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Old 19-12-2012, 19:35   #1
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So, how battery and electricity do I need?

I am trying to learn a little about electrical consumption, demands, and needs. I know very little about it, so that is why I am posting here.

1. First thing is, how do I figure out how many amp hrs would I consume, ballpark figure, so I can figure out how much of a battery bank do I need?

2. Any links to where I can read basic info on batteries, which and how many, chargers, how to charge, how much to charge, how often to charge, to what level discharge, how to figure what size alternator to use based on needs and size of battery bank, etc?

3. Adding solar. How do I figure what size do I need, what type of charger and controller, etc.

Thanks in advance.
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Old 19-12-2012, 20:14   #2
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Re: how to detemine battery and electricity need

First, make a list of all your electrical items, their amps and how long you will used each every day. This will give your daily amp-hr ( a spreadsheet is easiest). Then, depending on how much sun you will have available and how you will recharge, you can determine the house battery bank.

I bought about 370 am-hr used gel cell batteries, added two 185 watt solar panels and the cheap solar controller and we were usually charged by early afternoon if we had sun. Worst case was several cloudy/rainy days when we got down to 45% on the batts (suggest a power monitor like the Link Lite) We have a portable fridge/freezer and NO charging from the OB, only from the solar. We operate mainly off LED lights and stopped being conservative after the first month in the Bahamas. As it got warmer and higher fridge load, the sun was higher and the batts were charged earlier. I am sure others will provide other suggestions.
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Old 19-12-2012, 20:22   #3
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Re: So, how battery and electricity do I need?

Is there a rule of thumb for battery bank capacity selection? Something like "you want your amp hr to be 30 or 50% of the bank's capacity. I just pulled those numbers out of thin air BTW.
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Old 19-12-2012, 20:35   #4
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Re: So, how battery and electricity do I need?

Suggest you take a look at Nigel Calders electrical and mechanical manual

AND search this forum, there is plenty of stuff here about batteries, charging, solar, regulators etc.
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Old 19-12-2012, 20:37   #5
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Re: So, how battery and electricity do I need?

I'm no expert, but I think you want as many amps available to start with as the less % of your bank that you draw down the better. In other words, if you draw you batteries down to a 50% state of charge every day they will have a much shorter life than if you only drawn them down to say 70 or 80% and then fully recharge.
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Old 19-12-2012, 23:56   #6
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Re: So, how battery and electricity do I need?

I believe the answer is that you will find ways to consume all that you can generate. Whatever size you install, that is how much you'll need + a little more at the end of each day.

Though cheaper than in the past, solar equipment is still expensive and generally is only considered by people who desire access to free, continuous electricity. If that describes you, the answer is as much as you can manage to install without breaking the bank or hideously disfiguring your boat.
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Old 20-12-2012, 02:05   #7
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Re: So, how battery and electricity do I need?

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Originally Posted by ArtM View Post
I believe the answer is that you will find ways to consume all that you can generate. Whatever size you install, that is how much you'll need + a little more at the end of each day.

Though cheaper than in the past, solar equipment is still expensive and generally is only considered by people who desire access to free, continuous electricity. If that describes you, the answer is as much as you can manage to install without breaking the bank or hideously disfiguring your boat.
Solar is considered by 90% of cruisers, probably, and the problem is not generally cost -- it's cheaper than generators. The problem is where to put the panels.
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Old 20-12-2012, 02:23   #8
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Re: So, how battery and electricity do I need?

I personally found the book "Energy Unlimited" written by Reinout Vader very informative. A big plus of this book is that it's free.
Order the book
(Reinout Vader is the managing director of Victron, a brand that will be familiar to many here)
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Old 20-12-2012, 02:38   #9
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Re: So, how battery and electricity do I need?

As others have said, make a budget. It's not a science at all, but a rough guess will be very useful.

Then, you just have to work it out from experience.

One very important thing to remember is that lead-acid batteries start losing their real effective capacity the first day. So don't just add up the nominal A/H capacities of the batteries you intend to buy and assume you will be able to use those many amps over the whole life of them.

Remember also that the real practical capacity of batteries -- if you are on the hook for long periods of time without motoring or overnight charging off shorepower -- may really be only between 50% and 80% of their theoretical capacity, since it is not worth running the main engine and often not even a generator to get the batts from 80% to 100%, where the absorbtion rate falls way off.

Solar is really great because it is relentless and often will get the batts over 80% and keep them there when long off shore power. My dad doesn't have a genset on his boat but has a modest solar installation. He uses the main engine to bring the batts up when they are deeply discharged, then just lets the solar do its thing. It works great (much better than electrical life on my boat without solar).

So if you think you are going to use 100 A/H of power per day, and you want to be able to live for a day without any kind of charging, then you would probably like to have nominal battery capacity of no less than 400 A/H. That way you will be able to cover your needs even under some combination of batteries not giving full theoretical capacity and/or not charging to 100% every day.

I don't usually get a whole day of life out of my batts on the hook, but that's also ok because I have a big diesel generator which is extremely quiet (almost inaudible) so it's not a horrible tragedy to run it a couple of times a day. I have 420 A/H of batts (@24v, like 840 @ 12v), but I have a large boat with quite serious electrical demands -- separate fridge and freezer, central heating, lots of computers on board, inverter, etc., etc., etc., plus it is my habit to have my electronics running 24/7 at anchor in case of dragging and needing to get a quick getaway. If I don't use any central heat and don't use the inverter much, I can just about live for 24 hours without charging, but in real life this is rare, so I usually charge for a couple of hours in the morning and a couple of hours in the evening while dinner is being cooked and AC power is needed anyway.

If you have only main engine (especially if you only have standard alternator), or if you only have a portable generator for charging, then this is tedious, and adequate battery capacity becomes more important.

I am convinced more and more that solar is the way to go for anyone who (like me) spends a lot of time off the grid and not motoring around.
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Old 20-12-2012, 02:55   #10
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Re: So, how battery and electricity do I need?

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Originally Posted by Sailorman375 View Post
I'm no expert, but I think you want as many amps available to start with as the less % of your bank that you draw down the better. In other words, if you draw you batteries down to a 50% state of charge every day they will have a much shorter life than if you only drawn them down to say 70 or 80% and then fully recharge.
Glacier Bay has a document on how to size your battery bank. You actually get more amp-hrs out of the battery over its lifetime with discharging to 40%, with 50% nearly as good.

page 4 of the doc has the table I'm talking about.

https://www.google.com/url?sa=t&rct=...hc4Iec482JlNyg
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Old 20-12-2012, 04:06   #11
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Re: So, how battery and electricity do I need?

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Is there a rule of thumb for battery bank capacity selection?
It depends on battery technology (I would not buy anything other than lithium phosphate now) and is a compromise among price, weight, space, and capacity. It is also critical to ensure that the battery pack is large enough that the maximum load will not exceed the safe discharge rate or the acceptance rate while charging, though this should not be an issue in practice unless one is trying to use the smallest battery bank possible.

Assuming you will have some solar panels and use lithium phosphate batteries, I would consider any battery bank capacity nominally between one and three days' maximum expected use reasonable depending on how much you value price, weight, space, and capacity. If you prize some combination of price, weight, and space but don't care much about capacity (are willing to run a generator more than twice a day) then you might choose a smaller battery capacity. If you want to live without a generator and dread the idea of running the motor when you aren't motoring, then a larger battery bank might be appropriate.
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Old 20-12-2012, 04:42   #12
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Re: So, how battery and electricity do I need?

Can I add a dissenting voice here, especially as the OP is an electrical newbie

Drawing up energy consumption budgets is a very exacting process. Duty cycles, expected use time etc are all very variable. Then you have to factor in actual power devices use, against what is said on the label.

I seen spreadsheets that either massively overstate the energy requirements, or seriously understate them. Yes its easy to do things like lights etc. But refrigerator /freezer/autopilot, even the instruments and MFDs, can widely affect things based on actual usage patterns and real life current draw.

Then factor in Peukerts and acceptable discharge depth and I can make any spreadsheet give you 100% variation in bank size.


I tend to come from the other side.

First I look at recharge ability, what do I have now and what could I afford to have and do I have the space/time & technical ability to install it or run it.

Thats gives me a reasonable idea of charge ability and takes into account it I am a weekend sailor , a 48 hours from shore power sailor or a away from mankind type. Generally you can estimate recharge watts easier then consumption watts ( possible solar/wind being the hardest).

Then I look at potential cost and space of battery storage, what do i have and can I easily add batteries ( in some boats this is a major factor). Can I simply upgrade what Ive got or am I stuck with it.

That gives me battery size , recharge capability and potential peak energy storage.( remember battery size is irrelevant if you can generate enough power, see cars).

Only then do I look at how that energy might be consumed. Often I suck it and see and use real life measurements to actually see what power draws I have ( I have a nice logging 9000 point multimeter to help in that),

Focussing on bank size is the wrong thing to do. Adding batteries will not help if you cant recharge them

On two identical boats Ive seen poepl get away with 200Ah of batteries just fine and on another Id say 1000Ah wouldnt have been enough.

Dave
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Old 20-12-2012, 05:37   #13
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Re: So, how battery and electricity do I need?

Battery Capacity in Ah = 4 x Daily Consumption in Ah
Solar Capacity (W) = 4 x Daily Consumption in Ah

This will get you by with no generator and no alternator in the tropics with a few days of continuous cloud every few months. Based on real experience in the tropics, not theory.

If you have an alternator on your engine, you can reduce the battery capacity to 3 x for those cloudy days every few months.
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Old 20-12-2012, 05:40   #14
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Re: So, how battery and electricity do I need?

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Battery Capacity in Ah = 4 x Daily Consumption in Ah
Solar Capacity (W) = 4 x Daily Consumption in Ah

This will get you by with no generator and no alternator in the tropics with a few days of continuous cloud every few months. Based on real experience in the tropics, not theory.

If you have an alternator on your engine, you can reduce the battery capacity to 3 x for those cloudy days every few months.
This rule of thumb is voltage dependent. Is that at 12V? Or ....?
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Old 20-12-2012, 05:49   #15
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Re: So, how battery and electricity do I need?

12V. Have to adjust by factor of 2 from 24 V.
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