Electronics are wonderful as long as the electrons keep flowing, but they are useless once the power ends — and it always does eventually! The trick on a boat, particularly one in a hot saltwater environment, is to keep those electrons flowing from the boat’s batteries through various connectors, wires, switches, fuses, etc., to the right places and to not get sidetracked or blocked.
There are several unique factors to keep in mind with regard to charting electrics. It is not the same as wiring up a new light over the chart table! First, I have found it is important to isolate the electronics circuits from possible major surges caused by things like engine starting or windlass grinding. I have witnessed various electronic devices turning themselves off or on due to power surges, and low voltage is never good for sensitive electronics. Luckily, most modern electronic devices can handle a wide range of input voltages, including low-voltage situations, quite well.
On my boat I have a small fuse panel that resides right in the battery compartment. It is connected directly to the main large battery bank at the opposite end from where alternator and solar charging juice gets brought into the bank. Batteries act as filters for voltage spikes and a large house bank of batteries is fantastic protection. A very short fused lead connects the small electronics fuse panel directly to the battery bank. This panel is “always on,” meaning I only disconnect it for maintenance purposes. An entirely separate battery is used only for engine starting, which is usually the No. 1 routine action that can cause power spikes. Of course, the two battery banks can be combined using switches if required, but in normal operation they are kept separated with only a trickle charger from the main bank keeping the starter battery topped up. Normal starting only uses a tiny bit of capacity from the starting battery and if you are routinely draining that battery for some reason, your engine and/or starting system needs work.
I have learned through hard experience that it is far better to keep your electronics attached to power than to disconnect them. Many devices have small internal batteries that maintain critical memory, and those internal batteries can be difficult or impossible to replace. The job is also expensive and must be done by the factory in many cases. One issue I have right now is that my VHF radio lost its programmed MMSI number one winter when I had it disconnected, and the only way to have it restored is to send it to the factory — the repair charge would be greater than the radio is worth!
Some will argue that a fuse panel should not be in the battery compartment due to corrosion issues from batteries gassing and that the panel can’t be isolated using the main battery switch. All I can say is that I have used this system for decades on several different boats and have never found a serious corrosion issue. Your batteries should not be gassing that much anyway! If they are, it is time to closely examine your charging system.
Concerning not being able to isolate this panel using the main battery switch, I don’t consider it a serious safety issue. First, there is an inline fuse in the short wire from the batteries to the panel; second, each individual power line is fused in a position that is very close to the battery. Most of the lines also have fuses close to the electronic device. The wires and fuses are very small, so in the event of a catastrophic short somewhere along the line, either the fuse or the wire will burn out very quickly.
The “always on” electronics fuse panel provides other benefits. When there is an electrical emergency in some other part of the boat, you can safely turn off the main battery switch and know that critical navigation and communication devices will continue to work. Think of the situation in a boat fire. You might quite rightly believe that the electrical system is to blame, so you flip off the main switch as the boat fills with smoke and then you try to call the Coast Guard only to find the VHF radio isn’t working. You then grab a hand-held radio and reach someone who can help, but then you can’t tell them your position because the GPS and chartplotter have been knocked out. You want to keep critical electronics running as long as possible in an emergency situation.
Of course, when it comes to most things on board, it is important to have backup systems that don’t depend at all on the main boat electrical system. I don’t go anywhere without a small, hand-held GPS that runs on regular alkaline flashlight batteries — no rechargeables! I want this GPS to always be available no matter what and, in my experience, small rechargeable batteries are not reliable, have much less capacity than advertised and have relatively short lives. Good old alkaline disposable batteries, on the other hand, last for years on board and it is very easy to carry enough to last years. They are available everywhere in the world too. I have a hand-held VHF radio that can take ordinary alkaline batteries for this very reason, along with a hand-held depthfinder and even an old hand-held RDF! Even in today’s world with few navigational radio beacons, there are almost always commercial radio stations or airport radio beacons that can be homed in on when all else fails. In any case, think through how you would navigate if your main batteries were gone for some reason — it will happen!