

15052016, 10:17

#1

Registered User
Join Date: Sep 2014
Boat: 1980 Mariner36
Posts: 199

Could someone check my math please?
I'm just starting to scratch the surface and trying to learn the very basics of electrical and systems.
i was trying to determine the power draw an autopilot has on a battery system. after researching it i did find many examples to give me some estimate but no one converted that number into how many batteries you would need to set aside for an autopilot.
i found an online calculator and after messing about with it think its all starting to sink in but just want to be sure im getting it.
so here is the example...if im running an autopilot and lets say it draws 5 amps continuously for an hour X 8 hours it estimates about 92AH (over 20 hours) total draw. If i understand this correctly for an 8 hour sail that would equal 36.8AH draw from the battery (92AH/20 hours=4.6AH/hr X 8 hours=36.8) approximately.
https://www.batterystuff.com/kb/tool...toaload.html
from what i know so far we try not to drain our sailboat batteries more than 20%ish or is it 80%?. assuming 20%, if i had a single 110AH battery dedicated to autopilot from the information above...i'm estimating i would drain the battery about 34% of its capacity in 8 hours so i would need to charge it at least once during this time period to get it back to 100%ish charge.
so in other words...if i wanted to sail for 8ish hours with autopilot working the entire 8 hour time, one 110AH battery would be plenty for a pilot assuming i did charge it at least once? If i did not want to charge it during this time i should have a couple batteries?
i do realize these numbers are arbitrary and very general for the example. in reality an autopilot may draw much more or much less than that depending on circumstances.
much thanks....
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15052016, 10:32

#2

Senior Cruiser
Join Date: Feb 2007
Location: Southwestern Yacht Club, San Diego, CA
Boat: Searunner 40 trimaran, WILDERNESS
Posts: 3,042

Re: Could someone check my math please?
Things to consider: how heavy is the boat, how well balanced is the boat while sailing, how heavy are the seas and winds, HOW MASSIVE ARE THE SUPPLY CABLES TO THE DRIVE MOTOR (to reduce resistance and converting available power into heat), how large can you make the available battery bank, and what else will be supplied by that bank (running lights, radar, nav electronics, etc.). Don't forget to budget less than 50% of available supply so as to keep battery longevity in your favor.
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15052016, 11:16

#3

Registered User
Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch
Posts: 1,828

Re: Could someone check my math please?
Without solar panels or such charging there's no practical way to get your batteries much above 80%, then you draw to 50% before charging again. How much the AP draws depends also of weather and trim and such things so better to x2 that 5amps. 8h*10A =80Ah , 80Ah/30*100=266Ah battery
BR Teddy
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15052016, 11:53

#4

Senior Cruiser
Join Date: Oct 2007
Location: 29° 49.16’ N 82° 25.82’ W
Boat: Pearson 422
Posts: 12,381

Re: Could someone check my math please?
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.
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15052016, 12:06

#5

Registered User
Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 6,387

Re: Could someone check my math please?
Quote:
Originally Posted by er9
so here is the example...if im running an autopilot and lets say it draws 5 amps continuously for an hour X 8 hours it estimates about 92AH (over 20 hours) total draw. If i understand this correctly for an 8 hour sail that would equal 36.8AH draw from the battery (92AH/20 hours=4.6AH/hr X 8 hours=36.8) approximately.

Strange math.
Whatever the load is, autopilot, fridge, whatever,
X load in AMPS times # of hours = AH (amp hours)
This is what is taken out of your battery bank. Period.
How you switched to the 8 hour load into 20 hours makes no sense.
You've taken 92 ah out of your bank, not 36.8.
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Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)



15052016, 12:24

#6

Registered User
Join Date: Sep 2014
Posts: 317

Re: Could someone check my math please?
Quote:
Originally Posted by er9
so here is the example...if im running an autopilot and lets say it draws 5 amps continuously for an hour X 8 hours it estimates about 92AH (over 20 hours) total draw. If i understand this correctly for an 8 hour sail that would equal 36.8AH draw from the battery (92AH/20 hours=4.6AH/hr X 8 hours=36.8) approximately.

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.
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"If one does not know to which port one is sailing, no wind is favorable."
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15052016, 12:30

#7

Registered User
Join Date: Nov 2014
Location: Falmouth, UK
Posts: 169

Re: Could someone check my math please?
Quote:
Originally Posted by er9
so here is the example...if im running an autopilot and lets say it draws 5 amps continuously for an hour X 8 hours it estimates about 92AH (over 20 hours) total draw. If i understand this correctly for an 8 hour sail that would equal 36.8AH draw from the battery (92AH/20 hours=4.6AH/hr X 8 hours=36.8) approximately.
https://www.batterystuff.com/kb/tool...toaload.html

I think you may have misunderstood the web page tool calculation, possibly confused by the reference to the 20H rate. To clear that bit up first, the AH rating of a battery will depend upon the time taken to discharge it. Most reputable battery manufacturers use the 20 hour rate when specifying a batteries AH rating. So, assuming this is the case with your battery you can ignore the reference to 20H.
As I see it, your AH usage will simply be 5A x 8H=40AH.
The web site tool you used takes into account battery type and efficiencies and estimates the AH rating required to ensure you don't discharge the battery more than 50%. In your case, this is 92AH, so a 100AH battery would be fine.
I note though, that if your battery is a flooded type, and over 6 months old, the estimate is 110AH.
Hope this helps.
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15052016, 13:23

#8

Registered User
Join Date: Sep 2014
Posts: 317

Re: Could someone check my math please?
Quote:
Originally Posted by wiekeith
I think you may have misunderstood the web page tool calculation, possibly confused by the reference to the 20H rate. To clear that bit up first, the AH rating of a battery will depend upon the time taken to discharge it. Most reputable battery manufacturers use the 20 hour rate when specifying a batteries AH rating. So, assuming this is the case with your battery you can ignore the reference to 20H.
As I see it, your AH usage will simply be 5A x 8H=40AH.
The web site tool you used takes into account battery type and efficiencies and estimates the AH rating required to ensure you don't discharge the battery more than 50%. In your case, this is 92AH, so a 100AH battery would be fine.
I note though, that if your battery is a flooded type, and over 6 months old, the estimate is 110AH.
Hope this helps.

It looks like it's assuming a 100% starting charge though.
As skipmac said, 80% is more realistic.
40AH/0.3 (50% to 80%) = 134AH
That's 134AH at 5 hour discharge.
Which means about 170AH at 20 hour discharge.
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"If one does not know to which port one is sailing, no wind is favorable."
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15052016, 14:37

#9

Registered User
Join Date: Jun 2013
Location: canada
Posts: 1,699

Re: Could someone check my math please?
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?
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16052016, 02:44

#10

Registered User
Join Date: Nov 2014
Location: Falmouth, UK
Posts: 169

Re: Could someone check my math please?
Quote:
Originally Posted by KISS
It looks like it's assuming a 100% starting charge though.
As skipmac said, 80% is more realistic.
40AH/0.3 (50% to 80%) = 134AH

Yep, good point.
Quote:
Originally Posted by KISS
That's 134AH at 5 hour discharge.
Which means about 170AH at 20 hour discharge.

Can't get my head around the increase to 170AH. The 20 hour discharge at 5A is only 100AH, so 'theoretically' the 134AH could be reduced?
Having said that, I do agree with other posts suggesting in reality a very large margin should be assumed as there may be occasions when the AP is taking more current, in addition to the power demands of other equipment.
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16052016, 08:46

#11

Registered User
Join Date: Dec 2014
Location: the Med
Boat: Nauta 54' by Scott Kaufman/S&S  1989
Posts: 1,066

Re: Could someone check my math please?
40AH/0.3 (50% to 80%) = 134AH
This is correct. The rating of 134Ah implies that the battery delivers its capacity over 20h, which is 134/20 = 6.7Ah > 5Ah your absorption. OK!
Add the same math to other users.....i hope AP doesn't work fulltime. Set response to lowest in fair seas.
We mean lead acid deep cycle batteries. For a 30% cycle. It can be 50% with AGM, or solar topping, even 80+% with lithium.
But if you consider a smaller battery, its capacity gets lower than nominal by draining 5A...
.
Also, old battery has a diminished nominal value over time (soon) unless you top it to 100% frequently (shore, or solar)
A larger bank, seeing a lower % drain, INCREASES ITS NOMINAL CAPACITY
A reasonable house bank may range between 300 to 600Ah (agm) depending on size/users.
Absorption can be in the 150/250Ah per day, depending on many obvious factors....
Consider solar working 2/3 of nominal W for 6h/day, thus 5600 W solar can make you off grid :)
This summarizes my rules of thumb after one year long readings...
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16052016, 10:43

#12

Registered User
Join Date: Nov 2013
Location: Homeport: Oswego, NY
Boat: 1993 Sabre 362 #113
Posts: 351

Re: Could someone check my math please?
Quote:
so here is the example...if im running an autopilot and lets say it draws 5 amps continuously for an hour X 8 hours it estimates about 92AH (over 20 hours) total draw. If i understand this correctly for an 8 hour sail that would equal 36.8AH draw from the battery (92AH/20 hours=4.6AH/hr X 8 hours=36.8) approximately.

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.
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16052016, 16:57

#13

Registered User
Join Date: Dec 2015
Posts: 13

Re: Could someone check my math please?
Your math needs to consider also the dutycycle. A current of 5A (Ampere) is not drawn permanently, but only when the drivemotor of the autopilot is running to change the rudderangle. Once the rudder reaches the new position, the drivemotor stops and waits for the control unit to turn it on again when needed to make a new rudderangle correction. During such waiting period, the drivemotor consumes nothing, while the control unit will consume permanently probably less than 0.2A only.
If the boat is well balanced, the drive motor will operate average only 23 seconds each half minute, which means its duty cycle will be about 3sec/30sec = 10%. Accordingly to calculate the overall average consumption, You have to consider both the permanent consumption of the control unit (0.2A x 8hours sailing = 1.6Ah (Amperehours)) + (plus) the intermittent consumption of the drivemotor (5A x 8hours x 10% dutycycle = 4Ah) totalling 5.6Ah (Amperehours).
The dutycycle will change depending on sailing conditions. In rough sea the autopilot drive motor will operate more frequently, and the resulting dutycycle will be higher. In calm sea it will be lower.
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16052016, 17:24

#14

Registered User
Join Date: Jan 2015
Location: Georgian Bay, Ontario, Canada
Boat: iRWIN 43 Mk 3
Posts: 33

Re: Could someone check my math please?
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.
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16052016, 19:55

#15

Registered User
Join Date: Sep 2014
Posts: 317

Re: Could someone check my math please?
Quote:
Originally Posted by wiekeith
Can't get my head around the increase to 170AH. The 20 hour discharge at 5A is only 100AH, so 'theoretically' the 134AH could be reduced?

Forget the 170AH, I screwed that up...
You just need 134AH.
If you start with an 80% charged 134AH battery (@20 hour rating), draw 40AH out of it via the autopilot over 8 hours, you'll still have 50% left on the battery, which is what you want.
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"If one does not know to which port one is sailing, no wind is favorable."
 Seneca





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