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I want to be able to output lipo battery voltage to a display the remaining juice in the battery much like a fuel gauge. It has to work for 1s to 6s batteries and it would be ideal to work using an AVR microcontroller. A lot projects out there are focused on 1s batteries and they normally use a voltage divider connected to ADC. I know for a fact that a voltage divider will drain the battery quickly so you have to use a mosfet or something similar to pulse the voltage. I'm fine if the solution is close to accurate and not really intending on using TSSOP packages or other SMT ICs. I want to be quickly breadboard this and prototype as well.

Any schematics or thoughts about how I can achieve this variable voltage sensing ?

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    \$\begingroup\$ "I know for a fact that a voltage divider will drain the battery quickly" - This is very subjective. In reality, 10k or higher resistors will not be an issue really. The self discharge of the battery is likely higher at this point. \$\endgroup\$ – Gustavo Litovsky Dec 17 '12 at 23:04
  • \$\begingroup\$ Thats true. you have a point. \$\endgroup\$ – nixgadgets Dec 17 '12 at 23:06
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    \$\begingroup\$ From my experience, many people attempt to go to low power needlessly. The benefits of low power are usually more obvious for very low duty systems where active mode is rarely used. Otherwise, the active mode is really what takes out all the power. I doubt a few uA of current will make significant difference. \$\endgroup\$ – Gustavo Litovsky Dec 17 '12 at 23:09
  • \$\begingroup\$ If I get it right, you are in RC car models. Just use a 1 to 10 voltage divider with a total resistance of 100k and you are good to go. \$\endgroup\$ – Vladimir Cravero Dec 18 '12 at 8:23
  • \$\begingroup\$ close. im into autonomous robotics. \$\endgroup\$ – nixgadgets Dec 18 '12 at 21:20
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While measuring voltage is possible with an AVR, its non-trivial to calculate the amount of juice left in it. This is done using precise coulomb counting techniques. Maxim makes a variety of these ICs which can talk to the AVR over the Dallas 1 wire bus. These ICs are very low power and feed off the batter directly, which lets them stay on continuously even if the AVR is off. They generally include the coulomb counting algorithms which take into account the behavior of Li or LiPo cells, and can compensate for temperature and cell degradation and keep track of charge cycles.

You can look at the DS2780 for an example for such an IC, although it's only for 1S and can be used for 2S. There are other chips of the same family which work for larger arrays.

Note that if you want to be able to actually shut off the AVR entirely and prevent any leakage from the DS2780 via the 1 wire conection, you would need some sort of a level translating buffer sitting in between which you can tristate the output of. Ive used the TI SN74LVC1T45 for this (one in each direction) with success. . Opto isolation would also work but would be an unnecessary power consumer.

This is, however, not a DIP solution. Further, its generally best practice to keep this thing as close to the cell as possible - generally touching it - to make the temperature compensation work well.

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    \$\begingroup\$ So Coulomb counting measures ... Lipo suction? \$\endgroup\$ – Kaz Jan 25 '13 at 23:43

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