Could someone check my math please?

[knockabout]

a lot of good battery advice
an inexpensive 4 stroke generator will help out when nature
throws the spanner into the calculator
if you have a charger for your shore power adapter
run the charger off the generator to top up your main
batteries as well as your steer battery

oops! that's right you mentioned you have a cast iron main
with 120 amp alternator
if you run your motor for 1 hour theoretically who knows
if your steer batteries connected to your charge system it will
charge as well
it's hard to allow for the unexpected when you don't expect it

good luck with your sailing project

[er9]

Originally Posted by skipmac

To summarize the previous posts.

- If you are off the grid, that is sailing offshore or at anchor then you will probably have to use your batteries between 80% max State of Charge and 50% min SoC. The reason is it takes a very long time to take your batteries from 80% to 100% which is not usually practical on sailboats unless they have huge solar panels.

- This means on a practical basis you have only 30% of the batteries' capacity to use without damaging the batteries.

- So to use 100 amp hours between charging times you need 300 amp hours of battery capacity.

If you have solar panels, wind charger and other sources that usually put more charge into the battery as you use them then the calculations get a little more complex but I think this gives you the basic idea.

much thanks that did clear it all up nicely.

[er9]

Originally Posted by KISS

You've got two different concepts mixed up here.

If the pilot draws 5 amps for 8 hours, that means a total draw of 40AH, not 92AH.

So why is the calculator saying you need a 92AH battery?

Because battery efficiency depends on the rate of discharge.

The faster you discharge it, the less juice you get out of it.

For instance, a 40AH battery discharged over 20 hours will give you 40AH. Discharge that same battery over 5 hours, and you get much less than 40AH. This is why you need a 92AH battery to feed the autopilot 40AH over a 5 hour period.

Most batteries are rated at 20 hour discharge. So, if you buy a "100AH" battery, that means it will give 100AH if you discharge it over a 20 hour period. Less than 20 hours, you get less that its rated AH. More than 20 hours, you get more than its rated AH.

and to further clarify i beleive its rated 20 hour discharge for american batteries is based off a 5amp continuous draw if i'm not mistaken.

i guess my math took the long way around. the calculator did confuse things a bit and i didn't realize it was such a simple, direct equation 5*8=40amps. i suspected it might be that simple but i wasn't sure.

[er9]

Originally Posted by smac999

It's only going to draw power well turning... So daily use will very a lot and is only a guess

Also you do not want an ap battery. It will come off your house bank. Which will be sized for total use of boat

How are you charging them?

well i dont actually have an autopilot yet. i was just using it as an example because i happened to be researching them that morning.

i'm starting to read the 'boatowners electrical and mechanical' book. i recently bought my boat and would like to redo and establish a battery bank that will eventually meet all my anticipated needs in the future. for now i'm trying to anticipate the electronics, equiptment and devices that will be installed and being used in the near future so when i build and establish my battery bank its sufficient size at the start and there will be enough room to expand.

for the foreseeable future the only source of charging will be a plug in at the slip, the motor and maybe a small 100watt solar panel. eventually two or three years from now as money becomes available i'm guessing several hundred watts of solar and probably wind in addition to that.

[er9]

Originally Posted by Dave Lochner

er9, if you're not confused by now, let me help.

A 5 amp draw for 8 hours is 40 amp hours.

Generally, lead acid batteries should not be drawn down to more than a 50% State of Charge.

A Group 27 Battery that is new and at 100% state of charge has about 90 amp hours of capacity.

Back to the original question. You run some device for 8 hours that draws 5 amps. Subtract that 40 amp hours from 90 amp hours (100% SOC) and you have 50 amp hours left in the battery 50/90 is .55, the battery is at 55% state of charge. Time to start recharging the battery.

Think of this analogy. You have tank with 90 gallons of water, if you take out 5 gallons every hour, how much will you have left after 8 hours?

There is a whole bunch more that will add to your confusion, such as size of battery bank, charging sources, continuous vs. intermittent loads, etc. but we'll avoid that now and stick the very basic stuff, that is, how much electricity are you using per hour and how many hours you will be using that electricity. Once you're confident with those concepts, it will be time to move on to other concerns.

lol...much thanks. it does get confusing very quickly. some really good replies so far, things only experience could shed light on that a novice would overlook and probably be under prepared for.

[er9]

Originally Posted by Gamechanger

Reading the responses to your request to "Please check my math" has been very interesting. The subject of house battery capacity is one that I took longest to feel comfortable with. I spent a LOT of time with a specialty battery supplier trying to understand what capacity is needed AND what capacity is economically most advisable for my Irwin 43 MkIII.
My decision was to go with a 1250 Ah system. I do not yet have solar or wind generator capacity but that's in the cards in the future. I installed a 120 amp alternator.
Two key factors to keep in mind..... the first of which is mentioned in a few of the responses you've gotten.
1) Try to avoid bringing your batteries down below 50% of their capacity before recharging. Drawing them down lower than that will substantially reduce their usable lifespan (# of charge/discharge cycles). I'd urge that you get a Victron battery system monitor so that you can watch the drawdown % carefully.
2) If you don't have either solar or wind generation capacity then consider getting an external "smart" voltage regulator. It will lead your alternator charging through the bulk/absorption/float stages of the charging cycle. That will get you back up to 100% charge rather than the 80% that is what most people can get from their alternators.

Rest assured, your not alone in your uncertainty about what battery capacity you need on your boat.

thank you...good stuff to think about.

[TeddyDiver]

From an another thread
"I had a 300 Watt hydraulic drive unit which pushed 200 Amp-hours for 24 hours in a hurricane and only 20 Amp-hours in mild stuff. In this case it is easier to figure an average power consumption equal to half the peak power rating of the drive motor for the 24 hour calculation for a "bad case" of power consumption. You will then be well under 360 Amp-hours for 24 hours."
http://www.cruisersforum.com/forums/...tion-3838.html

[er9]

Originally Posted by knockabout

a lot of good battery advice
an inexpensive 4 stroke generator will help out when nature
throws the spanner into the calculator
if you have a charger for your shore power adapter
run the charger off the generator to top up your main
batteries as well as your steer battery

oops! that's right you mentioned you have a cast iron main
with 120 amp alternator
if you run your motor for 1 hour theoretically who knows
if your steer batteries connected to your charge system it will
charge as well
it's hard to allow for the unexpected when you don't expect it

good luck with your sailing project

huh...i knew about generators but didn't know thats how easy they worked. do the generators just plug into the shore power outlet on the boat? i would definately want one for long passages but will not be doing any for some time as i get the basic systems up to snuff.

[Dave Lochner]

er,

A good source of information that is mostly easily understood is this site.

Welcome To MarineHowTo.com Photo Gallery by Compass Marine How To at pbase.com

He has a lot of good information on electrical systems and a few other topics.

[Sandero]

I would be curious to see the actual current draw from an AP under duty in different conditions. The AP I have (Alpha3000) has the ability to control "yaw" and response to "sea state". In flat seas and motoring with the AP, for example... the helm is not moved at all. I imagine that the current draw is mostly then the processing of the compass data and the feedback data from the ram drive. It would be very low under these conditions. As seas build the AP will make more corrections to maintain course. Puffs of wind of course will cause the boat to head up for example and the AP needs to correct for this as well. So the heavier the conditions... the more work the AP does and the more current it draws to do this. With the Alpha you adjust to the controls to find the right balance.... between corrections and course.

And of course current draw is not an issue when there is charging of the batts taking place which exceeds or matches the current draws... such as motoring, wind and or solar.

What would be interesting would be to see a plot of AP current use compared to wind and sea states.