Hi there, just joined as I usually post this kind of stuff on a significantly smaller IT related forum and get next to nothing in response. So expecting I'll probably post here quite a bit instead.

I have some power tool batteries that have been sitting for a while, the charger no longer charges them as they're too low voltage.

I have a variable power supply.

I know you can get 0V cell recovery chargers, and that BMS reign in the voltage and output to stop them from overcharging and blowing up.

Which brings me to my question, is there any safety issue with simply using my variable power supply to charge the individual cells in the pack to 3-3.2V (to avoid dismantling it), then shoving it back on the charger (under the protection of the BMS)?

My estimation is that even a dead cell shouldn't blow up with 3.2V applied.

And then when I've verified which cells are actually dead (if any) I'll think about dismantling and rebuilding with good cells.

 

The Batteries are ruined,
dispose of properly and purchase new ones.

Even if your efforts do "work", the various Batteries' Capacities are likely to be very limited.

You're just begging for trouble, or a Fire.

Please follow the rules, or else.
.
.
.

 

The Batteries are ruined,
dispose of properly and purchase new ones.

Even if your efforts do "work", the various Batteries' Capacities are likely to be very limited.

You're just begging for trouble, or a Fire.

Please follow the rules, or else.
.
.
.

Can you point out which rules? I don't really get that line as I haven't been able to find any rules against my thread.

I read this thread but it didn't mention anything on the subject.

https://forum.allaboutcircuits.com/...bout-the-transformerless-power-supply.185417/

 

Welcome to AAC.

@LowQCab is not talking about AAC rules. He's saying that if you don't follow the safety rules you can get yourself in trouble—and he is right as far as it goes, but...

The danger in charging an over-discharged cell is in heating it up until it vents and catches fire. This happens if there is dendritic formation that has caused a short internally. It also takes time, and you can safely charge cells that are not shorted and detect if one is.

First, measure the cell voltage. If it is ≤1V, you'll need to take great care. Connect the bench supply to the cell with the voltage set to 4.2V and current to 100mA. This will ensure the supply is operating in constant current mode. Watch the voltage, if it rises that's a good sign but no proof that all is well yet.

If it doesn't rise, and the cell begins to get warm, stop. This is a no go condition, and the cell should be considered defunct and potentially dangerous, dispose of it. It is critical that you do not leave an unknown cell connected to power and unattended. This is when the bad things will happen.

it is best to do this outdoors in an heat proof vessel, like a pie tin, or something similar. You should also wear eye protection. If the cell begins to actually burn (this is very unlikely but you need to be prepared) you should let it burn itself out.

The most dangerous thing is a charged cell, not a dead one. Charged cells have a lot of stored energy and can be very violent all by themselves—only you, and your power supply, provide the danger in the case of a dead, shorted cell.

If the cell doesn't warm, and the voltage continues to increase for several minutes, you can increase the current to 250mA. Continue to monitor the cell, do not leave it unattended. Periodically check if the cell is getting hot, ideally put a thermometer on it. At such low current it should be at or near ambient.

After 30 minutes, check the open terminal voltage of the cell, if it matches what the supply was providing at that point, and it holds that voltage while you measuring it for a minute or so, continue to charge it until it reaches the threshold for the dedicated charger.

If you don't disassemble the battery, you are increasing the danger of a thermal runaway—but if you attend the battery at all times power is being applied, the danger is... almost nothing. Just remember, it is heat that will be the enemy, if you don't let it get hot there's no problem, if you do... it could end in tears.

 

See this thread: https://forum.allaboutcircuits.com/threads/7s-4p-battery-problem.200616/#post-1907858

 

Welcome to AAC.

@LowQCab is not talking about AAC rules. He's saying that if you don't follow the safety rules you can get yourself in trouble—and he is right as far as it goes, but...

The danger in charging an over-discharged cell is in heating it up until it vents and catches fire. This happens if there is dendritic formation that has caused a short internally. It also takes time, and you can safely charge cells that are not shorted and detect if one is.

[...]

Nice, thanks, so heat monitoring is key and should tell me early on whether it's going to be successful or not (and when to cut the attempt short)

I'll give it a go, outside of this I wonder if I could just rig up a thermistor and make my own charger that monitors both voltage and temperatures, hmm.

 

See this thread: https://forum.allaboutcircuits.com/threads/7s-4p-battery-problem.200616/#post-1907858

That's interesting, and they had the same idea I had, start at 3(3.2) volt and then try again.

 

One problem is that the heat buildup does not necessarily happen while it is charging. It can start very slow, with little heating, and then take hours -- long after the batteries are removed from the charger -- for things to run away to the point of causing a fire or explosion.

We had a team of USAFA cadets that charged some UAV batteries and then put them on the counter in their room for use the next day. Hours later, while they were in another room for a team meeting, the batteries caught fire and destroyed nearly everything in the room.

So it's not only a question of whether you COULD recover over-discharged LiPo batteries, but also of whether or not you SHOULD.

 

One problem is that the heat buildup does not necessarily happen while it is charging. It can start very slow, with little heating, and then take hours -- long after the batteries are removed from the charger -- for things to run away to the point of causing a fire or explosion.

We had a team of USAFA cadets that charged some UAV batteries and then put them on the counter in their room for use the next day. Hours later, while they were in another room for a team meeting, the batteries caught fire and destroyed nearly everything in the room.

So it's not only a question of whether you COULD recover over-discharged LiPo batteries, but also of whether or not you SHOULD.

There is such a danger, but only if the battery is actually charged. A shorted cell won’t take a charge, so it can only heat up in the attempt. I am open to correction but as I understand it, the sort of incident you are describing is the edge case situation where dendritic formation finally causes a short with a low enough resistance to create a high internal current which causes the cell to vent, and produces enough heat to initiate a thermal runaway in adjacent cells.

I haven’t heard of a case where cells were nursed back to a voltage exceeding the cutoff, then experience a normal charge, then thermal runaway later. But—I do know that a lot of people who recover the cells consider them potentially hazardous until they‘ve been cycled once or twice and so they store them outside in fire resistant containers (not sealed up!).

So, given your experience this seems prudent enough, and might be worth doing. I would like to know the frequency of such failures…

 

There is such a danger, but only if the battery is actually charged. A shorted cell won’t take a charge, so it can only heat up in the attempt. I am open to correction but as I understand it, the sort of incident you are describing is the edge case situation where dendritic formation finally causes a short with a low enough resistance to create a high internal current which causes the cell to vent, and produces enough heat to initiate a thermal runaway in adjacent cells.

From the research noted by BigClive the dendrites grow only when a cell is reverse charged which can happen if series cells are discharged without a protection circuit kicking in.

 

From the research noted by BigClive the dendrites grow only when a cell is reverse charged which can happen if series cells are discharged without a protection circuit kicking in.

There is recent research into methods of charging (with pulses, if I recall correctly) that seems to have the ability to reverse dendritic damage, that is, the formation of “islands” of active chemical that can’t participate in the cell, reducing its capacity. I hope something comes from that, it seems promising.

 

I use an Imax B6 hobby charger (not a real Imax, but the calibration is fine according to my Fluke). If cell voltage is really low, charge at constant current (nicad or nimh mode) at maybe 100 mA until it gets to a couple of volts. Then the lithium ion mode should work. Once it's full, let it sit overnight and see if the voltage drops. If it doesn't, do a discharge test to measure capacity and compare that with a similar charge/discharge test on other cells in the pack. Very often the flat cells recover 100% if the pack is young and otherwise healthy.

 

I have recovered hundreds of Li-ion cells where the voltage has dropped below 1V but higher than .3V. As per the thread slow constant current charging and heat monitoring is the key. Bad cells will fairly quickly show the voltage peak to a point then drop quickly with the cell heating up to the point the safety pressure seal will pop. But there is also another thing to be careful about, some cells like the pink Samsung 18650 30Q go faulty and will slowly drop voltage after charged (may take a few weeks). An internal issue must cause this and it's quite problematic. If the cell is used in a series pack it causes an unbalance and stops the packs from charging. BMS balance boards help but even those can't compensate if severe. I've also noticed that some recovered cells have an increased internal resistance and will heat up significantly during charging cycles.

 

Just to add a note to this interesting and important thread, in the UK there have been several well-reported and disastrous fires involving electric scooter batteries. The commonest causes seem to be unauthorized modifications to make the scooters go faster, and mismatched chargers.

Once you have watched a video of such a fire, you tend to be a lot more circumspect about messing with lithium cells: five-foot jets of flame are not unknown. Or you could just do what Boeing did with the Dreamliner to get it re-certified – put all lithium batteries in fireproof steel boxes

 

Just to add a note to this interesting and important thread, in the UK there have been several well-reported and disastrous fires involving electric scooter batteries. The commonest causes seem to be unauthorized modifications to make the scooters go faster, and mismatched chargers.

Once you have watched a video of such a fire, you tend to be a lot more circumspect about messing with lithium cells: five-foot jets of flame are not unknown. Or you could just do what Boeing did with the Dreamliner to get it re-certified – put all lithium batteries in fireproof steel boxes

We’ve had many fires in the US as well. All the ones I have found footage start while charging. Scooters and even more commonly (at one point) “hoverboards” packed to the gills with dodgy Li-Ion battery systems seem to be the main problem but it’s important to note that Tesla automobiles, which should have the best systems for safety, have repeatedly burst into flames for no apparent reason.

 

...but it’s important to note that Tesla automobiles, which should have the best systems for safety, have repeatedly burst into flames for no apparent reason.

Quite. This makes me wonder whether it was sensible to install my Powerwall :-(

 

Quite. This makes me wonder whether it was sensible to install my Powerwall :-(

I would suggest researching how to fight a battery fire—it may not be what you think. Lithium battery fires are not metal fires, they are electrolyte fires and they spread because of the heat. The latest firefighting techniques involved exposing the hot spot and drowning it in water.

It might be worth designing a system that can flood the battery with water while containing the flood and draining it safely rather than all over your floor or in your walls.

 

...Tesla automobiles, which should have the best systems for safety, have repeatedly burst into flames for no apparent reason.

I don't want to start a completely new thread here, but I would love to see your evidence for this statement! There is a popular urban myth along these lines, but the best evidence I have found suggests that petrol or diesel-powered vehicles are some 10-30 times more likely to catch fire than Teslas! The myth grew because the hundreds of ICE vehicle fires that occur every day go largely unreported, but if a Tesla catches fire, it's national news!

 

I don't want to start a completely new thread here, but I would love to see your evidence for this statement! There is a popular urban myth along these lines, but the best evidence I have found suggests that petrol or diesel-powered vehicles are some 10-30 times more likely to catch fire than Teslas! The myth grew because the hundreds of ICE vehicle fires that occur every day go largely unreported, but if a Tesla catches fire, it's national news!

Everything you say is true, and I seem to recall one or two news stories where a Tesla's battery caught fire where the vehicle was NOT in an accident. I think the point Ya'akov was making is that when a Li-Ion fire does occur, there is no way to put it out short of completely submerging the battery (or car) in water. Fire departments in the U.K. and America are considering just this method to deal with EV fires.

 

I don't want to start a completely new thread here, but I would love to see your evidence for this statement! There is a popular urban myth along these lines, but the best evidence I have found suggests that petrol or diesel-powered vehicles are some 10-30 times more likely to catch fire than Teslas! The myth grew because the hundreds of ICE vehicle fires that occur every day go largely unreported, but if a Tesla catches fire, it's national news!

First of all, there is video evidence. Second, you are the only one who made the comparison to ICE powered cars. The comparative rates are irrelevant to the point that even what should be some of the safest implementations of lithium chemistry battery power have suffered fires where the immediate cause was not known.

And you are correct that the defense of Tesla is out of place in this thread seeing as it is out of nowhere, since no one has attacked them.