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Old 20-01-2007, 21:45   #421
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Mike,

The 2 battery chargers you are refering to do not run off of 110V AC or a generator, but are a special chargers that converts 72V DC to 12V DC, thus your extra genset would need enough wattage to power one of the 8640watt battery chargers (you get 2 if you order the 21.5Kw genset). I'm not sure how big you would need to power a 8640W charger, but it seems like you might need at least a 8640W genset.

Above I mentioned that the 72V to 12V devices where battery chargers. As this was my idea in the first place and Lagoon "borrowed" it, I hope they are battery chargers and not just 2 DC to DC converters, as they would not be as good at keeping your batteries properly charged. In fact this a topic that needs to be discussed more, as a true battery charger charges batteries at 3 different rates; heavy at first but from 85% on to 100% it start to back off until at the end it is just trickle charging your batteries. Also, a true 3 stage battery charger is need to prevent crusting of the electrodes inside the batteries, which will greatly reduce its life. Thus in Steves (ESS105) figures we can't say that a 25 Amp draw for 1 hour will be recharged by a 25 watt charger in 1 hour, because of the 3 different charging rates. It will probably take more like 2 hours to bring the battery back to 100%. So in my pond analogy the flow into the pond is not really constant but varies from a max. of 6 gal./min all the way down to a trickle.

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Old 21-01-2007, 11:19   #422
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I think maybe it time to really look at the 420's energy consumption. Below I have listed the items that will be on my boat and the approximate wattage.

But first lets review the science: Watts = Volts X Amps

So: 120 watts = 12 Volts X 10 amps or 120 volts X 1 amp= 120watts
Thus you can see that a 10:1 ratio when switching between the 2 voltages.

Moving into the real life your, that would mean that a 1200 watt hair drier running on 120V AC would be rated at 10 amps, because of the above formula: 1200watts = 120 volts X 10 Amps

With this in mind I am listing all the energy consuming items below in 2 catagories; DC draw, & AC draw.

DC appliances: (12 volt)

2- Waeco Refridgerators.........48watts each or 4 amps each

1- Waeco Freezer..................48watts ............4 amps

40- Halogen lites ...............12 watts each(?)......1 amp each

1- Anchor windlass.............1000 watts ............83 amps

1- VDO Radio/CD stereo........200 watts ............17 amps

6- fans ........................6 watts each(?)...... 1/2 amp each

I will leave out nav stuff, and running lights, as I think your hours of peak energy use will come in the evening, just after dark, and not while you are out sailing. On to the AC appliances (120Volt):

1- water heater..............2000 watts..............17amps

1- micro.........................1200 watts .............10 amps

?-hair driers..............1500 watts(varies).... 12.5 amps each

1- air conditioner.............840 watts .............7 amps

1- blender ......................500 watts ............4 amps

You can add or subtract from my list to fit your onw needs. Let me know if I have left out anything that most boat would have.

In my next post I will try to see how much energy might be used in the high drain hour just before dinner. Does anybody have a guess as to how long the water heater (2000 watt) takes to recover from a 3-4 min shower.
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Old 21-01-2007, 11:34   #423
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After making the last post, the fridge figures didn't look right, so I went to the Waeco USA site, and rechecked to wattage. It worded as "average power
consumption", which worries me. Average of what? When its running the amps will be fairly constant after startup, and there is nothing to average, unless they are using some type of time frame with average use, but this skews to actual amps while running way down. Anybody have better figures to use for these Waeco appliances?
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Old 21-01-2007, 13:26   #424
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Fridges don't run constantly. They tend to run when the temp exceeds a certain threshold until they get cold enough to shut down. Therefore there will be peak draw which will be higher than your average. The average numbers for the Waeco may have some assumptions built in (like how often the door is open and what the outside temp is). In your dinner example, I guess you'll assume someone is making dinner and hence opening the fridge door, causing the motor to run.
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Old 21-01-2007, 13:51   #425
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Quote:
Originally Posted by Quiet Riot 420
Does anybody have a guess as to how long the water heater (2000 watt) takes to recover from a 3-4 min shower.
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This is taking me back so I hope my figures are ok. The specific heat capacity for water is 4.2 Joules per gramme per degree Kelvin (C). As the volume of 1kg of water is 1 liter, you can see it takes 4.2 kJ to raise 1 liter of water by 1 degree C.
Let's say the outside temp is 25C and you're heating water to 50C and you use 10 liters for a shower, than means you'll need 10*(50-25)*4.2 = 1050kJ to reheat the cooler water replacing the spent water. This would take a 2kW heater 525 seconds to heat - thats just under 9 mins.
I'm not sure though if 10 liters is reasonable for a shower.
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Old 21-01-2007, 13:56   #426
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Just found a website that stated a frugal shower uses about 1 gallon a minute. Let's say the showers on the L420 are not quite frugal and use an extra 50% - that's about 6 liters a minute. You'll need to double my previous calcs as your 3-4 min shower will use abou 20 liters
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Old 21-01-2007, 14:23   #427
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Low-flow shower heads use about 2 gallons (8 - 9 litres) of water per minute,compared to between 4 5 gpm used by conventional heads.
By restricting the flow and forcing the water through very small apertures, the low-flow showerheads aerate and increase the velocity of the water, creating a very fine but "wet" feeling spray pattern.



Hence, the 3-4 min. shower might be expected to use about 24 to 36 litres of water (not 20 l).
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Old 21-01-2007, 15:09   #428
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Steve,

I had come to the same conclusion as you, at the dinner hour the fredges and freezer will be opened repeatedly for such things a bevs, ice, snacks, and of course the cooks themselves opening for whatever. From these assumptions I would say that the refridges might be running for the whole hour and then some just to keep up with the 80 degree temps running in at each opening. I will assume the freezer might only come on once and run for say 20 min. Now all I have to do is find the true running amperage of the refridgerators, but I assume it will be similar to my 7 amp air conditioner draw, but at 12V.

If Gord & you are right and we assume a 30 litre (3-4 min) shower, this would tend to point to about a 1/2 recovery time per shower.

Should we post a sigh that says military showers only, use water for first wetting and final rinse only!

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Old 21-01-2007, 15:14   #429
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There's the other form of military shower - cold water only.
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Old 21-01-2007, 15:17   #430
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Quote:
Originally Posted by GordMay
Hence, the 3-4 min. shower might be expected to use about 24 to 36 litres of water (not 20 l).
Thx. What I neglected to factor in is that a hot/cold mix is likely to be used so this would lower the amount of water taken from the hot water cylinder.

Quiet Riot. In this thought experiment - remember the hot water can be heated well outside your dinner time rush hour. I presume the tank has pretty good insulation.
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Old 21-01-2007, 15:23   #431
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I think that the refrigerators and freezer are probably actually around 600 watts (that's 240 watts less than my 9000 BTU air conditioner), thus while running they would draw about 50 amps each as they run off of 12V.......600watts=12V X 50amps

What do you think?
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Old 21-01-2007, 15:30   #432
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They are all based on a motor driving a heat pump so there's no suprise if they're in the same ballpark. The only thing to remember is the fridge has a lot less air to cool than the aircon so it won't run for nearly as long.

How often do you foresee being in this scenario and how bad would it be to run the genset while all this is going on? Sooner or later you're going to have to replace the consumed Wh (I'm no fan of Ah) and unless you're going to be doing a lot of sailing, you're going to have to run that genset sometime.
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Old 21-01-2007, 15:32   #433
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I think the hot water heater is only about 10 gal., so you can maybe have 3 showers ready to go, but the heater will still come on and start to use energy before its first 10 gal is used up, thus the happy hour energy drain is on!

Hot and cold mix? Is it 50-50, 60-40, 40-60, any guesses for my to help with my analysis of happy hour consumption. (non-alcoholic)
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Old 21-01-2007, 16:34   #434
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I think it will be personal. In the tropics I would have mostly cold showers - very refreshing. Is the water heater switchable so you can defer the reheat?
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Old 21-01-2007, 19:02   #435
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This is the way the DC current usage could add up at happy hour (just after dark)

2 fridges running for 2/3 of our hour...........................66 amps

1 freezer running for 1/3 of our hour..........................17 amps

of 40 odd lights, lets say 75% are on.........................30 amps

stereo............................................ ......................17 amps

Fans an other misc.............................................. .....3 amps

Total amps DC used in one hour......133 amps

Now lets see how this affects the standard 2 battery system

2- 140 amp-hour batteries hold a total of 280amp-hours, so to take them down 20% would be .20 X 280 = 56 amp-hours used.

If you take them down 40%, that would be twice as much or 112 amp-hours used.

Now lets combine the drain on the batteries with the input from the 2X25 amp chargers or 50 amps/hour at peak efficiency, most chargers are no better than 90%, so lets say we get 45 amps/hour.

So in our happy hour we consumed 133 amps,(this is just DC, and is not counting showers and the like AC draw off your inverter) and replenished 45 amps. This means that our 2 house batteries are down a net 88 amps in 1 hour or 88Ah. This means that at the end of happy hour our batteries are at 31.4% discharged. If happy hour continued for another 1/2 hour the batteries would be down 47.1% and damage is occurring.

It seems fairly clear, that to be safe on a charter boat, that we either add batteries, add battery chargers, or both. The easy fix is add 1 or 2 more house batteries, but that is also the one that add the most weight.

What say ye all!

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