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Old 03-03-2016, 02:48 PM   #1
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Another electricity question

Sorry guys, need your help again. I've read some books, now trying to practice my understanding of batteries. Could you please verify my calculation below? Nothing is real, purely trying to work out the formula:

If I want to calculate how much battery do I need for overnight staying on a boat, suppose I don't run generator, one "night" I mean 12 hrs(some of the numbers are rounded):
One Refrigerator: 150w x 12hr = 1.8kwh per night(the 150w number I found on line, average)
One Freezer: 100w x 12hr = 1.2kwh per night
Ten Bilge pumps(assume they runs twice an hour, for 5 minutes per time): 100w x 10 x 24 x 5mins / 60mins = 2kwh per night
One Air conditioner(assume it runs continuously): 900w x 12hr = 11kwh

So in total I need: 1.8+1.2+2+11=16kwh

If I buy battery rated 300ah, I need: 16000wh / 12v / 300ah = 4.5
So I need to install at least 5 batteries(again, not counting discharge rate, 50% reserve etc).

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Old 03-03-2016, 03:06 PM   #2
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Yes, with the numbers you have presented, I think your calculations are correct. In practice, you would need double the batteries to keep discharge level to 50%. And you probably already know this, but a bunch of your numbers are much higher that they should be in any real situation.

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Old 03-03-2016, 03:19 PM   #3
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The bilge pump calculation surprises me. Mine almost never come on. Even for a wooden vessel, that's a lot of time on the pumps. Can you provide the details on these pumps? Also, the fridge and freezer won't be running continuously, if they are AC units. Also, I think it would be very taxing on your batteries if you were running an AC unit continuously.
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Old 03-03-2016, 03:30 PM   #4
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Thank you guys! Yeah, the refrigerator/freezer numbers I found on line, for Bilge pump and air conditioner, I really have no idea how long/often they would need to run, I guess it would have to depend on the condition of the boat(which I don't even have one) and weather etc, hence just throwing some random numbers just to get the calculation going.
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Old 03-03-2016, 03:34 PM   #5
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I also agree that the calcs look correct but the numbers are high. If you have ten bilge pumps running a total of 10 mins/hour then I think your boat may be sinking!

I don't think most people consider running an Air Conditioner from batteries - I think that they fire up a genny for that. Mine run on 220V so I have run a genny.

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Old 03-03-2016, 03:48 PM   #6
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The math looks right enough, but...

An AC would be a big draw. Believe that's where most would fire up the genset, charge batteries while at it. Ideally an AC wouldn't run continuously, though. But it also needs a water pump; can't tell if that's included in your theoretical 11kwh for AC.

(Fans instead would be a different but postulate-able draw.)

Our fridges don't run very often. although that's influenced by how many beers I retrieve (how many times we open the doors).

Our bilge pumps (almost) never run. If/when they do, I'm all over that to figure out why.

If you were to leave any electronics components turned on -- AIS transmitter for example, or a GPS/plotter so you can use the anchor alarm, maybe a VHF on monitor -- that'd be a continuous draw.

An incandescent anchor light can be a big draw (LEDs much better for that role).

Any other cabin or deck lighting would be a draw, assuming you stay awake -- and use lighting -- after sunset.

Water usage, fresh water pump would be a draw.

TV? Stereo?

Much of what I'm adding assumes you're actually alive and doing stuff on there... so the boat's just not out there enjoying the evening all by itself.

If it helps, I can tell you I didn't beat myself up compiling an energy budget (although I did eventually get a round tuit, posted it here somewhere). I just improved the quality of batteries in our existing banks (600 Ah in 2 banks) and got on with life. Gradually worked out that we can get by quite easily by running the genset for an hour in the am while cooking breakfast (electric cooktop, toaster, microwave, coffeemaker) and then again for a couple hours in the early evening while we prepare dinner. That keeps us more or less above 50% SOC on each bank all the time (and that's with a fridge on each bank).

AC is a separate deal, for us; we only run that when the genset is running... but we also tend to not need it toooo much when we anchor out... except for sometimes in July and August, and we can often just choose to not anchor when conditions aren't wonderful for that.

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Old 03-03-2016, 04:04 PM   #7
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Just a suggestion, but have you considered chartering a vessel and experiencing actual usage of the boat's systems? It might help you get everything in perspective. Even though boats are all different they do all have some similarities.
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Old 03-03-2016, 04:17 PM   #8
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I did realize the AC would be a huge draw comparing to other appliances. Even if it doesn't run continuously, likely need a gen.

Thanks for the advice! I plan to just read books, post insane questions on line for a couple of years(so pls bear with me for a while , then definitely join a club, then eventually buy my own boat and move off land. Not in a hurry!
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Old 03-03-2016, 04:19 PM   #9
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Your calculations are ok as far as they go, but your assumptions are off the wall:

1. Nameplate running amps on refrigerators assume full time operation. Most home, ie 120V refrigerators operate about 20% of the time. Marine, DC Danfoss compressor types operate 30-50% of the time. So your calculation of refrigerator watthours are several times too high.

2. Same with freezers.

3. If your bilge pumps are running that often and for that long, you need a new boat. A bilge pump should run for maybe 5-10 seconds every 3-4 hours if you have a conventional dripping stuffing box. Otherwise, bilge pumps should never run.

4. 900 watts is a pretty small A/C. The typical marine A/C is 16,000 BTU and takes about 15 amps at 120 V to run or 1,800 watts. A/Cs typically run only half the time or less, particularly at night.

5. I don't know of any 12V, 300 amp hour batteries that are commonly used in marine service. Even a big 8D (which I sure don't want to haul around) is only 220 AHs.

6. You absolutely can't ignore the 50% discharge rule of thumb and you should include inverter efficiency in your calcs as well.

So come up with some realistic duty cycle and wattage values for your expected usage and calculate again. You will probably never be able to run a decent size marine A/C from batteries/inverter. Maybe a small portable one like the carry on type, but it will still take some big batteries to do it all night.

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Old 03-03-2016, 04:33 PM   #10
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FWIW, I size my house bank to permit a discharge to 35%, which is admittedly more damaging than 50%, but only a little more so. Xantrex, which I consider to be an authoritative source, has published that recommendation.

More importantly, if you have 10 bilge pumps each pumping for 10 minutes per hour, you need to deal with that leak (or maybe multiple leaks?), even if your batteries can keep up. I have cycle counters on my 6 bilge pumps. I go months at a time without any cycles (and usually when it does cycle it is because a cleaner has open a laz hatch and sprayed water down there).

From what I have seen, not many people run AC off of battery power.
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Old 03-03-2016, 04:40 PM   #11
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Originally Posted by chicagoq View Post
Sorry guys, need your help again....
Ten Bilge pumps(assume they runs twice an hour, for 5 minutes per time): 100w x 10 x 24 x 5mins / 60mins = 2kwh per night....
10 bilge pumps??!! I`m genuinely concerned for you going boating. Unless of course, we are being played with here.
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Old 03-03-2016, 05:10 PM   #12
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Who on this forum has 10 bilge pumps? Figure one pump a few minutes a night worst case unless your packings are leaking like crazy. Even an hour a night is not much load percent wise.
Your math works but your total load is way high.
I suggest you drop the AC on batteries idea. That's one sure way to prematurely kill your bank. If you want AC away from shore, run the genny.
A typical fridge/freezer found on a 35-40 ft boat draws 5 or 6 amps at about 50% on, 50% off duty cycle.
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Old 03-03-2016, 05:16 PM   #13
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If you don't have a gen I would consider a Honda eu2000, a lot lighter than all those batteries and will run a AC all night. I have one we used on our 32' gas powered boat for running the AC at night, no noise in the boat and used less than a gallon of gas. The boat had a 6.5 Westerbeke we used when cooking and hot water but then we shut it off and fired up the Honda to watch TV, charge batteries and for AC. I've read on forums of people not liking the noise in a anchorage but we always anchor in areas with no other boats.

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Old 03-03-2016, 05:54 PM   #14
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All boats are different, but here's a shot at my numbers as a point of comparison. The items below are items that are typically operated at anchor:

12V: LED interior and exterior lights, stereo, TV, freshwater pump, multiple fans, multiple laptop and device chargers, electric head, 12V Norcold fridge/freezer, 2nd fridge running on 1000W inverter. Two bilge pumps are 12V but they never come on so they're not a factor in my electrical budget. I have lots of minor loads like tank monitors that really don't affect the numbers.

Propane: stove, oven and heat.

110V (Shore power or Honda Generator): 55A shore charger, 110V feed to Norcold Fridge, microwave, electric ceramic heaters.

My house bank is a 660AH bank of 6 golf cart batteries which gives me 330AH to play with. I use 150-180AH per day, so I can get 2 days without recharging if I'm careful. Typically, I run the generator 2-4 hrs per day to keep up, usually in the morning for coffee and evening for shower and dinner prep. If I had solar, I could probably do without the Honda gen.

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Old 03-03-2016, 10:56 PM   #15
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Originally Posted by BruceK View Post
10 bilge pumps??!! I`m genuinely concerned for you going boating. Unless of course, we are being played with here.
I agree. Now I realise you haven't even got a boat yet, I have to say it is hard for anyone to make solid recommendations, but for example re this bilge pump issue. I have an old fashioned stuffing box type stern prop-shaft gland, and it drips slowly, but I have only two electric bilge pumps, (one manual) and only one of those ever runs unless I activate the other manually, and the one that does only runs for say 1 minute about every 3-4 hours at most - less when at anchor or in the berth. My boat is described in the avatar, so that will give you some sort of idea re bilge pumps anyway. Others have already commented on battery and generator issues, but again, even there you might be getting several carts before their horses...just sayin'...

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