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Old 02-23-2018, 07:56 AM   #1
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Follow my wiring crazy - inverter and solar install thought

I've hit this topic a few times and with a pending 3 week trip to the Bahamas this summer I might finally act on it.

Right now I have 4 Optima AGM batteries that are hooked up in parallel. These are starting and house batteries combined. Each battery has about 66ah so in theory I have 264ah available. If you go by the 50% theory, I really only have 132ah available but I have a generator on a separate battery so I would think I could go to 40% with no issues and have 158ah available.

My thought is to replace my Promariner 60amp charger with a new charger/inverter like the ProMariner 02412. I could also add solar panels to my hard top which would supplement the charging.

If I did my math correctly, I should use about 50 amps per day estimating the following draws -

Fridge - 14 amps per day
Freezer - 14 amps per day
Misc outlets for TV, Satellite, etc... - 15 amps
AM/FM radio - 3 amps per day

In theory that gives me about 3 days before firing up the generator. If I add two 100 watt solar panel into the mix, then using a 400 watt estimate of "hours of power" per day calculation I should be able to create 800 watts or about 66 amp hours per day.

Does this make sense?
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Old 02-23-2018, 08:21 AM   #2
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You daily amp usage estimate seems low. What freezer and refrigerator do you have? 28 amps/24 hours for those would be pretty remarkable.
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Old 02-23-2018, 09:09 AM   #3
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I have an inefficient Tundra frig and a super efficient freezer. I canít get below about 90ah for the frig on my best day. Iím not sure on the freezer since Iím always focused on the frig, but I could probably get away with 25ah for it on days where it was just keeping things frozen. Itís about 3amps max per hour when running. Get an amp hour counter so you too can ruin your life watching each amp go by and searching for hours for that extra ghosting load hiding behind the bed because somebody did not turn their power strip off. No really.
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Old 02-23-2018, 09:30 AM   #4
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The fridge and freezer are separate and I think run at about 5 amps each. Not really knowing what the cycle time is, I figured about 3 hours total cycle time for each 24 hour period.

Most of my boat is set up for AC power so I have to deal with that. If I turn off the A/C's then I can save a bunch.

Here is my math -

Right now with everything running to include 3 air conditioning units for a weekend on shore power I am burning about 55kw/ 24 hours. That changes to 55000 watts. 55000/12v = 4583 amps. 4583/24 = 191ah with everything running.

When we are gone from the boat during the week, the A/C is in humidity mode and the fridge and freezer run. During that time, I average .44kw/ 24 hours. That's 440 watts. 440w/12v = 37ah.

If I add in misc. stuff then I average 50?
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Old 02-23-2018, 10:06 AM   #5
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I don't understand your last post and how it relates to DC usage unless you think you can run 3 A/Cs from an inverter. Forget about that!!

Your DC estimate for your fridge and freezer is too low. Three hours duty cycle time in 24 hours is unrealistic. In a moderate climate- 80 degree highs I ran a 50% duty cycle to power a combination fridge/freezer that used 4 AH when it ran.

If you run the same duty cycle for both units and each draws 5A when running then you will require 120AH in 24 hours. You might do better. When I am not onboard, opening and closing and putting new groceries in that need to cool down, my AH draw dropped in half, ie the duty cycle went to 25%.

Your other loads seem reasonable. So even if you get to 25% on average which is doubtful then you will probably use about 80 AH in 24 hours, but possibly 140 AHs.

All this means is that you will be running your generator more than you thought. A bigger charger will reduce genset running time, but that Promariner 02412 is only rated at 50A. You can only use up to 100A in any case as AGMs should be limited to AH/3.

So, I would expect you would need to run the generator every day for at least two hours. A solar panel would be good to push the state of charge up to 100% in your batteries after reaching 85-90% with the generator/charger.

Here is a rule of thumb for solar panels: Divide the wattage by 3 and that is how many amphours flat mounted panels will generate in a sunny day in a reasonable climate/latitude. Your two 100 watt panels then might hit 70 amps on a good day which might come close to keeping you in balance but certainly would cut back genset running time to every other day.

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Old 02-23-2018, 10:35 AM   #6
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The fridge and freezer are separate and I think run at about 5 amps each. Not really knowing what the cycle time is, I figured about 3 hours total cycle time for each 24 hour period.

If I add in misc. stuff then I average 50?
Your estimates are wishful thinking! If you're in the Bahamas in hot weather, your fridge is likely to run WAAY more than 3 out of 24. More like 18, and if it's a Vitrifrigo, more like 24 . Put an LED on the fridge that lights when it's running, see if it's ever dark!

Without an amp counter, you're guessing, and from past experience I can assure you, if you put an amp counter online, you'll be floored. We tend to guess where we'd like to see the values rather than what they actually are tallying up to!

The real problem is in recharging those batteries. Conventional thinking says that if you have a 500 AH bank and draw it down to 50% SOC that's 250 AH. So, with a 50A charger, I need to replace 250AH, that's 5 hrs of charging, right?? NOT!! To bring those batteries up to 100% SOC requires bringing them up to about 85-90% via bulk/absorb, then the remaining 10-15% in float. The problem is, the float takes a LONG time by comparison, so your recharge cycle is exponentially longer than the discharge. It's not linear. Add to that, if you only recharge thru absorb to float and then shut down charging when SOC reaches say, 90%, your bank capacity is reduced by a factor that is not equal to the SOC percentage, but more, and each successive recharge that does not bring the bank to 100% compounds the reduction in capacity.

You need to factor that into your amp hour calculations, because when you disconnect the shore power, you'll have 500AH available; the 1st time you recharge with a genset and stop at 90% SOC, you'll have 425 AH, the next time, 400, and so on- just as an example. This is where solar has great benefit as it will float the batteries to 100% and restore the full capacity over a long period. Additionally, chronic undercharging will sulfate the batteries, and you can permanently lose a significant percentage of capacity or kill the bank prematurely. Keeping the SOC above 60% will extend the life of the bank, and although you may get away with discharging down to 40% SOC, your bank will suffer. The deeper the discharge, the shorter the life of the bank. Check the mfr specs, you'll see that battery life is measured in cycles vs DOD - depth of discharge. They are inversely proportional, and again, not linear. e.g. the higher the DOD, the fewer cycles the battery will yield over its lifetime.

The takeaway should be that the amp hour load is one data point. The SYSTEM must be considered - every piece has to work together to function properly, so see the big picture.
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Old 02-23-2018, 12:11 PM   #7
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Your estimates are wishful thinking! .
Agree....I have very simple systems and requirements. On a cool day I'll burn 90 amp hours to run fridge, lights, stereo, TV etc. over 24 hours. On a hot day that jumps to 125 amp hours due to more Fridge cycling. Your battery bank seems small for serious cruising... I have 440 am/hr capacity so 2 days with no charging is my baseline..
I have a Victron gauge so this is real data..
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Old 02-23-2018, 12:45 PM   #8
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Maerin is exactly right. You need to charge up that last 10-15% which takes a long time and if you don't your batteries won't last. In my several years of full time cruising there really were only two ways to do this:

You could discharge to 40-50% and recharge to 85% for a couple of days and then pull up the hook, start the propulsion engine and let that engine's alternator charge up the batteries on the way to the next anchorage. If you have long enough to go, say 4-6 hours then they will be fully charged when you get there.

The other way is to have a couple of hundred amp hours of solar. Charge the batteries up to 85% in the morning with your genset and then let the solar panels bring them up to 100% during the day. Repeat, wash, rinse, repeat.

The OP's battery capacity is marginal and his shore power charger is as well. He needs to put a couple of more Optimas in, upgrade his charger to 100A and install 200 watts of solar. Even better would be to dump the Optimas and use that space for GC batteries. He can get 440 AH with four GCs for less than the cost of two more Optimas.

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Old 02-23-2018, 12:49 PM   #9
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So help me with my math. I must have missed something. I certainly don't plan on running the air conditioners on the inverter.

Here is my math again, and I see some changes already...

I take pictures of my power meter when I leave the boat and when I arrive to keep track of how much the marina is off when the bill me ( a whole separate story there...)

I leave the fridge, freezer, charger, 2 A/C powered fans, and all three air conditioners in humidity mode.

On 2/11 at 1632 my meter read 31400. When I came back and read it again on 2/17 at 0815 the meter read 31474.

For simple math, that's 74kw (74000 watts) burned over just more than 6 days. Total time measured = 164 hours (this also includes one night on the boat when we had everything running to include all 3 A/C's, the stove and the hot water heater, but I'll use that as an extreme measurement.)

74000/164 =451 watts per hour. 451w/110v = 4.1 amps per hour. 4.1 amps x 24hours = 98 amps per day.

When I am sitting plugged in and everything is running, according to the meter I burn 55kw every 24 hours. 55000/24 = 2292 watts per hour. 2292w/110 =20.8 amps per hour. 20.8 amps x 24 hours = 500 amps (this is everything running to include 3 air conditioners.

I'm not looking to use this for cruising since we are running out generator 99% of the time when we travel.

Right now 1kw is metered at $0.65 in the Bahamas. If I am using 55kw every 24 hours that is an extra $35.75 a day that I am spending. On our planned trip that is an additional $650 that I will spend.

I don't want to pole vault over mouse turds but if I can cut that cost and "trade" the expense of the equipment for it then it's a win.
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Old 02-23-2018, 12:56 PM   #10
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Why obsess over this. With a good sized inverter charger (100 or so charging amps) just run your genset to make up for any excess usage above one's dreams? Does Promariner offer a unit that can put out 100 amps of charging?

We commonly draw down the batteries by over 150 amps overnight. Especially when watching movies, running the diesel heater and using the microwave.

Then an hour or so of genset run time in the AM, cruising for 6 hours or so with genset off and back to square one. And no solar panels. Solar panels would help a lot if you are at anchor frequently as compared to cruising.
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Old 02-23-2018, 01:00 PM   #11
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Investing in off-grid energy systems is only cost effective for off-grid use, no matter how much they charge for mains. Appliances designed for mains power need to be replaced to be viable running off batteries.

Amps per any time period makes no sense, AH per 24 hours is what is relevant.

10A at 120V mains is 100+A at 12VDC.

Your current storage is not enough for a single little DC freezer unless lots of bright solar every day.

Any aircon requires a big genny running.
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Old 02-23-2018, 01:39 PM   #12
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So help me with my math. I must have missed something.

Right now 1kw is metered at $0.65 in the Bahamas. If I am using 55kw every 24 hours that is an extra $35.75 a day that I am spending. On our planned trip that is an additional $650 that I will spend.

I don't want to pole vault over mouse turds but if I can cut that cost and "trade" the expense of the equipment for it then it's a win.
I'm confused. I don't understand the point in calculating your kW usage in context with 12V battery capacity. If the objective here is to use the 12V fridge/freezer on 12V to cut back on shore power consumption, you're robbing peter to pay paul. The original post led me to believe you were discussing an off-grid situation and running on the battery bank, but your calculations are including general consumption of kW hrs from shore power. If your Bahamas stay is primarily in marinas, your 12V consumption is moot.

What is it you're trying to achieve?
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Old 02-23-2018, 01:49 PM   #13
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Yeah I'm confused also. I started to work out all the math and then I realized you were comparing apples and oranges.

If you're saying you're using 74000/164 =451 watts per hour, that would equal 451/12 = 37.58 amp hours PER HOUR if you want to get that from 12V. One would need a very large battery bank to be able to sustain that for even 12 hours.

I think you'll find that ONE fridge can easily consume 50-75AH per 24hours. 2 100watt solar panels could give you about 50-60 AH per DAY under reasonable conditions.

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Old 02-23-2018, 02:11 PM   #14
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I was using the kw reference since it was a known source of electrical usage.
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Old 02-23-2018, 02:27 PM   #15
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Wow! I am going to leave this thread, never to return, but one last comment:

You are confusing AC usage and amperage with DC usage and amperage.

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Old 02-23-2018, 02:52 PM   #16
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Wow! I am going to leave this thread, never to return, but one last comment:

You are confusing AC usage and amperage with DC usage and amperage.

David
You know, itís a shame that rather than helping somebody to understand where they are missing a vital link that you choose sarcasm.

Since David has decided that this is no longer worth his time, can anybody help me understand the difference between AC and DC amp draw? It was my understanding that it is still a measure of power used.

You donít know what you donít know unless you ask.

Is there a formula that I could use to convert AC to DC amperage?
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Old 02-23-2018, 03:06 PM   #17
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What is it you're trying to achieve?
My thoughts were if I could convert some of my AC usage via an inverter and solar panels then I would be able to reduce the cost of my stay at a marina slightly while adding a piece of equipment to my boat that could be useful later on down the road.
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Old 02-23-2018, 03:32 PM   #18
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OK, since you asked and I will apologize for my snarkiness, here is a bit of electricity 101:

The basic relationships are:

Watts are a measure of the rate of production of energy. Watts are volts times amps. AC shore power is 110-120v and DC battery power is 12V. So 100 watts AC is a little less than 1 amp and 100 watts DC is bit more than 8 amps.

Watt hours are a measure of total energy.

You can convert DC to AC using an inverter. But it takes about 10 amps DC (allowing for efficiency losses) to make 1 amp of AC.

With that electricity background maybe we can have a conversation. You said that power is expensive in the Bahamas and you run your generator to make your own. None of this has anything to do with your DC needs.

Let's assume that the incremental costs (ignoring capital costs and depreciation) of running your generator are maintenance and fuel. I will bet that the incremental costs are at least $.50 per KW. Fuel alone is probably more than half of that. Small generators are very inefficient and fuel costs in the Bahamas are high.

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Old 02-23-2018, 03:39 PM   #19
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Thank you David for the additional clarification. I didn't realize that there was such a significant conversion with regard to amperage.

With regard to my generator, I was meaning that for cruising purposes, the generator is always running. It would not be running at the dock.

My boat is almost 100% AC powered so anything that I do via DC power has to go through an inverter. Perhaps this is where I am missing the connection (bad pun...)
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Old 02-23-2018, 07:02 PM   #20
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Follow my wiring crazy - inverter and solar install thought

Iím do most of my boating in northern BC and I figure on a duty cycle of about 75% for each of my fridge and freezer. At 10 amps combined that works out to 180amp hours. We figure on 200-250amp hours per day, both estimated and measured. 80-90% of that is refrigeration. On a good day we can ďfully rechargeĒ our house bank from 435 watts of panels. Thatís almost because it doesnít take into account a longer more optimal absorb charge duration. The meter says itís at 100% SOC but it probably isnít fully charged.
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