BMS Cutoff Won’t Save Neglect (Delaying Recharging)

If the BMS has an undervoltage (UV) cutoff to protect the cells, why does delaying recharging still damage them?

Because UV cutoff only disconnects the load. It does not fix a cell that’s already too empty, and it doesn’t stop slow drains or chemistry from degrading while the cell sits near dead.

Here’s what actually happens if you delay re-charging after UV cutoff:

1. Parasitic drain keeps pulling it lower.
   • The BMS itself draws current (quiescent 1–3 mA is common). Trackers/screens draw more.
   • So after cutoff, voltage keeps creeping down. Sit long enough → cells fall well below the safe floor.
2. **Deeply discharged cells start to chemically deteriorate.
   • At very low potentials, the SEI (protective layer) breaks down → higher self-discharge and permanent capacity loss.
   • Copper current-collector dissolution can begin; on recharge, dissolved copper can re-deposit as micro-dendrites, causing internal shorts or rising self-leak.
   • Electrolyte decomposition and gas formation increase—swelling/pressure rise risk.
3. Series packs go out of balance and can reverse-bias the weak group.
   • The weakest parallel group hits UV first. Leave it sitting low while other groups are healthier → imbalance worsens.
   • In severe cases (especially if someone connects a load/jumper later), the weak group can be reverse charged—that’s fatal.
4. Some BMS cut only the discharge FETs.
   • There may be leakage paths or still-active circuits. Bottom line: cutoff ≠ zero drain.
5. Calendar aging accelerates at the extremes.
   • Storing near 0% SOC and/or at warm temps ages cells faster than storing around 50–60% SOC in the cool.

What “delay” means in practice

• Hours to a couple days at the cutoff? Usually recoverable.
• Weeks to months sitting near empty? Expect permanent capacity loss at minimum; at worst, cell damage that won’t safely recover.

Quick math (why “just a few mA” matters)

• 100 Ah pack, BMS draw 2 mA:
  • 0.002 A × 24 h = 0.048 Ah/day → ~1.44 Ah/month (~1.4%/mo) on top of normal self-discharge.
  • If you were already at the cliff, that’s enough to push cells below the safe region in a few weeks.

Best practice (don’t overthink it)

• If a pack hits UV cutoff, recharge immediately (gentle pre-charge if needed) to get the cells back above the release threshold.
• Never store packs empty. For storage, park at ~50–60% (LFP), isolate from loads, and check every 3–6 months.

Bottom line: The BMS saves you from an immediate crash, not from slow death. Sitting near empty is what kills cells—chemically and permanently. Recharge promptly.