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I'm building a mobility music box, powered by a car battery (12v, 74Ah). What can I build into this box to show the current charge state (read somehow from a display if the battery should be changed)?
Will a simple voltage or ampere meter be enough?

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closed as too broad by Leon Heller, PeterJ, Daniel Grillo, Lorenzo Donati, Dave Tweed Jul 5 '15 at 20:13

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ A Google search would have provided the answer. Question should be closed. \$\endgroup\$ – Leon Heller Jul 2 '15 at 18:51
  • \$\begingroup\$ @LeonHeller you are right and I googled. However as an non learned electronic there where to many different solutions and answers which I didn't understand, there were differences for different batteries, some said it could be done, some said it wouldn't and I ended up not getting anywhere, so I tried asking here to get a straight forward answer, which I got :) \$\endgroup\$ – Kimmax Jul 2 '15 at 19:22
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    \$\begingroup\$ As it stands, it is overly broad and shows no research effort. If you want us to help you show us what you came up with. It is not enough just to say you did search, show us the alternative you have found, show schematics or block diagrams, show the pros and cons you think you have discovered, etc. etc. \$\endgroup\$ – Lorenzo Donati Jul 4 '15 at 10:13
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A car battery has some very specific charge/discharge characteristics. In general, it is not simple to accurately measure the state-of-charge without resorting to physically measuring the specific gravity of the electrolyte. One can attempt to measure the voltage, but many factors will affect it's accuracy, including temperature, load, past loading, amount of time left to rest, etc. That might be fine for a low-accuracy idea of how much battery is left. Here is a fairly straightforward implementation. Here is a more rigorous one, and a very rigorous one. Google "12v lead acid battery indicator schematic" or similar.

One electronic way of doing it would be to measure the amps used by the music box and average this over time. If it's a 74A/h battery, and over the past week the music box has operated for 16 hours whilst drawing exactly 2A of current, then (16*2) = 32A/h of the battery capacity has been consumed. 32A/h / 74A/h = 0.43 * 100 = 43% of the battery is consumed. 100 - 43 = 57% remaining. Doing these calculations can be done many ways, most today utilizing a microcontroller such as the AVR, PIC, or Arduino.

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