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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 06-29-2010
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Battery consumption/charging calculator

I'm in the process of trying to figure out what my power demands will be if we take off cruising. I got to messing around and created a online energy calculator of sorts.

Sailing Adventure

I don't know a whole lot of experience estimating what realistic energy consumption and charging will be and would be interested in what other folks have experienced.

The big energy consumption items that pop to mind are refrigeration, electronics (maybe playing with a laptop a few hours a day, chart plotters, VHF radios, etc.), and autopilot (for sailing days -- which I expect to be much fewer than anchor days). Has anyone been able to get experimental data for how much each of these really use?

What about on the charging side? How much energy do you really get out of your solar panel? wind generator?
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Old 06-29-2010
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If you are going off cruising I would highly recommend investing in a true battery monitor. The Victron BMV-600 is under $185.00 and is perhaps one of the best investments a cruiser can make. it is well worth the expense. It will also tell you a LOT about your system that you might not have known like how fast your batteries actually accept a charge or how long it takes you to get from 50% state of charge back to 80% etc. etc.. They MUST however be programmed & used correctly in order to be even close to accurate..

No matter how much of an energy budget you do it will only be a ball park shot at best. Many items are variable draw and lights are on and off for varying times per day fridges draw more based on outside temps, new food, how many times per day you open it, autopilots also vary depending upon sail trim and sea state etc. etc.. It is very difficult to be accurate trying to count amps and turn them into Ah's...

I would be very surprised to see your fridge run only 25% of the day. Most I've come across cycle between a low of about 35%-80% depending upon insulation. 25% would be a very,very well insulated ice box opened infrequently.
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I've been through the same exercise. I don't see that there is any precise way to end up with an accurate load calculation. You can only get a ballpark number. I believe a battery monitoring system is an absolute necessity if you take off cruising.

In very basic terms wind or solar are very nice to have, but while away from shore power it's hard to beat a high output alternator for putting amp hours back into a battery bank at a rapid rate.

I'd spend money on a monitoring system, good batteries, and a high output alternator. If you have more money left over add solar or wind power.
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I'd second the purchase of a battery monitor as one of the best investments you can make when it comes to your boat's electrical system.
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Some thoughts on consumption as you mentions on your site going a "couple of days"..


Consumption:

As others have said...battery monitor first. The Victron BMV-601 can be had for under $200.00 but this is still double the $100.00..

The below calculations assume a 60% cycling of the fridge at 5 amps. This could be less or more depending on insulation. The 25 Ah number could be very slim depending upon what you use, you may use more or less:


Fridge: @60% cycling / 5 amps draw = 72 Ah's / 24 hr

Other draws: 25 Ah's / 24 hr

TOTAL / DAY = 97 Ah's

97 Ah's X two days = 194 Ah's consumed


Bank:

If you want to run two days at the above loads before recharging it looks like this:

194 Ah's needed... so.... GULP

650 Ah bank depleted to 50% DOD = 325 usable Ah's. (This is ONLY when the batts have been FULLY charged to 100% SOC)

Real world on the hook cruising:

650 Ah bank only recharged to 80% while on the hook charging by alternator is:

650 @ 80% of full capacity = 520 Ah bank

Depleted to 50% of total bank capacity of 325 Ah's leave you at:

520 Ah (80%) - 325Ah (to 50% total bank capacity) = 195 Ah of real usable battery capacity.

To use 194 Ah's over two days with no charge time in-between you'll need a 650 Ah bank to do that.

Using West Marine deep cycle group 24 batteries @ 75 Ah each it would require 9 group 24 batteries.

Using Sam's Club 6V golf cart batteries it would require 6.

If you plan to recharge every day cut the above number in half, or a 325 Ah bank. Only do this if you will be able to run the motor long enough to replenish the bank to 80% which can take many hours.

Of course a battery monitor may tell you that you only burn 72 Ah's a day. Guess work can be expensive and a good monitor can pay for itself if you were to over buy/build a system.

Alternator:

If using wet cells plan on max acceptance of about 25% of Ah rating so. Most recommend an alt of 20-40% of bank capacity.

650 Ah bank = 162 amp alt: This is not going to happen easily on a small motor so charge times to 80% will take a while.

325 Ah bank = 81 amps This is achievable on a small motor with a 100 amp alt.. Going bigger on the alt is fine as it works it less hard and never puts out full "spec" when hot..


Disclaimer: These number are all rough GUESSTIMATES but the basis for bank sizing theory is what you really want/need..
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Putting $ aside, is there another brand which you have dealt with that you would reccomend Maine, or SD? In particular, i wish to monitor my consumption with the onboard computer.
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Quote:
Originally Posted by Capnblu View Post
Putting $ aside, is there another brand which you have dealt with that you would reccomend Maine, or SD? In particular, i wish to monitor my consumption with the onboard computer.
Blu,

There are a number of monitors but the easiest and most widely accepted, at least here in the US, are the Xantrex Link series which is really made by made by TBS Electronics and the Victron. Blue Seas and BEP also make monitors as does Sterling Power. The best value is the Victron as it comes with all the wiring you need to hook it up and it is plug & play..

Don't know of any that will allow monitoring only through a computer as the computer would need to be on and powered up 24/7 and that can draw massive amounts of power and the Link or Victron are very,very low standby draw.

The Link Pro and the older version called the XBM can be connected and communicate with a laptop, with the extra communications kit, but it's still a sand alone unit if you need it to.

I own three Xantrex monitors a Link 10, XBM and a Link Pro. The older Link 10/20 units, made by Cruising Equpment Co., were very buggy in comparison to the TBS made units. The Xantrex XBM, Link Lite and Link Pro are all made by TBS and are very, very reliable. TBS used to also make the monitors for Victron but Victron is now making them elsewhere as TBS contracted with Xantrex to supply all thier monitors. I have installed a couple of Victron BMV-600's and they are excellent units and easier to hook up than the Xantrex models, and also less money.
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Maine,

Thanks for the great information. Its kind of amazing how big of a battery bank seems to be necessary.

I have kind of been operating under the assumption that for extended cruising, I probably don't want to run the engine every day to charge the batteries. But maybe this isn't a good assumption. I can buy a lot of diesel for the price of a wind generator and a few solar panels!

It is also interesting how difficult it is to generate enough energy from wind/solar to power the relatively minimal power consumption of a cruising boat. Makes wonder if it is also that difficult for the entire grid!
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I'd second the Victron. I'd point out that there are some monitors that do allow a computer interface, but as Maine Sail said, it would require a computer to be on and draw down the batteries without giving you too much more information.

I'd also point out that solar panels are probably one of the best ways to get some passive regeneration capacity. I've written about Solar Power on Boats and would recommend you read the article. Unlike using the engine, a solar panel doesn't require you to carry fuel. Unlike a wind generator, a solar panel requires far less maintenance once installed.
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Nice article Sailingdog.

I didn't know that was how some of the "smart" charge controllers work. That makes a lot of sense to have built in DC to DC converters.

I was looking at the KISS site for their wind generator. They apparently don't use or even recommend a regulator at all, which I find interesting. Basically, their argument is that batteries can accept 2.5% charge of their capacity indefinitely. So a 400 amp-hr bank can be charged at 10 amps continually without any harm.

So if I understand correctly, we could do the same thing with the solar panels. If we have a 400 amp-hr bank, we could probably pretty safely have a 80 Watt or even a 120 Watt panel without regulation. But we would be loosing out, especially in the situations where we end up with a shaded panel.
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