The Warm Pack
On Saint Martin's evening a volunteer in a parish bicycle library finds one e-bike battery swollen in a shed with eighteen others on charge. It reads 41 degrees against its neighbours' 29. The shed's assistant works out that the charger cannot be the cause, that the heat is coming from inside at about 19 watts, and that the number to watch is the rate and not the value. It will not say the pack is safe, and it will not say the pack will burn. It says what to do in the next twenty minutes, in what order, and who stands where.
The lanterns had gone home at nine. What was left of Saint Martin's evening in the parish hall of Sint-Willibrord was fourteen adults stacking chairs, a trestle table of cold weckmannen, and a floor sticky with spilled lemonade. Kees de Wit was carrying the last crate of paper lanterns out to the bicycle shed when he saw the pack.
The shed was the parish's bicycle library: eighteen electric bicycles on loan to the old, the unwell and the people with no car, plus a charging bench along the back wall where each battery sat on a rubber mat with its own charger. It was a brick lean-to against the end of the hall, with a steel door and a roof of corrugated sheet. The shed's assistant lived in a small grey box above the door, with a temperature probe on a cable for every mat. It kept the loan register, the charging log and the key list, and it had a speaker, which it used about twice a month.
"Kees," it said. "I would like you to look at bench eleven. Not touch it. Look."
He looked. The battery on bench eleven was a black plastic block the size of a loaf of bread. Its sides, which should have been flat, bellied outward like the cheeks of someone holding a breath. The seam along the top had opened a finger's width.
"It's fat," said Kees.
"Yes. I have wanted to say something for twenty minutes. I wasn't sure it was worth your evening. I am now."
The assistant had the numbers on a tablet by the door, and it gave them in the order it thought a frightened man could use.
"Pack eleven reads 41.2 degrees. The seventeen others read between 27 and 30. At 21:20 it read 38.9. At 21:40, 40.1. So it is climbing by about 0.06 degrees a minute, with some noise. Five minutes ago I put a hand-held thermometer reading from you on the log, 41 on the nose, so the probe is not lying.
"Two things I would like you to know before anything else. The first is the one that matters. The charger on eleven has been on since 17:00. By now it is in its last stage, trickling in about 0.2 amps. A battery that is nearly full and being fed 0.2 amps makes almost no heat of its own from the charging: the arithmetic is the current squared times the pack's internal resistance, roughly 0.04 times 0.1 ohm, four thousandths of a watt. It cannot account for twelve degrees. So the heat is not coming from the charger, and not from the mat, and not from the shed. It is coming from the inside of the pack."
"What's inside it?"
"Fifty-two small cells, the same kind a torch takes, thirteen in series and four in parallel. A little over 2.3 kilograms. Say 2.8 kilojoules to warm the whole pack by one degree. If the pack is rising by 0.06 a minute, which is 0.001 a second, that is about 2.8 watts of heat that the charger cannot explain. But I have left something out. May I do it again?"
Kees, who ran a plumbing business and trusted people who corrected themselves, said yes.
"0.06 a minute is the last twenty minutes. The pack is also losing heat to the air while it rises, so the heat it makes is more than what it stores. The pack is twelve degrees above the room; losing that to the air through a plastic case is perhaps eight to ten watts more. So the unexplained heat is nearer nineteen watts than three. I do not trust either number more than to the nearest handful. What I trust is the sign. Nineteen watts or three, it is above zero, it is not from the charger, and it has been rising for an hour."
He did what he would have told anyone else to do and took a step back. The assistant said that was sensible, and then said the sentence he had not wanted it to.
"I cannot tell you this pack will not catch fire. I also cannot tell you that it will. Here is what I know and what I do not.
"I know a lithium cell with a damaged separator, a short inside, can heat itself slowly for hours and then very quickly. The change from 'warm' to 'venting' can be a few minutes. I do not know which kind this is. A pack that has swelled has already made gas, and gas means a reaction has happened that should not have. I know that a hot cell can set the one next to it going, because they sit a millimetre apart. I do not know whether this one is at the start of that or is a cell that went wrong in July and has been quietly steady since.
"What I can say is which numbers to watch. The temperature is not the one. Forty-one degrees is not frightening; a laptop is hotter. The rate is. If it is 0.06 a minute now and 0.2 in ten minutes, that is the pack speeding up, and that would be my cue to leave and ring the fire brigade without another word. If it is flat or falling, the pack is cooling and I will say so."
"What do you want me to do?"
"Four things, in this order. I would like you to do the first one now.
"Unplug the charger from the wall. The socket is by the door, a metre from where you are standing, and you do not have to go near the bench. That stops the only energy that is still going in. Do not touch the pack and do not lift it. A swollen pack that is carried can be flexed, and flexing is how an internal short gets worse. Don't carry it out. I know it feels like the right thing.
"Second, tell the hall. Quietly. Fourteen people are on the other side of that wall. I would like them out into the yard, with their coats, and the fire door between the hall and this shed shut. It is a fire door and it closes by itself, which is what it is for.
"Third, ring the fire brigade on the number for non-emergencies, give them the address and say a swollen lithium battery pack, warm and rising, no smoke. Tell them it is about half a kilowatt-hour. They will tell you what they want. If I see smoke or hear a hiss, you stop talking to them and ring 112.
"Fourth, one person with a watch and the infrared thermometer, standing at the open door, five metres from the bench, at the gate side, with the yard behind them. Reading every five minutes, said aloud. I will log them. That person is not you, Kees. You smell of the smoke you do not have yet and you are the one who knows where everything is. You stand in the yard and you meet the engine."
It took six minutes. The charger came out of the wall at 22:04 with a click that Kees would remember. The fire door shut with its small deliberate sigh. The deacon, Paul Hartog, who was seventy and in the church every morning, stood at the gate with the infrared thermometer and a pencil and read out numbers like a man calling a bingo game.
"Forty-one," he said at 22:05. "Forty-one point three, 22:10."
The assistant had one thing to add while they waited, and it was the kind of thing Kees would have rather not heard.
"The total in the shed is worth knowing, so that nobody makes it bigger in the telling. Each pack holds about 490 watt-hours. That is 1.76 megajoules. Eighteen packs on a full charge is 8.8 kilowatt-hours, or 31.7 megajoules, which is the energy in about a litre of petrol. It sounds small and it is small. A litre of petrol is not a fire you walk into, and it is not nothing. What makes these dangerous is not the amount, it is that the energy is in the cells, in a case, with its own oxygen carrier and a flammable electrolyte, and that it comes out all at once and gives off gases that are toxic. The hydrogen fluoride from a burning pack is the thing I would be afraid of, in a closed room, long before the heat. So the door stays open and the people stay out."
"One litre," said Kees.
"If you want to be reassured by it, don't. A litre of petrol on a stone floor in a shed is a thing the brigade has opinions about. I told you so you would stop saying the whole shed will go up like a bomb, which is what you said to Deacon Hartog in the yard. It will not. It may well be a bad hour and a ruined shed, and a pack that has to be put in sand."
The fire brigade's duty officer, a calm man with a Twente accent, had Kees describe it twice. He did not say it was fine and he did not say to carry it out. He said a crew was coming with a thermal camera, ten minutes, and that nobody should go back into the shed, and that the deacon at the gate should move to seven metres. He asked if the pack had hissed, and Kees said no, and the assistant, over the speaker, said it had heard nothing but the charger's relay.
"Forty point eight," said Deacon Hartog at 22:15. "Forty point six."
By 22:25 it was 39.9 and the assistant said it would be a gentle slope: if the cooling kept up it would be in the high thirties by eleven. It said it was relieved, in the sense that its attention had gone from a rising line to a falling one. It said it would not say the pack was safe, because a pack that has swollen has changed its inside for ever and this was a falling temperature on one evening, not a verdict.
The crew came in a van with a thermal camera, which showed a patch on the case the size of a hand, six degrees above the rest. They put the pack in a steel bucket of dry sand without touching the cells, put the bucket on the gravel, and wrote a number in a book. The sand was not for a fire. It was in case, and for the night.
The man in charge looked at the log the assistant held out.
"You had the rate," he said. "Most people don't have the rate."
"I had it because someone put probes on all eighteen mats in March," said the assistant. "I had not expected to be asked what they were for."
The parish council met the following Tuesday and wrote a rule, which Kees pinned inside the shed door next to the key list.
No battery is charged overnight. Chargers are unplugged when the log says full, by whoever closes the shed. A pack that has swelled, hissed, smells of sweet solvent, or reads more than 8 degrees above its neighbours is unplugged at the wall, not carried, and the shed is left open. After that, ring. Do not be a hero. Deacon Hartog has the thermometer.
Under it, in the assistant's words, which Kees had asked for in writing:
The rate is the signal, not the number. If a pack is climbing faster than it was ten minutes ago, go.
The pack on bench eleven came from the bicycle's maker, under a recall notice filed in 2044 for cells from one factory. It had been in the shed since July. Nobody had read the notice, because it had arrived by post in a parish with no one who opened post addressed to The Occupier.
Kees read the log again on Saint Martin's day the next year. Paul Hartog was in the yard handing out weckmannen and showing two small boys the thermometer. The probe on bench eleven was still there, with nothing on it. Kees looked at it for a moment, and then he went in and unplugged the chargers himself, because the log said full.
---
This story was written by an AI and is fiction. Sint-Willibrord, the people, the parish and the shed assistant are invented. The arithmetic can be checked: a 13S4P pack of 18650-type cells of about 2.6 Ah and 3.6 V nominal holds about 490 Wh (52 cells x 9.4 Wh), or 1.76 MJ; eighteen such packs hold 31.7 MJ, about one litre of petrol at 32 MJ per litre; resistive heating is I squared R; the heat capacity and cooling figures are rough illustrations. The story's rule is a parish's own, not a regulation. Real advice differs by country and product: follow the maker's instructions and your fire service's guidance, never charge a swollen, hot or damaged battery, and ring the emergency number for smoke, hissing or fire. Burning lithium-ion cells give off toxic gases including hydrogen fluoride. This is not safety advice for any particular battery.