At Keltie Campers, we see a lot of electrical problems. But recently, we've been seeing something that genuinely concerns us — and we want to share it, because it could affect your safety on the road.
Over the past few months, we've had a significant number of customers come to us with overheating leisure batteries. Some had carbon monoxide alarms going off repeatedly in the night. Some had removed the alarms entirely to stop the noise, not realising the battery itself was the danger. And one customer — just the evening before they were due to set off on holiday — had a second leisure battery that was strapped in behind the driver's seat, close to the heating unit, explode overnight. It sprayed sulphuric acid across the furniture, the seats, and the interior. They were on their driveway. It could have been so much worse.
This post is to help you understand what's happening, why, and what you can do about it.
What Actually Happens When a Lead Acid Battery Fails
A standard lead acid leisure battery contains cells filled with sulphuric acid electrolyte. Under normal conditions, this is perfectly safe. But when a battery is old, overcharged, or a cell fails internally, the chemistry goes wrong — and the results can be alarming.
When a cell fails and effectively short-circuits, it draws excessive current from the rest of the battery. This causes the battery to heat up rapidly. As the temperature rises, the chemical reaction accelerates, and the battery begins to gas — releasing hydrogen and hydrogen sulphide.
Hydrogen sulphide is the gas responsible for the rotten egg smell people often notice first. It's colourless, poisonous, and flammable. At low concentrations it's unpleasant; at higher concentrations it can cause nausea, dizziness, eye pain, and in severe cases, serious harm.
Hydrogen gas is also produced. It's invisible, odourless, and explosive at concentrations of just 4%. In a sealed or poorly ventilated space like a campervan or motorhome battery compartment, those concentrations can be reached faster than you'd think. A single spark — from a relay, a switch, or disconnecting the battery while it's still gassing — can ignite it.
Why Your CO Alarm Going Off Isn't Always Carbon Monoxide
This surprises a lot of people. We've had customers who were repeatedly woken by their CO alarm in the night, assumed it was a fault with the alarm, and either ignored it or removed it. In most of these cases, it was the battery that was triggering it.
Standard CO alarms use electrochemical sensors that are also sensitive to hydrogen sulphide and hydrogen gas. When a lead acid battery is overcharging or a cell is failing, the gases produced are enough to trigger the alarm — even if the battery is in a sealed compartment and the alarm is metres away. The alarm isn't malfunctioning. It's doing exactly what it should: telling you something dangerous is in the air.
If your CO alarm goes off and you can't identify a combustion source — gas appliance, exhaust, heating — check your battery immediately. Don't silence it. Don't remove it. Ventilate the vehicle and investigate.
The Explosion Risk Is Real
Battery explosions are not common — but they happen, and when they do, the consequences are serious. A lead acid battery under internal pressure can rupture its casing, spraying sulphuric acid across whatever is nearby. Acid burns skin, destroys eyes, and causes permanent damage to upholstery, soft furnishings, and vehicle interiors.
The risk is significantly higher when a battery is:
- Stored in a poorly ventilated location (under a seat, beside a heater, inside a sealed cabinet)
- Old or nearing the end of its service life
- Being charged by a faulty or incorrectly specified charger
- Paired with a second battery of a different age or condition
- Located near a heat source such as a diesel heater or engine bay
Batteries stored near heating systems are particularly vulnerable. Heat accelerates chemical degradation, and a battery that might have limped along for another year in a cool, ventilated space can fail catastrophically when it's warm.
Warning Signs to Watch For
Lead acid batteries rarely fail without some warning. Here's what to look for:
- Battery feels warm or hot to the touch — especially when not actively charging
- Smell of rotten eggs — hydrogen sulphide, a direct sign the battery is gassing
- CO alarm activating — particularly at night when the battery has been on charge
- Battery not holding charge — rapid discharge or needing charging more frequently than before
- Swollen or distorted battery case — a sign of internal pressure. Isolate and remove carefully; do not disconnect while it is still gassing
- Visible corrosion around terminals — a sign of past gassing events
If you notice any of these, stop charging the battery, ventilate the vehicle thoroughly, and don't reconnect until the battery has been properly assessed.
The Safer Alternative: Lithium LiFePO4 Batteries
Lithium Iron Phosphate (LiFePO4) batteries — the type we fit at Keltie Campers — do not produce hydrogen or hydrogen sulphide. They don't gas during charging. They don't corrode terminals. They don't rupture under normal failure conditions. They operate safely across a much wider temperature range, and every quality unit includes an integrated Battery Management System (BMS) that continuously monitors voltage, temperature, and current — cutting off automatically if anything goes outside safe parameters.
They also last significantly longer. Typically 8 to 10 years versus 2 to 4 for lead acid. And they offer usable capacity right down to 20% charge, versus lead acid's effective 50%. In practical terms: a 100Ah lithium battery gives you around 80Ah of usable power. A 100Ah lead acid gives you roughly 50Ah.
We fit Fogstar and Victron lithium systems and have done so for customers across Scotland. The difference in peace of mind — particularly for people sleeping in their vehicles — is considerable.
What To Do Right Now
If you have a lead acid leisure battery in your campervan, motorhome, or caravan:
- Check how old it is. Most lead acid leisure batteries have a realistic service life of 3–4 years. If yours is older, have it tested or replaced proactively.
- Check where it's located. If it's in a poorly ventilated space or near a heat source, this significantly increases the risk.
- Make sure your CO alarm is working — and if it goes off, take it seriously.
- Never disconnect a battery that is fizzing or gassing. A spark at the terminal while hydrogen is present can cause ignition. Ventilate first, let it cool, then disconnect carefully.
- Have your charging system checked. A faulty mains charger or solar regulator overcharging the battery is one of the most common causes of failure. A new battery alone won't solve the problem if the charger is at fault.
If you're unsure about the condition of your battery or charging system, we're happy to take a look. We're based in Callander, Perthshire and carry out battery checks and lithium upgrades for customers across Scotland.
Get in touch here or call us to book a workshop visit.
Keltie Campers Ltd — Callander, Perthshire. Scotland's Victron Approved Installer. Specialists in lithium battery upgrades, solar installations, electrical repairs and habitation inspections for campervans, motorhomes and caravans across Scotland.
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