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Old 12th June 2011, 07:12 PM   #1
donnib is offline donnib  Denmark
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Default Measure total consumption Ah

I am about to build an boombox from car audio parts and an car battery i am interested to include in the box a way to see how much battery i have left OR how much Ah i have used so i know when i am abot to hit the 50% of the battery capacity.

I have not found a way yet. Any measurement equipment that can sample current and then make a sum in form of Ah or Watts ?

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Old 12th June 2011, 09:36 PM   #2
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That would only work if the battery was 100% charged for each use and when the battery was new. As the battery ages, the capacity will decrease which means the actual Ah capacity will be less than the specified rating.

If you drain the battery to the point where the amp or head unit shuts off and then recharge the battery relatively soon (don't leave it deeply discharged for more than a couple of hours), the battery won't be hurt (especially if it's a deep cycle or AGM/drycell).

If you want to build something, you'd need a datalogger and a way to measure voltage and current flow. The monitoring would have to be continuous.
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Old 12th June 2011, 09:40 PM   #3
donnib is offline donnib  Denmark
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Yes you are right and i could live with that i mean charging battery at the beginning then use it so i can use the measurement for something useful.

Unfortunately i did not want to build something on my own therefore i asked maybe somebody knew a device that could do this.
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Old 12th June 2011, 09:46 PM   #4
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I think the practical way of implementing this battery monitor is to sample the voltage and set a lower limit. Maybe use a Red-Orange-Green LED indication. I think "there's an app for this" in the LM3914 datasheet, but there's other ways to do it.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 12th June 2011, 11:31 PM   #5
WSJ is offline WSJ  United States
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I use a Link-10 battery monitor in my electric car.
Public Home | WAYNE WANG |, photo and video sharing made easy.
Click the image to open in full size.

User Interface with Graphic and Digital Displays
Link 10's splash-proof front panel allows for hands free operation and performs many functions.

Major functions include:
Digital Numeric Display - LED display shows numeric read out of volts, amps, amp-hours and time remaining, or % charce remaining.
Easy to read multi-color LED bar graph
Splash proof panels allow for outdoor mounting and hands free operation
Displays key historical battery information such as charge efficiency, deepest discharge, and average discharge
Compatible with 12 and 24 volt DC systems
Works with any battery type
Includes DC shunt (part#84-2010-00)
Low battery alarm contacts
One year warranty

Down load Link-10 manual:
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Old 13th June 2011, 04:50 AM   #6
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Perhaps something like this?
Doc Wattson Meter - DC Inline - $59.99

There are other cheaper versions if you look around a bit,I recall seeing one for like $25 or so.
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Old 13th June 2011, 07:10 AM   #7
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It's never my intent to stifle anyone's creativity, but I'm not sure the problem is being approached correctly. If you do include a wattmeter, what meaning do you give the readings? Car batteries are not spec'd for watts or amp-hours, they are spec'd for cold cranking amps. And if you plan to discharge to 50%, a regular car battery won't take kindly to such repeated abuse. The absolute easiest solution is a D'Arsonval voltmeter and knowing your battery.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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