Juice

We're on our 4th day here in Port Washington NY, waiting for a weather window to transit NYC and then go outside around the Jersey Coast. Weather predictions still say it could be a week before that latter part is possible.

So we're making the best of it, and are fairly comfortable tied to a yellow 'transient mooring' ball. It's an easy dinghy ride to the town dock, where there is a park for the dogs, plus downtown restaurants and stores. Or we can dinghy a little farther to the newer dock which is right opposite a large grocery store and a short walk from a mini-mall with a movie multiplex. Yesterday we saw 'Gravity' in 3D, which was cool, and soon we'll probably see 'The Butler'.

It's good that we have easy access to groceries, so we can buy perishables daily, as we only run our fridge for 2 hrs/day or so, which keeps its contents cool but not cold.

Which brings us to today's topic 'Juice', or electricity. Hanging on this mooring, without access to shore power and without running our main diesel engine, presents some minor challenges to maintaining a supply of electricity aboard Spray.

Let's start with a description of Spray's electrical systems. There are two, a 110 VAC system just like in a typical home, and a 12 VDC system as in a typical car. We can transfer power between the 2 systems, with a battery charger to convert AC power to DC power, and an inverter to convert DC to AC.

The 12 VDC system stores energy in a large 4D marine deep cycle battery, which we call the house battery.

This battery sits in the engine room to starboard of the diesel. Another similar battery sits to port of the diesel. It's the starting battery and is used only to crank over the main diesel. The idea is that if you drain your house battery too much using fridge, lights, etc. you still always have energy in your starting battery to start the diesel. Both batteries can be charged by the main diesel (via an alternator that is belt driven from the diesel, same as in a car) or via the battery charger which pulls power from the AC system. Additionally, the house battery can be charged by 3 solar panels I installed 2 years ago on the front of the flying bridge.

Now for some numbers on the DC system. It's typical to describe power flow using amps. Since its a constant 12 volt system you can easily get proper power units of watts by multiplying amps by 12 (P = I x V = amps x volts).

 

For charging the house battery the diesel's alternator can output about 50 amps, but would only do that much if the house battery was almost fully discharged. It outputs less as the battery charges, typically in the 5-30 amp range. The battery charger can supply 10 amps to the house battery, and the solar panels can output about 10 amps also, under ideal conditions.

 

For draining the house battery we have several devices. The primary one is the fridge, which draws 5 amps (and runs at least 50% of the time we have it on). Next is lights, but we've switched most lighting to LED bulbs which reduces that power draw considerably. For instance our 4 ceiling lights in the salon draw a total of only 0.8 amps. Other power draws are short term and intermittent, and usually through the inverter such as when we plug a small vacuum cleaner into the 110 VAC circuit. The vacuum draws a lot of amps, but usually for a short time.

 

Now let's do some energy calculations on the DC side. Energy is power x time and is usually expressed in amp-hours (multiply by 12 to get watt-hours). The house battery is rated to hold 200 amp-hours of energy, but to extend its life I try not to discharge it by more than half that (even less if I can). So if we arrive at an anchorage with the house battery fully charged (meaning the diesel's alternator did its job), we can then run the fridge full bore for 20 hours (5 amps x 20 hours = 100 amp-hours) before I would freak out, so we're good until we start moving the next day. But since we've been doing some short passages of 3-5 hours I've been being conservative by running the fridge all day and evening, but turning it off while we sleep, so it usually draws the battery down by 40-50 amp-hours between passages.

 

Compared to the fridge, lights and other loads are negligible so I don't worry about those. The fridge is my taskmaster.

 

Now to the 110 VAC side. The AC power sources are the inverter (converts energy from the house battery) and 'shore power'. When we are in a marina slip, shore power is exactly as described, with a cable connecting 110 VAC power from a dock fixture to a socket on Spray. Then we have the luxury of running all the gadgets we want, including a small electric space heater, and simultaneously charging the batteries via the battery charger. When we are at anchor or a mooring for an extended period, we have another source of 'shore power' :

This portable Honda generator converts energy from gasoline into nice 110 VAC, at a rate of up to 2,000 watts. A nice feature of this unit is that it contains an inverter which allows it to run slowly and quietly at low power levels, so its only loud when you need a lot of power. The cable you see connects this to the Spray's shore power socket.

 

The AC power can go to several places, such as anything we plug into an outlet (vacuum, laptop charger, power tools), or that space heater, or to Spray's 6 gallon water heater, or to the battery charger.

 

Now that we are at a mooring for an extended period of time, we adapt in 2 ways. First we restrict the fridge operation to 2-3 hours/day, which keeps things cool but not cold. We try not to open the fridge door too often. Next we run the red Honda for an hour or two each day to recharge the house battery (we run the fridge during this time). The house battery is a bit low now (70 amp-hrs below full) but we've been gaining a little each day.

 

For these battery recharges the Honda is only outputting 120 watts or so, so it runs quietly. That is unless we want to make some hot water. When we are moving, the main diesel heats water for us (its coolant passes through a coil in the hot water heater), but that's not an option now. So it's up to the Honda to electrically heat the water, which takes 1500 watts and makes the Honda roar pretty loudly for the hour it takes to heat 6 gallons of water. This morning we started that process before heading to shore to walk the dogs, and it finished just as we returned to the boat. So today is shower day.

 

As you can see, it takes some planning and effort to manage Spray's juice, especially when moored or at anchor for more than a day, but we have the tools we need and are making out fine.

 

Any questions?

 

 

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