3.7V Li-ion battery reads -0.1V ?? - Page 1
Forum section: electronics / repair

author: MathWizard
author_rank: Super Contributor
timestamp: September 13, 2025, 07:11:55 pm
content: I have a battery pack made of 2x 18650 batteries, that wouldn't charge, and I've disassembled them and found the BMS PCB.One reads 3.3V, but the other, reads as -0.1V, and I can see the polarity markings, it's really <0V now, so is there any hope for that battery ? Or should I look up where to dispose it.

author: tooki
author_rank: Super Contributor
timestamp: September 13, 2025, 07:18:50 pm
content: Quote from: MathWizard on September 13, 2025, 07:11:55 pmA lithium-ion cell that has dropped that low may be dangerous to charge. Chuck it.(As a rule of thumb, if a Li-Ion cell drops to about 2V, charging should not be attempted. Under 3V, it should only be charged with a very, very low-current pre-charge phase until 3V has been reached.)

author: ME
author_rank: Frequent Contributor
timestamp: September 13, 2025, 08:52:26 pm
content: bin it,even if you get charge into it the cells capacity will be well down so replace the cell!, for the price a fire is not worth it unless your a pyromainiac.

author: MathWizard
author_rank: Super Contributor
timestamp: September 14, 2025, 12:10:41 am
content: Ok so once it's that low, people don't try bypassing the BMS to charge them. I figured as much, and I wanted to try bringing it back up to 2-3V. But yeah, I'm not doing that inside, so forget it. But I'll keep the other cell the the BMS circuit, and see if I can get them working for something.For a while now I've wanted to design a charger for 9V NiMH batteries too, and for random Li ones, I'm guessing I can do it with my MCU's and all in assembly, it will make for a good learning project.

author: Cyclotron
author_rank: Unknown
timestamp: September 14, 2025, 12:50:35 am
content: Quote from: tooki on September 13, 2025, 07:18:50 pmAnd if, in the case of this one, it's gone negative voltage. A micro blackhole has formed. Feeding it will only cause the end of the universe.Or fires and burns and acrid smoke.

author: amyk
author_rank: Super Contributor
timestamp: September 14, 2025, 02:17:16 am
content: Quote from: tooki on September 13, 2025, 07:18:50 pmThen why do a lot of li-ion charging ICs have no problems with 0V cells, instead giving them a very low current to bring the voltage up?See this for some good information:

author: Cyclotron
author_rank: Unknown
timestamp: September 14, 2025, 02:36:29 am
content: Quote from: amyk on September 14, 2025, 02:17:16 amhttps://www.nature.com/articles/srep30248.pdfThere's an old axiom that goes "just because you can, doesn't mean you should."18650 cells are cheap as chips. I don't mind recovering and reusing good cells, but I no longer will fool with anything below 2.5 volts, and that's pushing it.Just slight damage in a cell will lead to thermal runaway. It's a wonder they allow them at all on planes at all.Back when I quit smoking, I started e-cigs *before they called it vaping*. I did a lot of work on 18650 setups and experienced two thermal runaways, and it was no fun either time.I personally will take risks I won't recommend to others. If you do it, let us know. I like Big Clive, and I hadn't seen that video.Thanks

author: Kean
author_rank: Unknown
timestamp: September 14, 2025, 03:08:27 am
content: It is worth noting that the discussion above mostly relates to single unprotected cells which have a voltage below 2V. With protected cells, they may read as 0V but only because the protection circuit has kicked it. In this case a low preconditioning/trickle charge at 10-25% can "wake up" the protection circuit, and charging will continue at that low rate until the battery voltage rises to a suitable level like 2.4V.If I was to try charging an unprotected cell, it would similarly be at a very low current which is unlikely to be dangerous even if left for hours or days. If the cell is bad, it likely will have a high internal resistance so the voltage will appear to rise much quicker than you would expect even at the low current. The charger may try to switch to a higher current but the cell won't really accept it. The charge current will be generating a small amount of heat due to the resistance, but nothing like the heat dissipated by a good cell charging at normal currents. The voltage will collapse again when charging is terminated.

author: Cyclotron
author_rank: Unknown
timestamp: September 14, 2025, 04:19:01 am
content: Quote from: Kean on September 14, 2025, 03:08:27 amI've been meaning to buy aMC3000. You can program them and do some pretty complex stuff.

author: MathWizard
author_rank: Super Contributor
timestamp: September 14, 2025, 07:34:16 am
content: Yeah when I was making a schematic of the fan these batteries are out of, I couldn't understand how the charger and battery were in parallel, and have the motor, allowed to try running at the same time.Then I found out about the battery management circuit built into the battery, it actually measures -0.127VI'll leave it to Big Clive to revive these cheap batteries. I trust my PSU's can limit down to a few mA, but after almost having a little 380mAh Li battery burst from overheating, this would not be an indoor activity.

author: tszaboo
author_rank: Super Contributor
timestamp: September 14, 2025, 08:17:04 am
content: Quote from: Kean on September 14, 2025, 03:08:27 amWhen it's over discharged, it grows dendrites. If it didn't short yet, even then, the cell might be damaged. Maybe it's already 95% on it's way to be shorted out, you charge it another 3 times and it goes up in flames. Not worth it.

author: tooki
author_rank: Super Contributor
timestamp: September 14, 2025, 10:18:29 am
content: Quote from: amyk on September 14, 2025, 02:17:16 amBecause that job is often left to the protection IC, which cuts off the battery if its voltage goes too low. Better charger ICs do prevent it.For example, the cheap TP4056 chips don’t prevent it. They happily apply a precharge (their datasheets call it a trickle charge) indefinitely.Datasheet:1809261820_TOPPOWER-Nanjing-Extension-Microelectronics-TP4056-42-ESOP8_C16581.pdf(Many manufacturers make these chips; that’s just the most detailed datasheet I could find.)In contrast, the LTC4056 from Linear Technologies, which I strongly suspect is the chip the TP4056’s are based on,dolimit it, by shutting off if the trickle charging doesn’t increase the cell voltage to above the 2.8V threshold within 25% of time of a full charge. They call this “defective battery detection”.Datasheet:405642f.pdfCompare the state diagrams in the TP4056 and the LTC4056 datasheets (on page 10 of each, by coincidence; screenshots attached below) and you’ll see the former missing a transition from trickle back to standby.Consequently, IMHO a TP4056 should only be used with batteries with built-in protection ICs, and not with bare cells. (Or it must be paired with a protection IC, which some charger boards do.)Quote from: amyk on September 14, 2025, 02:17:16 amYeah, I’ve seen that already. It doesn’t change the fact that recharging a deep-discharged cell carries elevated risk compared to a healthy cell, and IMHO it’s just not worth it, given how cheap the batteries are, and how ferocious (to use one of BigClive’s favorite adjectives) Li-Ion fires are.

author: tooki
author_rank: Super Contributor
timestamp: September 14, 2025, 10:28:01 am
content: Quote from: Kean on September 14, 2025, 03:08:27 amJust to be clear, a protection circuit cannot regulate current, it simply disconnects the cell. Current (and voltage) regulation is strictly left up to the charger circuit.Quote from: Kean on September 14, 2025, 03:08:27 amThe internal resistance of a worn-out (but otherwise healthy) cell goes high. But the risk with deep-discharged cells is dendrite growth, which causes internal shorts, which would present aslowinternal resistance.

author: Kean
author_rank: Unknown
timestamp: September 14, 2025, 11:17:05 pm
content: Dendrite growth is certainly an issue, but if the battery is reading near 0V it would have to accept a significant charge before becoming a hazard again. It is definitely something to consider if attempting this, and I wouldn't recommend it unless you know the history of the cell and it has no visible damage. Most of the time I find they don't accept a useful charge. I personally am very cautious with 18650 cells, and mostly work with LiFePO4 these days.My main point was that a low voltage reading may not be due to severe over-discharge, but just due to the battery protection circuit. Ideally you would try measure the unprotected cell to see the actual voltage, and basic precautions should be taken.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: September 15, 2025, 11:57:43 pm
content: Quote from: Kean on September 14, 2025, 11:17:05 pmof course. the issue is that people are watching youtube and thinking they're smart by "reviving" their battery. They then continue charging it and using it, unaware that by being overdischarged it may be at a higher risk of developing internal short (and if that happens when it's fully charged, it's certainly a hazard!).If PCM/BMS has disconnected the pack and the worst individual cell is still above 2.0V then the risk is low.for anything that's gone lower than that, I would err on the side of caution and replace it.

author: MathWizard
author_rank: Super Contributor
timestamp: September 18, 2025, 12:21:43 am
content: I didn't get the name of the BMS IC in the battery, so Tooki they don't normally do max any current limiting or anything also ? And they aren't normally a programmable MCU either ?? IDK, I'm new to them.I need to get the datasheet for these batteries, and the motor. I want to convert it to run off a standard 12V SMPS, instead of 7V4. So I'll try a soft start circuit to allow for the startup current and stall currents of a brushed DC motor. The motors fairly big for a battery powered thing. I know it's idle current, but I'll have to check on a scope to what the startup looks like.Now the motor current flows into a lowside MOSFET, controlled from an MCU. I think it just uses PWM for different speeds, I don't see current or voltage monitoring of anything, and the MCU and it's circuits get power from an 3V3 regulator. I'll have to see if it all works in LTSpice.The motor has some snubber diode and small cap on it. IDK how bad the voltages spikes from it would be, or how fast the diode is, it's an SMD PCB. But the output cap voltage rating of most 12V SMPS, is probably 16V or maybe 25V. So IDK if that will be an issue or not. I'll have to look at it all with a scope.What's the output impedance of a typical 12V 2A wall power pack ?

author: rogerggbr
author_rank: Contributor
timestamp: September 18, 2025, 08:20:22 am
content: Back when I was an apprentice, when something was this bad, we would just write "FUBAR" on it and then you are sure no one would try to fix it. (Flipped Up Beyond All Repair)

author: tooki
author_rank: Super Contributor
timestamp: September 18, 2025, 03:18:03 pm
content: Quote from: MathWizard on September 18, 2025, 12:21:43 amPart of the problem is the very inconsistent, nonstandardized terminology. That and the fact that different amounts of “management” occur depending on the application.With that said, a single protected cell contains just a protection IC, which simply disconnects the cell if voltage goes too low, too high, or if current goes too high. (Some monitor temperature too.) Emphasis on “simply disconnects” — it is ONLY a SECONDARY safety system, and is not intended to be part of the normal charge and discharge management. Think of this as a circuit breaker: if it’s tripping, something is already wrong.Battery packs normally contain a protection IC per above, or a chip/circuit that combines that function with cell balancing. Some add battery authentication and battery health monitoring. Note that the responsibility of charge/discharge management (other than balancing) is still not the responsibility of this chip/circuit. (These are the ones most commonly called “battery management systems”, but people mistakenly think it encompasses more.)The charger is what actually manages the everyday charging functions. If it is functioning properly, the protection in the battery should never engage.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 18, 2025, 07:46:25 pm
content: I've been meaning to buy aMC3000. You can program them and do some pretty complex stuff.I've had mine for a couple years, and I don't know how I got by without it. All charging parameters areselectable and adjustable, for ALL cylindrical, rechargeable cells. And it even has a USB port and BLU tooth.Sure it costs a lot more than your run-of-the-mill chargers, but you get what you pay for.

author: tszaboo
author_rank: Super Contributor
timestamp: September 22, 2025, 10:16:35 am
content: Quote from: tooki on September 18, 2025, 03:18:03 pmI was selecting an IC, a Li-ion charger for USB. It seems simple enough. I went through about 15 datasheets, even from TI and other western manufacturers. To this day, I cannot tell you if the IC supports things like UVLO and other protection features like this, or it requires an extra IC to perform these essential protection features. Nothing on the datasheet, no application note.The sheer amount of irresponsibility blows my mind.