AC500 + B300 cell voltage imbalance and power LED blinking at 100% SoC

Product Discussion BLUETTI AC Series
1

Hello Bluetti community,

I’ve been using an AC500 + B300 (without the S) as a backup power source in Standard UPS mode for a few months.
Everything worked smoothly, and recently I decided to add an extra B300 to the combo to increase the capacity.

So I got a new B300 and upgraded the BMS firmware first. I disconnected the old B300 from the AC500, connected the new B300, upgraded the BMS firmware on the new B300 through the app, then connected both the old and the new batteries to the two connection slots of the AC500 (shutting down the unit completely every time before making any cable connection changes). Now I have an AC500 + 2xB300 combo where both batteries run BMS firmware 1021.07.

Everything worked fine after the setup. I gave the combo a full charge + discharge and confirmed that the SoC was in sync between the two batteries and the backup capacity had actually doubled.

However, the next day, I noticed that the power LED on the new B300 was sometimes blinking. Usually, it was a series of 9 blinks, although sometimes I counted 7 or 8 as well. After some diagnostics, I found out that it only happened when the new B300 was charged to 100%. Apparently, when going from 99% to 100% SoC, the voltages of the individual cells in BMS Maintenance start to become really unbalanced. As the charging stops and the battery discharges back to 99%, the voltages seem to even out a bit and the LED blinking stops. Then the cycle repeats once AC500 decides to charge the battery back to 100% from grid. There are no alarms in the AC500.

Note this cell imbalance is only observed on the new battery and only when both batteries are connected to the combo. The old B300 has perfectly balanced cell voltages from 1% to 100% SoC, with no more than 0.02V difference between any two cells. The new B300 also has perfectly balanced cell voltages when going from 99% to 1% SoC, as well as when charging back from 1% to 99%, but at 100% the voltage readings start to go haywire.

I’ve also recorded a video that captures the error cycle nicely. It shows the AC500+2xB300 combo connected to grid input and carrying a ~450W load, and the new B300 goes from 99% to 100% SoC: bluetti-b300-troubleshooting - Google Drive

Things I’ve tried so far (with no luck):

  1. Discharging the whole combo until shutdown, then charging the whole combo to 100% without any load.
  2. Disconnecting the old B300 and running only AC500 + new B300. Discharging it until shutdown, then charging to 100% (no LED blinking observed in this case and the voltages in BMS maintenance look pretty even).
  3. Discharging the new B300 until shutdown, disconnecting it and charging it with the T500 AC adapter, without interruption, and without any load.
  4. Swapping the battery connection slots on the AC500.

The combo works well overall, it’s just that these LED blinks concern me a bit.

Firmware versions:
DSP: 4057.03
ARM: 4048.02
HMI: 6052.07
BMS: 1021.07 (both batteries)

Questions:

  1. What do the 9 blinks mean? Is it an error code indicating cell imbalance?
  2. Is it normal for a new B300 to have unbalanced cells like this?
  3. Is the imbalance likely to even out after some use, and can I do anything to help the battery “settle in”?

old-b300-soc-100-grid-input
old-b300-soc-100-grid-input2008×1446 672 KB

old-b300-soc-100-no-grid-input
old-b300-soc-100-no-grid-input2101×1487 706 KB

new-b300-soc-000-shutdown
new-b300-soc-000-shutdown2139×1547 849 KB

new-b300-soc-001
new-b300-soc-0011951×1375 628 KB

new-b300-soc-100-grid-input-blink-start
new-b300-soc-100-grid-input-blink-start2078×1592 787 KB

new-b300-soc-100-grid-input-blink-end
new-b300-soc-100-grid-input-blink-end2034×1626 1020 KB

1 Like
2

Your explanation is very concise and clear.
As you suspected the 9 blinks from the B300 indicate the stop of charging caused by a single cell voltage too high during balancing.
.
Cell number #10 has a significant voltage shift delay since at 0% SoC it is at 2.70 while at 100% SoC it is at 3.41.
Consequently, it causes the BMS in the last phase of charging to continue balancing until the first cells reach a voltage level that is too high at 3.70 volts, and at that point the safety protection intervenes.
.
A first suggestion is to try to balance the cells by leaving the B300 to perform several 99-100% cycles and checking if cell #10 slowly starts to increase in voltage in order to better align to the others.
.
In any case, let’s wait for @BLUETTI for considerations on possible firmware corrections or the advice to proceed with the return of the B300 unit.

3 Likes
3

Hi @bandersnatch , Your detailed description has made us fully understand the issue you’re experiencing.
@ndwr 's explanation is very professional and accurate, and these suggestions are correct. :+1:
We’ve checked that your B300’s BMS firmware is already up-to-date, and there is currently no new firmware available to address this issue. It seems that the battery was just overcharged, and you are already recalibrating the battery, which is the best solution we can offer at this time.
Please don’t worry, just ensure that you don’t continue charging once the battery is fully charged.

4

The combo is currently set up as a grid-tied UPS, and it gets automatically charged from grid whenever the SoC drops. I don’t control the charging process manually. The way the new battery currently behaves is it gets charged to 100%, but some cell voltages keep growing. Then, as cell #6 reaches 3.70V, charging stops, the battery goes offline for a second, and the power LED starts blinking. The battery then goes online again and the voltages start to drop back to reasonable levels. So it looks like the BMS is taking care of terminating the charging process. Do you suggest using a different charging approach for the new battery?

5

Another interesting insight: when I only use the new B300 with the AC500 (disconnecting the old B300), cell #6 gets charged only to 3.60V, then gradually drops back to 3.40V. So the overcharge/imbalance issue has never occured so far when using it as a solo battery because the cell voltage never reaches 3.70V.

6

@ndwr Thank you, your advice was spot on.
I disconnected the old battery and let the new battery run solo, since in this case it was not reaching high voltages and triggering the overvoltage protection. Slowly but steadily, as it went through a lot of 99-100% cycles, the individual cell voltages started to balance out: the standard deviation of voltages across the 16 cells would decrease by 0.005-0.010V every day. After five days, I connected two batteries again and now the combo runs smoothly without any errors.

1 Like
7

Thank you for the update on resolving the issue. Those are valuable information for other users who may encounter the same situation and can trust the BMS’s ability to keep the cells balanced.

1 Like