A "Current" Affair
The first indication of trouble we saw was smoke on the horizon. Closer investigation revealed a boat on fire, with one crewman sitting on the bow as flames engulfed the boat. As we approached, it became clear that the boat was beyond salvation. After we picked up the crewman, he told us the boat had old wire-and-pulley steering, and one of the wires had broken and fallen across the exposed battery terminals. Sparks flew.
Batteries can be a serious hazard, so if it is time for a refit of your boat, then it is time to take a close look at the batteries. On modern boats, nothing works without the batteries. Engines will not start or run, nor will electronics systems for navigation and communication.
You can tell that the batteries need replacing if engines are reluctant to turn over, particularly on a cold day, or if the boat runs out of power early on when moored for the night. And if you’re replacing the batteries, then it might be time to switch to the latest lithium-ion version. They cost more than lead-acid batteries but should have a longer life (seven years plus) and better performance.
On a modern boat, there are likely three banks of batteries. The first starts the engine, the second supplies most of the domestic needs, and the third is likely dedicated to the generator. (A fourth may be forward to power the bow thruster.) There is a lot of weight in batteries, so it is common to find them aft, stowed low in the engine compartment. That is good for stability but can be bad if the boat is flooding. The batteries can become one of the first things to go underwater, just when you need power for the VHF radio and bilge pumps.
So, the batteries need to be installed well clear of the bilges, and clear of hatches and other openings in case you have to open the engine compartment at sea or when it is raining.
If you plan to relocate your batteries, then use the shortest possible heavy-duty cables between them and the engine’s starter motor. You want to avoid any voltage drop between the two ends of the cable, to reduce starting problems and the risk of chafing that could cause a short circuit.
No matter whether you are moving the batteries or leaving them in the same place during a refit, they need to be secure. A lot of power can be stored in a battery, and a short circuit can be devastating. The batteries should be firmly tied down with the terminals protected. The clamping system needs to bolt the batteries in place and stop all movement. A clamp can be fitted across the top, with the lower ends fixed to a strong point. The clamping system needs to be strong enough to hold the battery in place even against violent boat motion.
Apart from possible damage to the batteries themselves, any movement can strain the connecting wires that carry the high current loads of the engine starter. If the batteries do move, even slightly, then the strands of the cable can break, or the terminals can become disconnected. Either problem is likely to increase the load on the remaining strands. The wire link can heat up and, in a worst-case scenario, sparks can lead to fire. And because the cables are mainly out of sight where they connect to the batteries, you may not discover the problem until you see the smoke.
As for connections to the battery, not only do these need to be secure, but they also need to be protected, to prevent anything metal from touching them and causing a short circuit.
Also check out the battery terminals, where the main wires connect the batteries to the electrical system. The terminals should be corrosion-free and tightly fitted onto the battery connections. It is normal to grease these terminals to reduce the chance of corrosion, but with the trend toward low- or zero-maintenance batteries, they may not receive the attention they deserve. The terminal connection may be clamps or screws, and should be tight with the heavy-duty connecting wires having only a short, unsupported span. The rest of the heavy-duty cable run should be equally well secured to prevent movement and chafe. This setup can be particularly important on a planing boat, where there can be considerable pounding in waves.
On smaller boats and tenders, the batteries may be in a plastic box where they are not a tight fit. Even wedging them into the box may not be good enough because wedges can shake loose.
There should be a battery isolation switch mounted close to the batteries, allowing the main power supply from the batteries to be switched off when the boat is not in use. This shutdown reduces the risk of fire and prevents stray current from leaking power and causing electrolytic action on the hull fittings and stern gear. There may be a connection from the battery that bypasses this isolating switch: the feed to the electric bilge pump and, possibly, an alarm system (they would stay on when the boat is not in use).
I once did a survey on a fishing boat and pointed out to the owner that the battery was not secured. I met up with him sometime later and asked how the boat was doing. He said it caught fire and was a total loss. The loose battery was the cause.
The moral of this story is: Don’t take chances with batteries. They are vital to the boat, and they demand your close attention during a refit.