Eight modules, fifteen cells each. Ten of those cells throttle the charging of the other ninety-five. Here is which ones, how we know, and what can be done about it.
The pack is not worn out. Four modules out of eight are flawless: sixty cells without a single anomaly, after more than three hundred cycles. A fifth is just as clean.
Three modules carry the entire problem, and within those three, ten cells. When one of them reaches 3550 mV the BMS reduces the charge current — not for its own module, but for the whole pack. Five healthy modules are throttled by ten cells.
A module's identity is its barcode, never its address: each chain renumbers its modules from 1, so there are two “module 1” in this installation.
| Barcode | Pack | Addr. | Affected cells | Which ones | Median spread | Overvoltage | Undervoltage | |
|---|---|---|---|---|---|---|---|---|
| K220803C30160322 | older | 1 | 4 | 6 · 7 · 12 · 14 | 108 | 498 | 86 | in service |
| K220803C30160323 | older | 2 | 0 | — | 6 | 0 | 0 | in service |
| K220803C30160319 | older | 3 | 5 | 2 · 6 · 10 · 12 · 15 | 45 | 416 | 179 | in service |
| K220803C30160324 | older | 4 | 1 | 15 | 57 | 29 | 59 | isolated |
| Y230202C30371426 | newer | 1 | 0 | — | 2 | 1 | 1 | in service |
| Y230202C30371425 | newer | 2 | 0 | — | 2 | 1 | 1 | in service |
| Y230202C30371431 | newer | 3 | 0 | — | 2 | 1 | 1 | in service |
| Y230202C30371427 | newer | 4 | 0 | — | 2 | 1 | 0 | in service |
Read the median spread column. The five healthy modules sit between 2 and 6 millivolts; the other three are at 45, 57 and 108. The same contrast shows in the overvoltage counters: 498 for the first module against one for each of the four in the newer pack, which have nonetheless taken ten times more cycles.
All sixty cells of the newer pack sweep between 168 and 191 mV, without exception. The inventory is closed.
On 5 September the console was connected to the three modules of the older pack while the alarm was rising. What follows is the record, to the second.
| Time | What happens | |
|---|---|---|
| 13:04:48 | cell 7 of module …0322 at 3526 mV, cell 6 of module …0319 at 3541 | below threshold |
| 13:05:08 | the same two at 3555 and 3575 mV | level 3 crossed |
| 13:05:13 | the BMS drops the allowed charge current from 50 to 30 A | five seconds later |
| 13:15:10 | cells fall back to 3499 and 3522 mV, alarm clears | the limit stays at 30 A |
| 13:22:40 | limit 30 → 40 A | after 7 min 31 s |
| 13:26:39 | limit 40 → 35 A, leading cells at 3563 and 3548 mV | it trips again |
| 13:38:10 | limit 35 → 40 A | after 11 min 31 s |
The most useful finding is in the fourth row: the alarm clears well before the limit recovers. Looking for the alarm in the logs therefore misses the bulk of the phenomenon — the alarm lasts minutes, the throttling lasts hours.
The thresholds are not values we chose: they are the ones the hardware declares, read from sysinfo on the LV-HUB.
The answer decides what balancing can give back. Two measurement points on the same cell at two different currents settle it.
| current per cell | spread | |
|---|---|---|
| 13:04:48 | 6.33 A | 163 mV |
| 14:05:42 | 0.04 A | 46 mV |
The slope gives ≈ 18.6 mΩ of excess internal resistance on the leading cell, twenty to forty times that of a healthy one. The intercept, ≈ 46 mV, is the true charge imbalance.
Independent check: this line predicts 312 mV at 100 A pack current. The largest spread actually recorded over thirty-seven days is 280 mV.
So of the ~190 mV of spread under charge, only 46 mV are an imbalance — the rest is lost capacity. A perfect rebalance would bring the trip from 3555 down to about 3510 mV: below the threshold, but with only forty millivolts of margin.
Not from wear. The newer pack has taken three hundred and forty cycles in a year and its sixty cells are intact: cycling does not damage these modules. And the control module shared the chain, the currents and the days of the two damaged ones without a scratch.
The two terminal cells went down to 2953 and 2939 mV, below the low-voltage alarm threshold. A lithium iron phosphate cell taken there under load loses capacity irreversibly. These cells did not age, they were discharged too deep.