Lithium battery on the tender: where to charge it and how to spot a damaged one

Electric tenders, battery-powered outboards, e-foils, shore scooters — there are more of them on yachts every season. Along with them, a traction lithium battery comes aboard, and that's a different class of risk from anything we're used to.

The UK Maritime and Coastguard Agency updated its guidance on fire safety and storage of small electric vehicles in January 2026. Here's the practical part.

Why this isn't an ordinary fire

One property defines everything else: a lithium-ion battery fire can sustain itself without any outside supply of oxygen. That's exactly what the guidance states.

Some unpleasant conclusions follow from this:

• Smothering it and cutting off air won't help. The trick that works on petrol and oil doesn't work here.
• A CO2 extinguisher will knock the flame down but won't stop the process. The reaction keeps going inside, and the battery will catch fire again, sometimes tens of minutes later.
• You need water, and a lot of it. Water cools, and cooling is the only thing that actually stops thermal runaway. A boat doesn't carry that much water.
• The smoke is toxic. This isn't "smoky," this is poisoning.

The conclusion for a yacht is simple: a fire like this isn't something you put out — it's something you prevent. And decide in advance which way the battery goes overboard if it starts.

Signs that mean don't charge it

Australia's maritime authority puts it plainly: a battery that's damaged or warm to the touch must not be put on charge.

Add to that what you can see for yourself:

• swelling of the casing, even slight;
• a dent or an impact — dropping it on the dock counts;
• a smell, sweetish or chemical;
• traces of moisture or salt in the connector;
• heating up with no load;
• the battery discharges faster than usual or refuses to take a full charge.

Any one of these signs means: don't charge it, take it off the boat, hand it to a specialist. Not "I'll try it once more."

Separately: a battery that's been in salt water is always considered damaged, even if it still works. The salt inside keeps doing its work for weeks.

Where to charge it

In descending order of correctness:

1. Ashore, off the vessel. The ideal option, and usually available at a marina.
2. On open deck, away from hatches and escape routes, under supervision.
3. Never in a cabin, a locker, the engine room, or underway unattended.
4. Never at night, while everyone's asleep.

Three more rules that get broken often:

• Only the original charger. A third-party charger of the same rating won't do — the battery has its own protocol.
• Don't leave it on charge once it's full. A socket timer settles this.
• Don't charge it right after heavy use. A hot battery needs to cool down first.

Where to stow it on passage

The spot has to answer one question: how fast can this thing go overboard.

• close to the side, not deep inside the cabin;
• not on or next to an escape route;
• secured so it doesn't slam against a bulkhead in a seaway;
• kept apart from fuel and gas bottles;
• connector protected from spray.

One thought that sobers most people up: if the battery sits in a stern locker under a cushion, you have no way of throwing it overboard in a fire. That's exactly the wrong spot.

Winter storage

The end of the season is when it's worth spending half an hour.

• Inspect for swelling and dents in good light, from every side.
• Leave it at about half charge. Not empty, not full: both full and empty age faster.
• Take it off the vessel. A cool, dry place ashore, not a living room.
• Don't leave it in freezing temperatures. A frozen battery can't be charged until it's warmed back to room temperature.
• Check it once over the winter to make sure the charge hasn't dropped to zero. A deeply discharged lithium battery often can't be recovered.

What to prepare in advance

• A decision on where it goes overboard. Work out the route: from the stowage spot to the side, in gloves, without snagging on anything.
• Heat-resistant gloves near the stowage spot.
• A smoke detector in the compartment where the battery is kept.
• A talk with the crew. Everyone should know that a smell or hissing from that locker means calling for people, not opening the lid facing it.

What to photograph if you need a replacement

Traction batteries aren't interchangeable: each has its own connector, its own communication protocol, and its own dimensions.

• The full battery nameplate — manufacturer, model, voltage, capacity.
• The connector, close up, from both sides.
• The battery in its mount, so the fit and fastening are visible.
• The nameplate of the motor or tender it fits.
• The charger and its nameplate.

With this set, the battery, the charger, and the adapter can all be sourced correctly — and it's immediately clear if the model has been discontinued and a replacement is needed.

In short

A lithium fire sustains itself and can't be doused with the water a yacht carries, so it isn't put out — it's prevented. Swelling, an impact, moisture in the connector, heating with no load — don't charge it. Charge ashore or on open deck under supervision. And stow it so the battery can go overboard in ten seconds. ```