2 - 55.1
3 - 54.6
4 - 54.6

8 hours ago, chrisc said:

Do these cell voltages tell you anything?

Provide this picture at 56.4v please

13 hours ago, chrisc said:

The booklet for the SHOTO battery states 56.4 bulk and 54 float

If I reduce the bulk to 56V, then the batteries stop at 98%

Its 08h00 now, and ....

The PACE BMS has 3 trigger points that reset SOC to 100%.

1. The first is if any cell reaches OVP (normally 3.65v)

2. The second is if the battery reaches the battery OVP (normally 58.4v for 16 cell battery)

3. The last is based on a combination of battery voltage and charge current (usually set at when the charge current is below 2A and the voltage is above 57.6v).

All of the above can easily be changed with PBMS tools. All of those standard setting are to high. Have left setting 1 and 2 unchanged on my batteries but have changed no 3 to 56v and 0.5A).

The ideal position to be in is the 3rd trigger above as this avoids any over voltage alarms.

When the BMS triggers 100% SOC it closes the charge MOSFET and it will continue to balance the cells even though charging has stopped. Conversely, if the 100% SOC is not triggered the BMS will only balance the cells if a charge current is supplied.

There is another factor that might influence those erratic SOC values. What is the state of health of each of your batteries? If one is say at a SOH of 80% if means that the SOC is calculated based of 80AH capacity as opposed to 100AH which can also result in the not so uniform SOC readings. I know on the SA pictures they are reported as 100%, has this been verified by actual BMS readings?

I also see that the battery capacities are different (on the SA pictures), this will also contribute to the SOC differences, albeit not by much.

Edited July 28, 20251 yr by I84RiS

8 hours ago, chrisc said:

Do these cell voltages tell you anything?

Cell voltages look fine, but the voltages in the picture is still at the flat part of the voltage curve and probably meaningless to look at.

What does look odd here is the low charge rate of battery 2, at that SOC it should be charging at a rate closer to the other 3.

Is it expected for the cycle count to be so vastly different between the 4 batteries?

I would remove battery 2 from the bank to see if this had any impact on the remaining 3 when they are used without no 2.

I assume you are using open loop, ie no comms between inverter and batteries?