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Electricity and Power

“Here’s your drink Krabby” said a familiar voice.


“Thanks” I replied, “You had an electrical question for me, didn't you?”


“Yes I did. I am sure you noticed, I made it a double because this could take a bit of explaining. Electricity is uncharted waters for me and I am having a little trouble understanding the electrical systems on the boat I recently purchased. Last year you wrote an article about an electrical system that someone wanted to run Christmas lights with and I think I understood what was going on - but my system seems more complicated. There is a 20KW generator, a 3000 watt inverter with a 100 amp 24 volt charger and a 100 amp 24 volt alternator on the main engine. This is all hooked up to a 800 amp 24 volt battery bank. Everything seems to work okay, especially when the boat is at the dock and plugged into shore power, but when the boat is underway or at anchor there are some things that run on the generator and others on the inverter."


“My questions are, why do some things run on shore power or the generator only and others on the inverter? How does the battery bank fit into the system and when do the batteries get charged and for how long?” Queried my electrically challenged friend.


“Great questions. You know more about electricity than you think. You have all the information about the equipment and its capacity. Sounds to me like you could use a little tutorial on the fundamentals of your marine electrical system. It appears that you have a well engineered system that allows you to manage and store electrical energy when the boat is away from shore power. The answer to your first question is that the equipment on your boat is divided into two groups depending on load and duty cycle. For example: the interior lights are low load and continuous duty, so they run off the inverter/battery bank; the hot water heater is high load and a high duty cycle, so it can only run off the generator or shore power. This is because the inverter might be able to handle the load of just the hot water heater, but the amount of energy it would use would deplete the battery bank too quickly. The lights, on the other hand, could run for hours, the same for the refrigerator, and (one of my favorites) the ice maker. By the way, let's check and see how that component is working, my drink is at low tide.


There are some high-load pieces of equipment that run off the inverter because they have a low duty cycle, (like a microwave for heating up a cup of water for a quick hot toddy). The beauty of this system is that you can use the inverter to run light loads, by consuming stored energy from the batteries on demand, without having to start the generator. When you need the high load equipment such as the stove, water maker, water heater, etc. you run the generator and then the inverter changes to charging mode and recharges the batteries. The amount of time needed to charge batteries depends on how much energy you used and how much energy the charger is producing to recharge.


You have a 100 amp charger and 800 amp battery bank. In a perfect world, that -by the way- only happens on a boat if it is raining hundred dollar bills, it would take 16 hours to completely recharge the batteries if they were fully discharged. It is 16 hours (not eight) because batteries (in that same perfect world) absorb about 50% of the charge. It is not a good idea to run the batteries down all the way, but to recharge when you are running the generator for your high load equipment like the water maker, stove or hot water heater. If you run the generator for two to three hours in the morning and evening, you should be able to replenish the batteries when you are at anchor. Of course, some times you use more power then others, so the amount of charge time varies. You have a charging monitor that tells you when the batteries are fully charged, and you have an idea from that same monitor how much of the battery capacity you have used - so with a little practice you can get the hang of just how long the generator needs to run. When you are underway the main engine alternator will charge the batteries, but you will still need the generator for the high load equipment.


Now, how's that ice maker working? I’ve worked up enough thirst for another drink."

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