Deal with the urgent part first. If you can smell hot plastic or scorching on board, isolate the electrical supply at the battery switches and disconnect the shore lead, then investigate. Do not keep using the circuit to see whether the smell returns. That smell is insulation or a terminal getting hot enough to break down, and the step after that is fire.
Once everything is isolated and the boat is safe, the useful question is what was overheating and why. The cause is almost always a connection or a conductor being asked to carry more than it can.
Isolate both DC and AC. Turn the battery isolators off and unplug from the pontoon rather than just switching the boat breaker, because a fault upstream of that breaker will still be live. Ventilate the space, and keep an extinguisher to hand while you do it.
If there is visible smoke, discolouration or a component that is too hot to touch, treat it as a fire risk and get off the boat, then call for help. A boat is a confined space with limited exits and a great deal of flammable material, and lithium banks in particular do not give you long to reconsider.
Electrical heat is resistance in the wrong place. On boats, resistance appears where connections have loosened, corroded or been made badly in the first place, and where cable has been asked to do more than it was sized for.
Owners often assume a fault serious enough to smell would have blown something. A high resistance joint does not work that way. It can dissipate a great deal of heat locally while the total current stays comfortably under the fuse rating, so the protection never operates and the terminal quietly cooks.
This is why a fuse that has not blown proves nothing at all, and why a smell must be investigated rather than dismissed because everything still works.
Heat damaged cable and terminals are not repairable by re-tightening. Once insulation has softened or discoloured and once a terminal has annealed, the affected section should be cut out and remade properly with tinned cable, correct crimps and the right protection.
It is also worth understanding why that joint failed, because a single overheating terminal on an otherwise sound boat is unusual. More often it is the first symptom of an installation that has been extended over the years without the supply being uprated to match.
We would say this about very little, but overheating electrics genuinely is not owner repair work. The fault is by definition in a circuit carrying enough energy to cause damage, it may be on the AC side where a mistake is lethal, and the damage is frequently more extensive than the visible scorching suggests.
Have it inspected before the boat is used again. We carry out fault finding and electrical safety inspections along the South Coast, and where the underlying installation is the problem we can plan and stage a rewire rather than patching the one joint that gave itself away.
Most overheating faults trace back to how the installation was built and extended. Correctly sized cable, tinned conductors, proper crimps, terminals that get checked rather than assumed, and protection matched to the cable rather than to the appliance all but eliminate the problem.
A periodic electrical inspection catches high resistance joints before they announce themselves, and on an older boat it is usually the most useful money you can spend on the electrics.
No. The joint that overheated is still there and will do it again, usually under the same load. Nothing has repaired itself in the meantime.
Not until the cause is identified. Leave it disconnected, because an unattended boat with an overheating circuit is the worst version of this problem.
You often cannot by smell alone, which is exactly why both should be isolated. Establishing which side is at fault is part of the diagnosis.
Yes. A lead acid battery being overcharged can smell distinctly unpleasant, and a hot or swollen battery of any chemistry should be treated as urgent and dealt with by a professional.
Leave the boat isolated and get in touch. We inspect overheating faults and put the underlying installation right.