If ur trickle charging there can’t be much drift. You check on it.
In my experience, once you’re back above 2.5 V to 3 V per cell, which happens nearly instantly even at low current, you can hook the BMS back up and let it take care of things. I suppose maybe it’s theoretically possible that you have one p-group with practically no capacity, and it spikes in voltage in the minute or so that it takes to get back above 2.5 V, but I’ve not seen or really heard of that happening. Also, at that point, I think that the P-group would basically no longer be a danger. That would mean it holds basically zero energy even at a full charge, so there isn’t any energy to actually cause a problem.
Molicel doesn’t site it as possible afaik, the data sheet only mentions that the pack should not be allowed to drop below 2.5v and that any leakage current that over draws on the cell can damage it. As in their be dragons here. No reliable testing has been done on low packs and theoretically the lower voltage state could mess up the passivating layer the ions pass through - what happens is a huge unknown.
Cells start their life at 0 volts, then the initial conditioning of the cell isn’t just to charge it, it also causes one time chemical changes in the cell and is done in very specific and deliberate methods. Where the charge can be safely recovered from after this is not a matter of debate so much as a set standard to prevent un intended chemical reactions in the cell that can cause it to work and fail in an unknown way.
Finding and recording that stated charge voltage is the biggest goal here. It changes for each chemistry, construction, and use case so it would be nice to have the “do not cross” line established with good sources. Building a test that could actually determine a cell’s performance when allowed to drop below the stated minimum safe charge voltage is a side quest and interesting.
Mind posting the pdf or link to it too? When i get time to organize the main wiki post it will make it so much easier. @me so i can find it easy cause ill not if its buried in the discourse
Rant: fuckkng over working weird shifts, 3am starts are better than 5pm but yuck. Kills all my free time
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Just DeepSeek slop
Me:
Many cell data sheets cite that you should not charge a cell that has dropped below 1v. That seems extremely low, what are the risks involved with charging a cell that has dropped to around the 1v mark?
DeepSeek:
You’re absolutely right to question this—1V is an extremely low voltage threshold for a lithium-ion cell, and charging a cell in this state carries severe risks. Most reputable manufacturers specify 2.5V–3.0V as the absolute minimum safe cutoff, with anything below considered hazardous. Here’s why charging a ~1V cell is dangerous and what happens at that level:
Why 1V is a Critical Danger Threshold
-
Copper Dissolution in the Anode
- Below ~2.0V, the anode’s copper current collector begins dissolving into the electrolyte.
- At ~1V, this dissolution becomes severe, leading to metallic copper shunts that can pierce the separator, creating internal shorts.
- If you attempt to charge, these shorts can cause localized heating and thermal runaway.
-
Irreversible SEI & Electrolyte Breakdown
- The Solid Electrolyte Interphase (SEI) layer decomposes at low voltages, exposing fresh lithium to further reactions.
- The electrolyte breaks down into gases (CO₂, CH₄, C₂H₄), leading to swelling and pressure buildup.
-
Lithium Plating Upon Recharge
- Even if the cell “takes” a charge, the damaged anode may plate lithium metal instead of intercalating it, increasing the risk of dendrites and internal shorts.
-
Catastrophic Failure Modes
- Swelling & Venting: Gases from electrolyte decomposition can rupture the cell.
- Thermal Runaway: Internal shorts can trigger rapid self-heating, leading to fire/explosion.
- Sudden Voltage Collapse: The cell may appear to recover but then fail catastrophically under load.
Why Do Some Datasheets Cite 1V?
- Ultra-Conservative Safety Margin: Some manufacturers set 1V as a “point of no return” to account for measurement errors or brief transient dips.
- Legal Liability Protection: They want to ensure no one tries to revive a deeply degraded cell.
- Chemistry-Specific Factors:
- LiFePO₄ (LFP) can sometimes tolerate slightly lower voltages (~1.5V) due to a more stable structure, but standard Li-ion (NMC, LCO, NCA) cannot.
What Happens If You Try to Charge a 1V Cell?
-
Initial “Recovery” May Be Misleading
- A very low-current charge might bring voltage back to ~2.5V, but the cell is already chemically compromised.
- Capacity will be permanently reduced (often by 50% or more).
-
Hidden Internal Damage
- Even if the cell charges, copper dendrites may have already formed, creating a time bomb that fails later under load.
-
Thermal Hazards Increase with Cycles
- A “recovered” cell may work briefly but overheat, swell, or vent in subsequent charges.
When Might a 1V Cell Be Salvaged? (Not Recommended!)
- If the cell was at 1V for only seconds/minutes (e.g., accidental short-circuit), and it rebounds to >2.5V when the load is removed, maybe it’s recoverable.
- Use a lab-grade charger with a pre-charge mode (trickle <0.05C) and monitor:
- Temperature (abort if >5°C rise).
- Voltage response (if it doesn’t climb steadily, stop).
But in 99% of cases, a cell at 1V is permanently unsafe and should be disposed of properly.
Best Practices for Dead Cells
- Never Force-Charge Below 2.5V (for standard Li-ion).
- Check Open-Circuit Voltage (OCV):
- <1.5V? → Dispose immediately.
- 1.5V–2.5V? → Attempt only with extreme caution (still risky).
- Look for Physical Signs: Swelling, leaks, or hissing means DO NOT CHARGE.
Final Verdict
Charging a 1V lithium-ion cell is playing Russian roulette—even if it “works,” the cell is structurally compromised and could fail violently later. Always prioritize safety over salvage attempts when dealing with deeply discharged cells.
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More, for shits and giggles
Me:
So lets say i have a cell that has discharged to between 2-2.5v. It has been at this low of a voltage for something like a couple of weeks. What are the risks involved with recharging this cell and is the cell now likely to be damaged? If so, how badly might it be damaged and what are the long term risks of continuing to use this battery?
DeepSeek:
Charging a Li-ion Cell That Has Been at 2.0V–2.5V for Weeks: Risks & Long-Term Effects
A lithium-ion cell sitting at 2.0V–2.5V for weeks is in a dangerous but potentially recoverable state. The risks depend on:
- How low the voltage dropped (closer to 2.5V = safer, closer to 2.0V = riskier).
- How long it stayed there (weeks is worse than days).
- Cell chemistry (LiFePO₄ is more forgiving than NMC/LCO).
Immediate Risks When Recharging
-
Copper Dissolution (Anode Damage)
- Below ~2.5V, the anode’s copper current collector begins dissolving into the electrolyte.
- If the cell was at 2.0V–2.5V for weeks, some copper may have already degraded, increasing internal resistance.
-
SEI Layer Breakdown
- The Solid Electrolyte Interphase (SEI) degrades at low voltages, exposing fresh lithium to side reactions.
- This leads to gas generation (swelling) and capacity loss.
-
Lithium Plating During Recharge
- A damaged anode may plate lithium metal instead of properly intercalating it, creating dendrites that can cause internal shorts.
-
Potential for Thermal Runaway
- If copper shunts or dendrites have formed, charging could trigger localized heating, leading to venting or fire.
How Badly Is the Cell Damaged?
| Voltage Before Recovery | Likely Damage | Recovery Chance |
|---|---|---|
| 2.5V | Moderate SEI degradation, slight copper loss | ~70–80% capacity remaining if recovered properly |
| 2.2V–2.4V | Noticeable copper dissolution, SEI damage | ~50–70% capacity remaining (higher internal resistance) |
| 2.0V–2.2V | Severe copper damage, possible lithium plating | <50% capacity, high risk of failure |
- If the cell was at 2.0V–2.5V for weeks, expect:
- 20–40% permanent capacity loss (even if it seems to work).
- Increased internal resistance (reduced power output, faster voltage sag).
- Higher self-discharge rate (may lose charge faster when stored).
How to (Carefully) Attempt Recovery
If you must try to salvage the cell:
-
Use a Lab Power Supply or Li-ion Charger with Pre-Charge Mode
- Set current to 0.05C–0.1C (e.g., 50mA for a 1000mAh cell).
- Do not exceed 3.0V during initial recovery.
-
Monitor Temperature Closely
- If the cell warms by >5°C, stop immediately—it’s unsafe.
-
Check for Physical Damage
- Swelling? Hissing? Leaks? → Dispose of the cell.
-
After Recovery, Test Capacity & IR
- Use a battery analyzer to check:
- Actual capacity (vs. original).
- Internal resistance (if significantly higher, the cell is degraded).
- Use a battery analyzer to check:
Long-Term Risks of Continued Use
Even if the cell seems to work after recovery:
Reduced Cycle Life – It will degrade much faster than a healthy cell.
Higher Risk of Swelling – Due to ongoing electrolyte breakdown.
Potential for Sudden Failure – Internal shorts may develop over time.
Recommended Action:
- If capacity is <70% of original → Replace the cell.
- If internal resistance has spiked → Dispose of it safely.
- If using in a multi-cell pack → Do not mix with healthy cells (imbalance risk).
Final Verdict
- 2.5V for weeks? → Recoverable but degraded (expect ~70% capacity).
- 2.0V–2.3V for weeks? → High risk, likely unsafe for long-term use.
- Best practice: If the cell was below 2.5V for more than a few days, replace it for safety.
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Aaaannnd just in case anybody was still unsure
Me:
When dealing with an over-discharged battery, what should my primary concerns be? Should I consider the datasheet that says I can charge a 1v cell and just send it?
DeepSeek:
Dealing with Over-Discharged Batteries: Primary Concerns
Your top priorities should be:
- Safety (risk of fire/explosion)
- Cell Integrity (irreversible damage)
- Performance (reduced capacity, higher resistance)
Should You Trust a Datasheet That Says “Charge a 1V Cell”?
Short answer: No, absolutely not—unless you’re in a controlled lab setting with safety measures.
Why Datasheets Sometimes List 1V as a “Recovery Threshold”
- Ultra-conservative safety margin (they expect you to discard the cell anyway).
- Legal liability protection (so they can say “we warned you”).
- Best-case lab conditions (perfect BMS, slow charging, immediate voltage rebound).
Real-World Risks of Charging a 1V Cell
It might “work” (voltage recovers, cell takes a charge).
But you’re gambling with:
- Copper dendrites → Internal short → Thermal runaway.
- Plated lithium metal → Sudden failure during next charge/discharge.
- Swelling/venting → Toxic fumes or rupture.
Unless you have thermal monitoring, a fireproof environment, and a very slow charger, “just sending it” is dangerous.
What Should You Do Instead?
Step 1: Assess the Cell
-
Voltage Check:
- >2.5V? → May be recoverable with caution.
- 1.5V–2.5V? → High risk, attempt only if necessary.
- <1.5V? → Dispose of it.
-
Physical Inspection:
- Swelling? Leaks? → Immediate disposal.
Step 2: If Attempting Recovery (2.0V–2.5V)
- Use a lab power supply (not a regular charger).
- Set current to 0.05C (e.g., 50mA for a 1000mAh cell).
- Stop if:
- Voltage doesn’t rise after ~30 mins.
- Cell warms more than 5°C.
- Swelling/hissing occurs.
Step 3: Test Before Reuse
- Measure capacity (expect 20–50% loss).
- Check internal resistance (if >2x original, discard).
- Never use in a multi-cell pack without balancing.
When to Just Dispose of the Cell
Voltage stayed <2.0V for days/weeks.
Recovery attempt caused heating/swelling.
Capacity is <50% of original.
Lithium-ion cells are cheap compared to the cost of a fire.
Final Advice
- Datasheets are not user manuals—they’re worst-case limits.
- 1V is a “point of no return” for safety, not a recommendation.
- If unsure, recycle the cell properly.
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Some of the cells ( Panasonic 18650BD) I used back a few years ago had dropped to 1V a cell. I purchased 100 used public scooter packs, in conditions between good, flat, or full of water. I’m still working my way through the collection. Sure, new cells are better, but I’d rather spend time skating than working to pay for it.
I’ve been lucky with used cells so far but I keep an eye for elevated self discharge and check the BMS data when I discharge a pack more than half.
When I made a mates board, I did his glass enclosure about 6mm thick and glassed the deck heavy too. Safe as houses 
I guess it all comes down to your risk appetite.
I am all for taking big risks if i’m 100% in control and completely aware of the dangers.
But in the circumstance we are describing here, even if you were 100% in control of the charge and discharge of the questionable cell, you have NO way of knowing the damage that has occurred within the cell and no way of knowing if or when it will decide to catastrophically fail.
It really is Russian Roulette and if you keep said batteries in your home, you’re putting your home and your family on the line for cheap thrills.
If you can’t be sure of the danger, you have to assume the worst in my opinion. And who wants to treat a battery as if it could explode at any given moment? That doesn’t seem like a fun time.