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unbusted67 · 15-07-2009, 11:58

Can someone explain amp hours to me? I have a digital readout for my batteries (I'm too lazy to get up and see what brand it is - I'm in the v berth and it's in the quarter birth, can you really blame me?). Is it an indication of how long your batteries will last while they are drawing the current amount of amps? If I have my fridge on and It says 4 amp hours does that mean I could run my fridge for 4 hours? Is this a dumb question?

Vasco · 15-07-2009, 12:06

How long you can run anything depends on your battery capacity. The 4 amps you see is the power requirement of the fridge. If you had a 100 amp battery you could (in theory) run that fridge for 25 hours (100/4) but in practice it's not good to run your battery down more than 50% and in real life your fridge would cut out before you run the battery flat as the voltage would be too low. There are many tutorials on the web on 12v dc systems It would explain it better than I can and you would have all the answers you need.

BlueSovereign · 15-07-2009, 12:21

Quote:

Originally Posted by unbusted67

Can someone explain amp hours to me? I have a digital readout for my batteries (I'm too lazy to get up and see what brand it is - I'm in the v berth and it's in the quarter birth, can you really blame me?). Is it an indication of how long your batteries will last while they are drawing the current amount of amps? If I have my fridge on and It says 4 amp hours does that mean I could run my fridge for 4 hours? Is this a dumb question?

Let me get this straight: you own a 38 foot yacht, have been on this site for 3 1/2 years, and you are (and I quote) "too lazy" to get up to check a meter in another berth???

Mate, whatever you do ... pleeeeeeease never sail in any location I might be in!

Yours faithfully

William aka 'The PIRATE'

PS: The answer above sums it up nicely. I hope you can find the time to follow his suggestion as there is plenty of threads that can provide you with the knowledge I recommend you acquire.....

**GordMay** · 15-07-2009, 12:38

1 Amp-Hour = 3,600 coulombs*,
hence
4 A-H = 14,400 C (4 x 3600)

* One coulomb is the amount of electrical charge in 6.241506 × 10 to the 18TH electrons.

Dogs that bark at mail carriers, joggers running by the house, or cyclers on the street naturally have their barking reinforced. They see the mail carrier, they bark, and the mail carrier leaves. The dog thinks, "Boy, I'm good. My barking made that person leave." In modifying the dog's behavior, we need to overcome this reinforcement.
So, too, with lazy dogs.

See also
➥ http://www.cruisersforum.com/gallery...p?i=3180&c=514

slomotion · 15-07-2009, 12:45

Nevertheless, I think it’s important to make at least some effort to confuse people:

The amp hour rating for a device like a light is pretty meaningful because you typically turn it on/off manually and you can more or less expect it to require the rated amps for each hour that it is on.

OTOH, a refrigerator is typically controlled by a thermostat such that it cycles on and off in a manner that would make an amps per day @ X ambient temperature a much more meaningful number.

And of course, the conventional way of rating battery amp hour capacity is to determine the cumulative amps available when evenly discharged over a period of 20 hours rather than 24.

Hope this helps.

**GordMay** · 15-07-2009, 13:19

Quote:

Originally Posted by GordMay

1 Amp-Hour = 3,600 coulombs...

If electricity comes from electrons, does morality come from morons?

slomotion · 15-07-2009, 13:49

Quote:

Originally Posted by GordMay

If electricity comes from electrons, does morality come from morons?

Yes! Because it is well settled:

1 moral hour requires 3600 x 6.241506 x 10 to the 18th morons.

unbusted67 · 15-07-2009, 13:52

Quote:

Originally Posted by BlueSovereign

Mate, whatever you do ... pleeeeeeease never sail in any location I might be in!

Oh you don't have to worry about that, I'm too lazy to go any where to.

And I can't seem to get my engine to start, something is wrong with the batteries. They are overcharged with morons.

sailorboy1 · 15-07-2009, 14:02

Normally only way to get the morons out of the batteries involves rum, chicken, and vodoo. Might be able to do a new moderm method of hooking to your computer and allowing them to run the internet waiting for people who have nothing else to do than find something to "trash talk" to them about :-)

**David M** · 15-07-2009, 14:07

Don't confuse current with power. Sometimes people do. Current is measured in amperes and power is measured in watts. You need to know potential (volts) and current (amperes) in order to calculate power. Current (amperes) times potential (volts) equals power (watts), for DC. For AC its the same plus you need to know the power factor, which usually is not a significant multiplier.

One amp-hour is one amp of current flowing for one hour. If the voltage is 12 volts and the current is 1 amp-hour. You are supplying 12 watts of power to a load for one hour. Or one watt-hour. Make sense?

To me watt-hours make more sense when measuring the total amount of power that has been supplied to something or removed from something, like a battery.

Just yesterday I overheard a conversation. One of the store employees who works for the largest retailer of marine parts, was telling a customer that his laptop draws as much power as his power drill. He was comparing amps at 12 volts DC to amps at 120 volts AC. I couldn't stand listening to the conversation any longer because he was giving his customer wrong information...so being how I am, I corrected him, politely of course.

Starbuck · 15-07-2009, 14:32

Blue Sovereign, I've read snotty responses to honest questions before, but your rebuff was truly unnecessary. The bone you chose to pick didn't even have anything to do with the original poster's question. You're not going to win Miss Congeniality that way. If that were the answer to my first post, I'd turn tail, never come back, and bad-mouth this place to every sailing friend I know.

Gord, you're usually a wealth of knowledge, but philosophizing about dogs and operant conditioning was less than helpful, and the information you posted about electricity was too esoteric to be of any practical value to the OP. Methinks you're the one hitting the bong today, Skipper.

*** *** *** *** ***

Unbusted, if your fridge says 4 amps, that doesn't mean that it could run for four hours: that's the amount of current it draws running full-tilt. If it were to cycle on and run full-tilt for an hour, it would have consumed 4 amp-hours of power.
(4 amp draw from the batteries for 1 hour = 4 amp-hours of power consumed)

Now of course the compressor motor doesn't run 24/7: it cycles on and off, controlled by a thermostat in the refer box. Let's say it runs 30 minutes out of each hour: it will consume 2 amp-hours of power in that same hour.
(4 amp draw x .5 hour = 2 amp-hours consumed)

If your box is super-insulated and you live on Seattle, it may cycle on for only 15 minutes in that hour, using only 1 amp-hour of power.
(4 amp draw x .25 hour = 1 amp-hour)

As has been suggested, when that compressor motor cycles on, it probably is not pulling all of 4 amps for its whole cycle, so those figures are only to provide some hard numbers to work with.

So: the refer runs 20min. in each hour, and the current draw is four amps. In 24 hours, that's about 32 amp-hours consumed.
(4a x .33 x 24hrs = ~32 amp hours)

Once those amp-hours have been drawn from your batteries, they need to be replaced. A battery charger might put out (let's say for this example) a constant 5 amps @120v AC. That's the same as saying 50 amps @ 12v DC. The charger will have to run .64 hours (about 39 minutes) to replace the 32 amp-hours.

The charger is only 80% efficient, so some of that energy is converted to heat (the charger will be warm to the touch), so it will take closer to 45 minutes to actually get 32 amp-hours of energy back into your batteries.

That's the theoretical cycle. In the real world, the batteries' own internal resistance will increase as they are charged, limiting the current that they will accept as they "fill". The topic of charging inefficiencies is a whole chapter unto itself.

Good luck, and here's to continued education of our own systems, no matter where on the knowledge scale we're starting from.

Jeff

MarkJ · 15-07-2009, 14:47

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