MacGregorSailors.com

Hi all. I was wonder about the amp/hour ratings on batteries. Am I correct in thinking that a fully charged 200amp/hr battery with a load of, lets say 2amps, will supply power for 100hrs? Also, what should the voltage be of a fully charged battery, and one in need of a recharge? Is it worth while to install one of those built-in volt meters to check levels of both batteries?

From my understanding you should only use 1/2 of the rated amp/hrs. If the voltage drops below 12v's you should replace soon.

Full charge is, I think, 13.5v's

Volt meter? if it is digital, yes, or you can just use your volt meter.

As far as checking battery(s) state of charge whilst on the water is concerned, there are heaps of very clever, very expensive gadgets that can be wired in that give you all the info you need.

....or you can do what I do and go get a $10 multimeter and check the voltage with that. On trips I just have a look at them each morning and if they get below 12.5, it's time to do some charging.

Thanks for the info. A lot of good stuff at that site Moe.

I had to pull start my Suz 50 this weekend. Too many movies the night before and an elec. fan while cleaning the breakfast dishes until we got underway. Started first try, I was amazed. BTW we spent 3 days in the Marquesas Keys, I will have pics on my site in the next couple of days.

later...
Hawg

On a boat or a remote home you depend on wind, water, solar or mechanical (generator, ect.) to supply your electrical needs. We all know you need to store up all this power for use when the wind doesn’t blow, the sun doesn’t shine or the diesel tank is too low. Where do we do that? In a battery bank! Batteries are not only sold by size (physical) but also by capacity. This capacity is rated in Amp Hour units, just like your gas tank in your car, you can go only as far as your fuel capacity and your Miles per Gallon will carry you. How do we determine our Amp Hours usage?

Let’s look at a very simple example. You have a cabin fan that is going to run all day, a light for 4 hours in the evening and an anchor light for 10 hours –

Cabin Fan – uses 1 amp so 1A * 24Hours = 24 AHrs
Cabin light – uses 1.5 amps so 1.5A * 4Hours = 6 AHrs
Anchor Light – uses 2 amps so 2A * 10Hours = 20 AHrs

So for one 24-hour day you would use 50 Amp Hours (24+6+20). If you have a battery with a rated capacity of 150 Amp Hours you could sit at anchor for 3 days before you ran out of “gas”. You can use our worksheet to help determine a rough estimate or if you have an amp meter on your system already, you can turn on your individual components and see what they each draw. Times the hours you think you’ll use each per day, add them up and you have your personal Amp Hour requirements for 24 hours. For boaters you will want to do two calculations, one for at anchor and one for under sail.

What’s next? Matching your daily needs to battery capacity and charging capabilities.



Determine your boats power usage by completing the chart below
Typical 12 volt equipment power requirements

Use the estimates below or by calculating usage by multiplying the current used by each device times the expected hours of usage per day

Equipment /Amp Draw/Hours used at anchor/AmpHours at anchor/ Hours used under sail/AmpHours under sail/Your AmpHours at anchor/Your AmpHours under sail
Refrigeration 2.5-20/ 24 or 5/ 60-100/ 24 or 5/ 60-100
Auto Pilot 0.5-10/ 0/ 0/ 24/ 12-240/
Ham Radio (transmit) 1-(20)/ 1-5/ 1-100/ 1-2/ 1-40/
Color TV 3-7/ 3-5/ 9-35/ 0/ 0/
20w Fluorescent Light 1.5/ 4-6/ 6-9/ 2-12/ 3-13
Cabin Fan (1) 1/ 4-10/ 4-10/ 0/ 0/
Tri-Color Nav Light 2.5/ 0/ 0/ 7-9/ 17-23
Stereo 1/ 5-15/ 5-15/ 3-10/ 3-10/
Radar 4/ 0/ 0/ 4-7/ 16-28/
12v Water Maker 4/ 2-4/ 8-16/ 2-4/ 8-16
VHF Radio (transmit) 0.5-(10)/ 12/ 6-10/ 24/ 12-15
GPS 0.5-1/ 0/ 0/ 24/ 12-24/
Depth Sounder 0.5/ 2/ 1/ 2-6/ 1-3/
Anchor 2 8/ 16/ 0/ 0/
Anchor Windless 80/ 0.1/ 8/ 0/ 0
Fresh Water Pump 7/ 0.3/ 2/ 0.3/ 2
Bilge Pump 8/ 0.2/ 4/ 0.2/ 4
Computer 2 - 8 1-4/ 2-32/ 1/ 2/
Spreader Lights 8/ 0.1/ 1/ 0.3/ 2/
Propane Valve 1/ 1/ 1/ 1/ 1/


Typical Power Consumption

If you have an amp meter in your power distribution panel, record the amps used by each piece of equipment
Multiply it by the number of hours used per day. Total the equipment Amp Hours to obtain the daily
Amp Hours used. Add 20% to obtain the Amp hours that must be put back in the batteries per day to keep even.
Calculate the usage both at anchor and underway. Call or email us with the results.



12 Volt DC Systems
Formula: 12 volt inverters require approximately ten (10) amps DC input for each 100 watts output power used to operate an AC load.
Example: How many DC amps will a 12-volt inverter require to operate three 500-watt quartz lights, or a 1500-watt electric heater?
Answer:
1) Total watts = 1500
2) 1500 watts/100 (from formula) = 15
3) 15 X 10 amps (from formula) = 150 amps. This is the DC current the inverter will use to operate the 1500-watt load. NoteIf these 150 amps are drawn from the battery for one hour, 150 amp hours of battery power will be used.
To support 150 amp hours of battery power 300 amps of battery capacity should be used for maximum battery life and performance.

Thanks Matt, just what I was looking for.

we spent 3 days in the Marquesas Keys

and I had to work. This is one of my cruising destinations. Just 30 (?) miles west of Key West. Did you post a log of this on your web site?

Matt. If a Honda 50 has a 10 amp alternator is it replacing the usage you use in these definitions at the rate of 10 amps per hour? I ran my battery down once and after about an hour or just a bit more the battery seemed to have decent power to start the motor which I did a few times before returning back to the dock. Just seems that the battery was in good shape although I didn't have any way to tell if I had a full charge.

Amp-hour is an energy unit and generally the amp-hour spec for a battery is based on a 20 hour discharge. You may get a different "amp-hour" calculation if you discharge slower or faster. When the battery voltage gets down to 10.5 Volts it's considered discharged.

Daniel