To try to put the whole thing into perspective, an aquantence of mine is a firefighter and while they don't always know the cause of the fires they are called to he is not aware of any fires caused by batteries. By contrast he has lost count of the number of washine machine and tumble drier incidents, usually unattended at night.
Battery fire!
- Thread starter Jeremy
- Start date
I'll second that, tumble dryers are particularly risky when left unattended.
I didn't want to cause any panic with my indications of only charging when being in and awake, and I'm referring to all charging, laptop and other batteries included, not just bike batteries.
I just think the sensible position is that if you are in and awake, the very, very slight offchance of something going wrong won't matter, and the same applies to running any electrical appliance.
.
I didn't want to cause any panic with my indications of only charging when being in and awake, and I'm referring to all charging, laptop and other batteries included, not just bike batteries.
I just think the sensible position is that if you are in and awake, the very, very slight offchance of something going wrong won't matter, and the same applies to running any electrical appliance.
.
Jeremy,
Let us know if you find out any more about the cause. I have added a warning to the thread on Parallel Charging in the Battery FAQs forum. In the long run maybe we should have a summary of the process and the dangers in that forum.
How is the house smelling?
Nick
Let us know if you find out any more about the cause. I have added a warning to the thread on Parallel Charging in the Battery FAQs forum. In the long run maybe we should have a summary of the process and the dangers in that forum.
How is the house smelling?
Nick
As I mentioned before, I don't think that the risk from a properly designed, manufactured and approved battery and charger is really very significant. My set-up is very home-made!
A dedicated charging area sounds like a good idea, if I had a fireplace I'd have been tempted to use it. The snag is that the amount of energy in the pack is significant, so you really need some containment to stop exploding cells flying off.
I'm currently discharging the "good" sub-pack and it looks like it's fine. Some of the cells at one end have damaged sleeving from the heat, but none are distorted and there is no indication of venting. I won't use this cells in the bike, as I suspect they will have suffered long-term damage, but will no doubt find another use for them.
One possible cause is the difference in wire lengths. The dual Schottky was fitted at the charger end of the lead and the sub-pack that failed had wires that were about a foot shorter than the "good" sub-pack. This may have been just enough to ensure that that sub-pack took a little more charge current than the other one, resulting in the imbalance. In some ways, not having the diodes might help, as it would allow self-balancing, perhaps.
The failure mechanism once the cells get hot is easy to see. The sleeve on each cell body is thin and as soon as it gets hot it splits and shrinks back. This allows adjacent cells to short, so massively increasing the amount of energy being dissipated. The "good" sub-pack has clear evidence of the way that heat almost completely removes the cell sleeves, as several cells at the "hot" end show severe signs of this, with large areas of cell case exposed. There are slight indications that some of these cell cases shorted against the aluminium case, even though they were wrapped with additional tape (which has melted). Luckily, this sub-pack was ejected from the case by the pressure from the cells in the pack that failed. This is most probably what saved them from also blowing up.
The smell has virtually gone, thank goodness, although there is still a big burn mark on the path. I found some other cell debris this morning, bits of perforated metal etc, about 6ft from the main fire. It looks like they were thrown there from one or two of the most severe explosions.
I'm considering my options for a replacement pack now. I could go for the same set up again, or I may opt to try something else. Whatever I do the pack will have a lot of built in protection systems.
Jeremy
A dedicated charging area sounds like a good idea, if I had a fireplace I'd have been tempted to use it. The snag is that the amount of energy in the pack is significant, so you really need some containment to stop exploding cells flying off.
I'm currently discharging the "good" sub-pack and it looks like it's fine. Some of the cells at one end have damaged sleeving from the heat, but none are distorted and there is no indication of venting. I won't use this cells in the bike, as I suspect they will have suffered long-term damage, but will no doubt find another use for them.
One possible cause is the difference in wire lengths. The dual Schottky was fitted at the charger end of the lead and the sub-pack that failed had wires that were about a foot shorter than the "good" sub-pack. This may have been just enough to ensure that that sub-pack took a little more charge current than the other one, resulting in the imbalance. In some ways, not having the diodes might help, as it would allow self-balancing, perhaps.
The failure mechanism once the cells get hot is easy to see. The sleeve on each cell body is thin and as soon as it gets hot it splits and shrinks back. This allows adjacent cells to short, so massively increasing the amount of energy being dissipated. The "good" sub-pack has clear evidence of the way that heat almost completely removes the cell sleeves, as several cells at the "hot" end show severe signs of this, with large areas of cell case exposed. There are slight indications that some of these cell cases shorted against the aluminium case, even though they were wrapped with additional tape (which has melted). Luckily, this sub-pack was ejected from the case by the pressure from the cells in the pack that failed. This is most probably what saved them from also blowing up.
The smell has virtually gone, thank goodness, although there is still a big burn mark on the path. I found some other cell debris this morning, bits of perforated metal etc, about 6ft from the main fire. It looks like they were thrown there from one or two of the most severe explosions.
I'm considering my options for a replacement pack now. I could go for the same set up again, or I may opt to try something else. Whatever I do the pack will have a lot of built in protection systems.
Jeremy
In my multi pack experiments with AA cells, I had switching for the pack charging so the packs could be charged individually or in groups. I found that when charging two or four packs, the charge self balancing was much better without Schottky diodes, and in the final range extending battery version using 2 x 32 cells in parallel, there's no Schottky diodes and I charge in parallel all the time.
.
I've now finished discharging the "good" sub-pack. It seems to give about 3.5Ah, which would indicate, perhaps, that it didn't quite get a full charge. I'm even more sure now that the problem was a charge imbalance between the two sub-packs, together with my poor choice of "trickle" charge current.
The photo below shows the damage to the cell sleeving I mentioned. This stuff just seems to disappear almost once it splits and shrinks away.
If I choose to use packs like this again, then I will definitely fit some more robust inter-cell insulation.
Thanks for the confirmation of the parallel charge direct connection thing, Flecc. I suspect that, despite all the technical advice from the cell suppliers that this shouldn't be done, it may well be that it's better than using parallel diodes for charging, at least. I'm still inclined to think that it might be better to use a pair of Schottkys for discharge connection, as cell balance would be much less of a problem for this anyway.
Jeremy
The photo below shows the damage to the cell sleeving I mentioned. This stuff just seems to disappear almost once it splits and shrinks away.
If I choose to use packs like this again, then I will definitely fit some more robust inter-cell insulation.
Thanks for the confirmation of the parallel charge direct connection thing, Flecc. I suspect that, despite all the technical advice from the cell suppliers that this shouldn't be done, it may well be that it's better than using parallel diodes for charging, at least. I'm still inclined to think that it might be better to use a pair of Schottkys for discharge connection, as cell balance would be much less of a problem for this anyway.
Jeremy
A quick update with some interesting findings. I've been sifting through the burnt wreckage and can now piece together what most probably happened. Having cleaned up the inside of the alloy case, I've found some deep pits caused by arcing from several cells to the case. The case was originally electrically isolated from the packs.
I think the sequence was pretty much as I have already guessed, except that the cells in the hot sub-pack also shorted to the case once the shrink insulation and tape failed. The evidence for this is that the end cells in each sub-pack seem to be OK, as the soldered wires came off. The cells that exploded are all ones from the centre.
These cells are pretty tough! As an experiment, I checked the terminal voltage of the intact cells and found that they were all sitting at 1.25V. This seemed to show that they still had some charge, so I've just discharged them all at about the 0.3C rate, into a big resistor. All of the cells seem to be at about the same state of charge and gave about 2Ah when discharged down to 1V (they are rated at 3.7Ah). These cells are very burnt looking, with no insulating sleeves and lots of black soot around the end. I'm frankly amazed that they are working at all!
I've just put them on charge (at a very low rate!) to see if they do actually accept a charge. If they do, then I will discharge them again to see what their true capacity is. Although not particularly objective, this should give an indication of what the capacity loss is from such extreme heat exposure, which might be reassuring to those who have just got packs a little bit warm.
Jeremy
I think the sequence was pretty much as I have already guessed, except that the cells in the hot sub-pack also shorted to the case once the shrink insulation and tape failed. The evidence for this is that the end cells in each sub-pack seem to be OK, as the soldered wires came off. The cells that exploded are all ones from the centre.
These cells are pretty tough! As an experiment, I checked the terminal voltage of the intact cells and found that they were all sitting at 1.25V. This seemed to show that they still had some charge, so I've just discharged them all at about the 0.3C rate, into a big resistor. All of the cells seem to be at about the same state of charge and gave about 2Ah when discharged down to 1V (they are rated at 3.7Ah). These cells are very burnt looking, with no insulating sleeves and lots of black soot around the end. I'm frankly amazed that they are working at all!
I've just put them on charge (at a very low rate!) to see if they do actually accept a charge. If they do, then I will discharge them again to see what their true capacity is. Although not particularly objective, this should give an indication of what the capacity loss is from such extreme heat exposure, which might be reassuring to those who have just got packs a little bit warm.
Jeremy
Even though I was able to parallel charge my packs, I still didn’t have too much confidence in my process especially after Jeremy’s experience.
On my first attempt, I only had one of the packs thermistor connected, and that happened to be the pack that peaked first. But if the other pack had been the one to peak first, it would have been overcharged.
Since both packs already have thermistors attached, I figured, why not hook them both up. I know having two thermistors in parallel would half my resistance and therefore make the charger think my pack is warmer than it is. This means that instead of my charger shutting off at Tmax>50, it would do it at Tmax>35. If one pack got hot as the other stay completely cold, than the max T difference that could take place before the charger shut off would be T=45 for the hot pack and T=20 for the cold. And 45 degrees is well within my comfort zone.
Anyhow, I gave it a try last night and it worked very well. When the charger shut off, the packs temp was 35 and 36 degrees. When I charge the packs separately, the pack would get to around 41-42 degrees.
The only down side of this is you won’t get a full charge. But because of this, I don’t have to worry about the voltage in one pack dropping due to overcharge and therefore having all the current from the charger go into the overcharged pack.
I think I’m getting about a 90% level of charge with this method, which is probably better for the life of the battery. And if I really wanted a full charge, I can always leave it on trickle for a few hours.
On my first attempt, I only had one of the packs thermistor connected, and that happened to be the pack that peaked first. But if the other pack had been the one to peak first, it would have been overcharged.
Since both packs already have thermistors attached, I figured, why not hook them both up. I know having two thermistors in parallel would half my resistance and therefore make the charger think my pack is warmer than it is. This means that instead of my charger shutting off at Tmax>50, it would do it at Tmax>35. If one pack got hot as the other stay completely cold, than the max T difference that could take place before the charger shut off would be T=45 for the hot pack and T=20 for the cold. And 45 degrees is well within my comfort zone.
Anyhow, I gave it a try last night and it worked very well. When the charger shut off, the packs temp was 35 and 36 degrees. When I charge the packs separately, the pack would get to around 41-42 degrees.
The only down side of this is you won’t get a full charge. But because of this, I don’t have to worry about the voltage in one pack dropping due to overcharge and therefore having all the current from the charger go into the overcharged pack.
I think I’m getting about a 90% level of charge with this method, which is probably better for the life of the battery. And if I really wanted a full charge, I can always leave it on trickle for a few hours.
Related Articles
-
MTF Enterprises announces acquisition of EMU Electric Bikes- Started by: Pedelecs
-
Wisper 806T folding bike wins Which? ‘Best Buy’- Started by: Pedelecs
-
Sustrans calls for protected cycle lanes- Started by: Pedelecs
-
Amazon launch their first UK e-cargo micromobility hub- Started by: Pedelecs
