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Old 24-08-2012, 07:31   #1
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Confused on Amp Hours Usage

confused on amp hours usage

I am using a macbook pro 17inch screen.

the power cord that comes with it is

input 100-240v---1.5A 50-60 hz
output 16.5-18.5v ---- 4.6A max



If I am running this on a 1000 watt inverter from 12 volt batteries how many amp per hour does it use????


only have a 12 volt meter connected to batteries and they are draining faster than I expected. batteries are brand new and running only computer


any help appreciated
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Old 24-08-2012, 07:40   #2
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CaptainKJ

Your MacBook Pro (I just swapped mine for a Retina), uses about 100 watts. Your inverter is likely only about 85% efficient. This means you are using about 120W or 10AH to power your MacBook. Hope this helps. You might consider using an Auto Adapter for your MacBook direct to 12 V to get a little more efficient use of batteries. This helps because you not only get gains back on the efficiency of the inverter, but on your Mac power supply which also about 92% efficient.
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Old 24-08-2012, 07:45   #3
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Re: confused on amp hours usage

The maximum listed on the adapter is not typical of the average use.
A good battery monitor or clamp on multimeter will tell you.

At a guess with a 17inch screen I would be expecting about 3.5A on a 12v dc to dc adapter or a bit over 4A when using an inverter, as you are.This is running,but the batteries fully charged.
This can reduced if you drop the brightness down.

Amp hours is simply the number of amps multiplied by time. So 4A for 2 hours is 8AHrs.
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Old 24-08-2012, 07:46   #4
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Re: confused on amp hours usage

max draw of the Mac power supply is 1.5A

Power = voltage x current
P = 110V x 1.5
= 165 watts

working backwards for 12 v:

P = I x E
165 / 12 = 13.75 amps

Drawing from your inverter will also have some efficiency losses - say 10%, so factor that in as well and you could be looking at up to a 15A draw

Of course, that's a max reading. Actual power draw will vary based on state of charge of the Mac battery pack. Also note that you should unplug the power supply when not actually charging the Mac. The power supply is essentially a transformer so will draw power all the time - even if the Mac batteries don't need it.

You might consider getting a current meter to get more accurate readings so you can plan out your usage and recharging cycles.
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Old 24-08-2012, 07:48   #5
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Re: confused on amp hours usage

thanks for quick reply
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Old 24-08-2012, 08:07   #6
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Re: confused on amp hours usage

Yes, high horsepower modern CPU's chew up a lot of juice. In my experience, most laptops draw about 10-15A DC. Do the math, if you run it for example 5 hours per day that's at least 50-75AHs. Only way to know exactly, as other has suggested, is check it with a meter.

Laptops are the highest draw we have both at home and one the boat. Even our larger fridge at home (all 12v) only draws a max of about 6A when it cycles up (about 25% duty cycle).
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Old 24-08-2012, 08:26   #7
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Re: confused on amp hours usage

If your inverter were perfectly efficient (it is not), based on the output rating of the power cord you would be drawing about aboput 5.5 amps. (If you were to run it for an hour, that would about 5.5 amp hours.) But you are converting from DC to AC then back to DC, each of these conversions may only be say 50 to 85% efficient - so you could be drawing 10 to 15 amps or more.

That's kind of a lot, actually. For perspective, my ammeter now says 6 amps with the fridge running and three cabin lights on.

Have a look at a book called "The 12 Vollt Doctor's Practical Handbook" by Edgar J Beyn - probably available at your local boatie shop.

Good luck!
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Old 24-08-2012, 08:45   #8
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Re: confused on amp hours usage

Quote:
Originally Posted by Panaseaya View Post
each of these conversions may only be say 50 to 85% efficient
Modern electronics aren't quite THAT bad! Yes, there are losses, but the efficiency of the conversions between AC and DC will be more like 85%-95%. 90% efficiency (a 10% loss) is probably a good average to expect.

Otherwise, Paperbird has all the math the OP needs. It really is as simple as remembering one formula (watts = volts x amps) and then applying simple, grade-school algebra.
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Old 24-08-2012, 09:01   #9
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Re: confused on amp hours usage

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Originally Posted by denverd0n View Post
Modern electronics aren't quite THAT bad! Yes, there are losses, but the efficiency of the conversions between AC and DC will be more like 85%-95%. 90% efficiency (a 10% loss) is probably a good average to expect....

Yes, 10-15% loss is more representative of a good inverter (maybe not a small el cheapo for your car). Xantrex specs many of their current products at 90% efficiency.

But....that's not all the losses have with this set-up. Your Mac's power supply is then converting again from AC to DC with some loss. Don't know the efficiency of this particular power supply, but in general I've seen a range of about 70% to 85% given for PC power supplies. (wow, multiply that across all the PC's on the planet...)

So, If you assume 90% efficiency for the inverter and 80% efficiency for the power supply then you are losing a whopping 30% !!!

Another factor to be aware of is that many PC power supplies are not very smart and will continue to draw some energy even when the PC is disconnected! So, best to turn off the inverter or disconnect the PC power supply from it when not in use.

Something to consider is getting a DC-DC power adapter for your Mac. This would eliminate most of the conversion losses. It too is going to have some loss, but it should be considerably less.
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Old 24-08-2012, 09:48   #10
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Re: Confused on Amp Hours Usage

Everyone with a cruising boat needs a charge monitor. I like the Link-10. If you bring your batteries up to 100% full daily, then the "Link 10" (or whatever brand), will re-0 its algorithm daily, and remain incredibly accurate. They measure V, A, Ah, and TTG at a given use level.

Also, due to higher line V, your electronics will work better/longer, and you can easily get > 10 years of reliable life from a good house bank this way. (Like Trojan L-16s)

To know the A. use level of ANY device, shut off EVERYTHING going into or out of the batteries, EXCEPT that ONE device. Then switch the "Link-10" to A. If it reads 4.0 A., then in one hour that computer used 4.0 Ah...

IF you don't always have an inverter on anyway, the computer will use about HALF as much power if you avoid using the inverter, and power the computer with A DC to DC sort of step up transformer, AKA "car adaptor", with a cigarette lighter plug & wire, to hook to the boat's DC system. Do unplug it when not in use.
There are numerous websites that sell these car adaptors, in a hundred varieties.

You will be looking at 12V DC in, and 16 V (or what ever you want), DC out. Then match the computer's in plug shape. Just get the right requirements.

Try to run all low V devices this way, rather than crank up a power hog inverter. Then If you think "CONSERVATION" on everything on the boat, you can run the entire boat on solar, like we do!

Mark
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Old 24-08-2012, 09:58   #11
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Re: confused on amp hours usage

Quote:
Originally Posted by Panaseaya View Post
... Have a look at a book called "The 12 Vollt Doctor's Practical Handbook" by Edgar J Beyn - probably available at your local boatie shop.
Good luck!
A couple of Edgar J. Beyn’s books are available (free) online:

“The 12 Volt Doctor's Practical Handbook”
http://www.smer.fi/docut/12volthandbook.pdf

“The 12 Volt Doctor's Alternator Book"
http://kb-kbh.dk/shipslib/el_ombord/...ndbook_ocr.pdf

I haven’t found:
“The Twelve Volt Doctor's Project Book”
“The Twelve Volt Doctor's Troubleshooting Book”
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Old 24-08-2012, 10:25   #12
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Re: Confused on Amp Hours Usage

Using a volt meter to read state of charge is at best an approximation. I only have a volt meter too (digital). its fine reading SOC at no load on the battery, after the battery has had time to rest.

But as you use power from the active battery the voltage reading will drop . The more current the more slump. You can't really use a volt meter to measure SOC under dynamic conditions. Just does not work. Over time (weeks / months) You do get a handle on what a normal reading is.

For Me the volt meter is fine. I pretty much have a simple electrical system so the loads day to day are constant.

Using a volt meter, you'll want a switch so you can measure the resting bank for SOC. Then you'll have useful information about the resting bank

BTW, I know that in general my working battery bank will have a SOC roughly 0.2V higher then the displayed voltage when under load. Based on lots of observation
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Old 24-08-2012, 10:28   #13
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Laptops run from dc bat power. So no need to convert dc to ac (inverter) and back to dc in the laptop, as others have replied.
Unplug in electrical storms.
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Old 24-08-2012, 12:09   #14
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Re: Confused on Amp Hours Usage

thanks for all the replies,,, one problem is I can not find a dc hookup for the macbook pro, have been looking, only keep finding ac/dc converters, apples does not seem to have them
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Old 24-08-2012, 12:31   #15
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Quote:
Originally Posted by belizesailor
Yes, high horsepower modern CPU's chew up a lot of juice. In my experience, most laptops draw about 10-15A DC. Do the math, if you run it for example 5 hours per day that's at least 50-75AHs. Only way to know exactly, as other has suggested, is check it with a meter.

Laptops are the highest draw we have both at home and one the boat. Even our larger fridge at home (all 12v) only draws a max of about 6A when it cycles up (about 25% duty cycle).
Shocking, isn't it? When I realized that my MacBook Pro was using as much power as my toaster, I switched to using the iPad whenever I'm on the hook. Nicest part of that is I can plug it directly into a 12v cigarette lighter socket.
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