British Scientists Investigated German Submarine Batteries — Then Discovered Why Crews Died So Fast
In the late summer of 1941, the North Atlantic had seen its share of Hubot, but never one like this. Just off the coast of Iceland, a German submarine suddenly surfaced under a plume of chaotic white spray. Allied air crews expected the usual evasive dives, frantic gunfire, or an attempt to flee. Instead, the boat, later identified as U570, halted dead in the water.
Across its deck, sailors stumbled like men poisoned. not beaten. A white flag rose shakily where an anti-aircraft gun should have been firing. From the air, the Hudson bombers crew watched something unprecedented in the Battle of the Atlantic. A fully operational hubot surrendering without a single lethal hit.
No flames, no crippling damage, no depth charge pattern close enough to explain the chaos unfolding on deck. Yet German sailors were clawing for fresh air, collapsing beside open hatches that should have been sealed in combat. Something inside the submarine had overcome an entire crew in minutes. So when British boarding officers finally stepped below, drawn from the Royal Navy, the Naval Intelligence Division, and the small but growing circle of British Scientific Advisers, they recognized panic prisoners before, but never on this scale. Many of the
captured crew had been coughing violently when taken aboard the escorting ships, some stumbling as if drunk, others semi-conscious and barely able to stand. There had been no attempt to fight on the surface, no effort to man the deck gun, no last burst of defiance, no demolition charges were set, no valves open to scuttle the boat in deep water.
Only bewildered men desperate to get out. Meanwhile, back in London, captured German Ubot was seen as floating archives of enemy technology. Every piece of equipment, from torpedo pistols to radio sets, was a potential clue in the larger struggle for the Atlantic. But U570 would offer something different.
As the first survey of her compartments continued, the initial excitement over intact torpedoes and documents faded, replaced by a quieter, more unsettling curiosity about the air itself. By the time investigators reached the passage leading toward the battery spaces, the metallic taste on their tongues had sharpened. Paint around certain vents seemed oddly discolored.
The men exchanged brief uneasy looks. Whatever had happened here was not just a matter of machinery and tactics. It felt like a warning written in gas instead of ink, and it was one they could not afford to ignore. The journey of U570 after her surrender is deliberately quiet. Towed first toward Iceland, then onto a British port under escort, she travels not as a victorious raider, but as a sealed container of secrets.
Orders from the Admiral T emphasize discretion. This hull represents a rare chance to look inside the German undersea campaign while it’s still underway, and every hour she remains afloat increases her value to British planners. When engineers from the Admiral T, the Royal Navy’s engineering branch, and the British Scientific Service finally step aboard, the scent that greeted the original boarding party is not entirely gone.
Ventilators have been run continuously. Hatches stand open whenever the tide and security allow. Yet a faint metallic, faintly choking tang still hangs in the air, concentrated along the lower passages. It is not strong enough now to injure, but it is persistent enough to trouble men who know machinery and the smells it ought to produce.
They begin with the familiar. Diesel engines are examined for blast damage and improvised modifications. Auxiliary machinery and compressors are checked. Torpedo racks are inspected and photographed. Radio stations and encryption gear receive particular attention in case anything can be tied to the wider signals intelligence effort that is tightening around the yubot arm.
Nothing in these upper compartments looks catastrophically damaged. There are no twisted frames, no gaping seams, no signs that the boat has taken a mortal wound from outside. Only when the investigators descend toward the lower deck do they find the first physical trace of what made the crew abandon their vessel while it was still buoyant.
Around several ventilation openings and pipe joints, they notice discolored streaks running down painted steel, a thin greenish pattern etched by corrosion. The shapes are telltale to anyone familiar with industrial chemistry. Chloride staining, the fingerprint left when salt water, metal, and electricity have interacted in the wrong way.
Meanwhile, in a nearby office, translated excerpts from the Yubot’s log books and interrogation notes begin to align with what the engineers are seeing. The patrol reports mention no direct depth charge hits strong enough to flood the battery cells, but they do note heavy shocks and severe jolts that shook instruments from their mounts.
Captured officers recall irritation of the eyes and throat, crewmen collapsing without obvious wounds, the air turning bad in a matterof minutes. To British scientists accustomed to lead acid installations, the implication is alarming. Seawater intrusion and damaged batteries producing chlorine gas in a confined space.
Yet even that explanation feels incomplete. Chlorine can a man quickly, but the near total breakdown of the order aboard U570 suggests something more than a single toxic episode. To understand how a trained crew could be pushed so rapidly past the point of discipline, the investigators will have to move from the captured hull into the controlled environment of a laboratory and deliberately recreate the conditions that turn this submarine into a trap.
A single spark is all it takes to turn theory into something the men in white coats can feel in their lungs. At the Admiral T experimental station, a sealed test chamber has been prepared with the same dimensions and airflow characteristics as a section of U570’s battery installation. Inside, mounted on a steel rack and wired to measuring instruments, sits a German lead acid battery cell identical to those recovered from the submarine.
Outside the thick observation window, British scientists, naval engineers, and medical officers watch as the experiment begins. Seawater droplets are misted cautiously over the casing, replicating the kind of brief intrusion that might follow a warped seal or a hairline crack after a depth charge near miss.
Almost at once, the first whiff of chlorine hits the sampling tubes. The gas is sharp, stinging, unmistakable to anyone who has worked around electrolysis or industrial cleaning agents. Instrument needles climb. Concentration figures are noted. Within a short time, the test atmosphere inside the chamber reaches levels known to cause serious irritation and respiratory distress in unprotected personnel.
But then the sensors spike again, this time in another column of figures entirely. Hydrogen levels begin rising rapidly, produced by normal battery, now trapped in a deliberately restricted air flow. Under ordinary conditions, forced ventilation would carry it away, keeping the concentration below dangerous limits.
Inside a submarine that has just been slammed by explosives, however, fans can fail, ducts can twist or disconnect, and vents can be obstructed by debris. The test chamber simulates precisely that scenario with reduced extraction and damaged ducting. The dual graph lines on the monitoring panel start to tell a more complex story. Chlorine climbs to levels that would drive any conscious sailor out of the compartment within minutes.
Hydrogen, invisible and odless, accumulates above. When mixed with air in the right ratio, as the scientists demonstrate, by triggering a controlled ignition source, the result is a sudden flash and a concussive wave that rattles the chamber walls. In a real battery room, such an event would mean burns, shrapnel, and possibly a cascading failure as other cells are ruptured by blast.
For the men watching, the implications are sobering. The crew of U570 were not facing a single simple hazard. They were caught in a dual crisis. Each component feeding the other, chlorine poisoning driving men from their posts, and hydrogen building silently behind them, threatening to turn the air itself into fuel. If this could happen in a controlled test with one cell, the question no longer seems academic.
Deep under the Atlantic, in a closed hole battered by weapons, a crew would have almost no margin for error and very little time. If you’re enjoying this story, hit like, share it with someone who also loves hidden chapters of history, and subscribe for more hidden stories of World War II, and after you finish the documentary, share your thoughts in the comments below.
I’d love to hear your take. Armed with the data from the laboratory, the investigative team returns to the captured submarine with a sharper set of eyes. The lower deck of U570 is no longer just a cramped engineering space. It is the stage on which their gas curves played out in metal and sweat. The battery room itself is a long, low cavern broken by narrow aisles and metal grings, the banks of cells slotted into place like blocks of solid weight.
Even with the boat now in harbor, stepping through the hatch brings an instinctive tightening of the chest for men who know what once hung in this air. The faint smell that has lingered since the day of capture still clings to the corners, diluted but not gone. Around several battery caps, residue marks show where vapor once escaped in concentrated spurts along bulkheads and around vents.
The same greenish corrosion seen earlier appears in spreading patterns, radiating from points where moisture must have seeped or been driven by shock. Moisture lines on paint and insulation suggest a brief intrusion of water, enough to set off the chemical chain, but not enough to register as obvious flooding.
German Ubot rely on two enormous sets of lead acid batteries, each consisting of dozens of cells arranged in series,weighing many tons in total. They generate electrical power, but also heat and chemical byproducts. Under normal running, ventilation fans sweep hydrogen out of the compartment, and engineers watch electrolyte levels and casing integrity.
On a calm day, with everything functioning, the room is simply hot, noisy, and unpleasant. The investigators now picture a different sequence. Depth charges detonate nearby, slamming the hull with sudden violent force. Battery caps jump in their threads. Vent lines crack at joints. Electrolyte sloshes against lids and seams.
Fine spray or vapor is driven into places it should never reach. Any one of these events might be manageable in isolation. Combined in a confined volume of air, they become catastrophic. To test air flow, British engineers run smoke through the compartment and observe how it pulls and twists in the spaces between cells and deck heads.
Within minutes, they see hydrogen’s path mapped out in slow, curling trails toward the upper corners, exactly where instruments predicted it would collect. Chlorine, heavier and more insidious, would move lower, hugging floors and settling in the very roots a man would take to reach a valve or inspect a leak.
Meanwhile, the investigators cross-check these physical observations with German medical notes and incident reports. Accounts of men stumbling, vomiting, collapsing without visible injury start to match the concentrations calculated in the lab. Panic, it seems, did not arise from fear of the enemy above, but from the immediate choking reality of an atmosphere turning hostile around them.
The battery room ceases to be just another compartment. It becomes a reminder that the most serious danger aboard some submarines was not always outside the hull. Long before the story of U570 is fully written up in British files, other pieces of evidence begin drifting in from different corners of the war.
At interrogation centers around the country, specialists in language and psychology question captured yubot crews about their patrols, their equipment, and their emergencies. Most of the answers concern torpedoes that failed, engines that seized, or periscopes that cracked. But scattered among these technical complaints are quieter references to something else.
Episodes in which the air itself seemed to rebel against the men who depended on it. British intelligence officers working through stacks of transcripts and seized paperwork start to notice a pattern. Several boats report crew casualties following flooding or after severe jolts, even where no external damage appears in the patrol summary.
German medical logs describe men with burning lungs and reddened eyes, coughing fits that leave sailors doubled over, and sudden respiratory collapse in compartments that were not directly hit. A few accounts mention a gas alarm without specifying the source, followed by strict orders not to discuss the incident outside the boat.
The Germans, it becomes clear, are not ignorant of the risk. Training manuals warn crews not to reenter battery compartments immediately after certain types of damage, and chemical defense pamphlets circulate within the service. But war at sea is not a controlled environment. When a convoy battle is at its peak, captains are forced to choose between ideal safety procedures and the demands of survival.
Battery fumes have sickened crews aboard U83, U167, and other boats noted in Allied files. Some submarines have experienced unexplained fires or explosions in internal spaces, later attributed, at least in part, to gas buildup. Few commanders are eager to file reports that suggest their own equipment turned on them.
Incidents that do not destroy the boat outright are often minimized or wrapped in technical language. In a fleet fighting for prestige as well as strategic effect, admitting that a key system contains a deadly flaw is professionally and psychologically difficult. The result is an unspoken half acknowledged problem dangerous enough to produce warnings and drills but not faced head on.
British scientists and analysts now compare these scattered references with the hard numbers from their experiments. The same battery configurations, the same ventilation layouts, the same operating regimes recur across the German submarine force. What distinguishes EU fascini is not its design but the visibility of its failure.
Its crisis unfolded in daylight, under Allied air coverage, ending in surrender rather than disappearance. In another ocean quadrant, under cloud and distance, a crew overcome by fumes might simply never report in again. Written offers lost to depth charges or mines. With each file they open, the British researchers become more convinced that they are looking not at an isolated accident, but at a systemic vulnerability running through the heart of the Yubot campaign.
To test just how far that vulnerability can go, the Admiral T experimental station moves beyond single cell trials and prepares afull-scale reconstruction of a Yubot battery bay. In a reinforced building on the British coast, technicians assemble rows of captured German cells into banks that mirror the layout taken from U570s plans.
Sensors are threaded through the installation. Ignition sources are carefully positioned. Air circulation is set to match the volume and flow rates of an actual compartment deep inside a submarine hull. The first phase of the experiment looks deceptively ordinary. Engineers run the batteries through charging cycles similar to those used when a submarine surfaces at night to refresh its electrical reserves.
Heat builds slowly. Caps vent small amounts of hydrogen as a normal byproduct of charging. Under standard conditions with all fans operating and ducting intact, that gas would be swept out and diluted before it reached dangerous levels. The instruments on the test rig confirm this.
Concentrations remain low within accepted safety margins for an industrial installation. Then the scenario shifts. Operators simulate a series of depth charge explosions by subjecting the structure to violent mechanical shocks. The whole assembly jumps in its mounts. Vent lines flex and in places deliberately weakened joints crack.
One or two battery caps rattle loose by a fraction of a turn. Cooling air flow is reduced to replicate damaged fans. Now the sensors tell a different story. Hydrogen levels begin to creep upward, then climb with increasing speed as the gas finds fewer roots of escape. At around 4% hydrogen by volume, the atmosphere inside the mock compartment enters the explosive range.
At 8% it becomes violently so. The test team safely behind shielding triggers a tiny spark using a relay similar to those employed in wartime electrical systems. The reaction is instantaneous. A sheet of flame races across the ceiling of the compartment. A sharp concussive blast echoes through the building.
When the smoke clears, several of the battery casings are cracked or deformed. exactly the kind of damage recorded in a handful of German incident reports where crews survived to describe what had happened. For British analysts, the demonstration closes a loop. If chlorine drives men from the space, forcing them to shut down equipment or seal hatches, the resulting lack of ventilation allows hydrogen to accumulate still faster.
The two hazards are not separate problems, but interacting forces, each making the other more lethal. Meanwhile, aboard the real U570, now recommissioned as HMS Graph and operated cautiously by British crews, even careful management produces occasional hydrogen spikes during heavy charging. The lesson is stark.
Every submarine that relies on such batteries carries along with its torpedoes and fuel the potential for an internal blast that needs no enemy weapon to set it off. By the time the last test results are collected, there is little room left for comforting interpretations. In a series of confidential reports circulated within the Admiral Ty, British scientists set out their conclusions in language that is clinical but carries an unmistakable weight.
German submarine batteries as installed and operated in boats like U570 present a dual hazard. Chlorine gas triggered by seawater intrusion and hydrogen accumulation driven by disrupted ventilation and shock. Each is dangerous on its own. Together, under combat conditions, they can turn a vital power source into a rapidly unfolding emergency.
Meanwhile, intelligence officers review captured German manuals and technical instructions with fresh eyes. They find detailed procedures for managing routine gassing, including guidance on running fans and opening specific vents. There are also warnings about re-entering compartments after flooding or damage.
What they do not find is a comprehensive doctrine for handling a combined chlorine and hydrogen crisis in a closed space under threat. Crews are told to ventilate hydrogen by operating electrical equipment and fans, but chlorine calls for evacuation and ideally shutting down sources of ignition.
In practice, these responses pull in opposite directions. The analysts reconstruct the likely sequence aboard U570 using interrogation notes, log entries, and their experimental data. They conclude that chlorine almost certainly struck first, released when shock loosened seals allowed seawater or electrolyte to contact exposed components.
The gas would have produced burning lungs, streaming eyes, and a sense of suffocation within minutes, especially among men already exhausted by long dives and attack maneuvers. Officers trying to restore control would have faced a cruel choice. Send engineers back into the compartment to investigate or seal the space and hope that whatever was happening inside did not spread.
As hydrogen levels climbed unseen above head height, any attempt to restart fans or switches carried a risk of ignition. Leaving systems off, on the other hand, would allow concentrations to rise stillfurther. Under such conditions, the interior of the boat turns into an adversary in its own right.
The collapse of discipline described by captured German personnel no longer appears mysterious or dishonorable. It looks like the predictable human response to a situation in which every option seems to make things worse. For British planners, the verdict shapes not only their understanding of one surrendered submarine, but their view of the yubot threat as a whole.
Here is a weapon system that exerts enormous pressure on allied shipping, yet carries within its har weakness that no amount of tactical skill can entirely erase. Chemistry cares nothing for training or ideology. In the quiet offices where these findings are read, there is no sense of triumph, only a clearer picture of the brutal, impersonal mechanics that govern life and death beneath the waves.
When the key findings are presented to senior naval commanders, they do not arrive as a dramatic revelation, but as a sobering confirmation of what some had already suspected from scattered reports. German hubot, the predators of the Atlantic, whose periscopes and torpedoes dominate so many strategic discussions, carry with them a flaw that lurks out of sight.
It is not a matter of poor construction or deliberate neglect, but of the basic chemistry tied to the batteries that make submerged operations possible. Under the wrong combination of damage, shock, and circumstance, those same systems can turn deadly with a speed that rivals any external weapon. HMS Graph, the former U570, continues in British service for a limited period after her capture, offering an unusual opportunity to observe German design in regular use.
Crews assigned to her learn her controls, test her diving performance, and evaluate her noise signature. Yet, many of the men who work below decks carry a quiet awareness that this submarine once came close to killing its own compliment without a single direct hit tearing open the hull. The faint chemical taint that seems to linger in some recesses, whether real or imagined, reinforces that impression.
As the war grinds on, other hubot disappear in the North Atlantic and beyond, recorded in logs and official histories as missing cause unknown. Some are almost certainly destroyed by escorts, mines, or aircraft. Others may succumb to mechanical failure, navigation errors, or simple bad luck. But in light of the British investigations, another possibility enters the margins of analysis.
that a number of submarines were lost to internal events that never left debris on the surface or witnesses in the water. A battery explosion deep below the thermocline or a compartment filled with choking gas far from any help leaves little trace. By the late stages of the conflict, both sides understand more about the hazards of operating complex machinery in confined spaces under pressure. Procedures improve.
Ventilation is refined. Protective equipment becomes more common. None of these changes fully removes the underlying risk. Every Navy relying on large lead acid installations carries some version of the same burden. When HMS Graph is finally retired and scrapped after the war, the decision is routine, one among many quiet demobilizations.
The steel is cut, the compartments open to daylight. The peculiar smell of enclosed machinery slowly fades. Yet the story written inside her hull persists in reports, training lectures, and the memories of those who studied her. It is a reminder that beneath the roar of engines and the impact of depth charges, much of undersea warfare is decided by forces that cannot be seen.
Mixtures of gas, pressure, and human tolerance. Long after the last yubot has been broken up, the lesson remains, lingering like a trace of something sharp on the air.
