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When using my various electronic devices, they always seem to know how much charge is left in the battery. They also know when the battery is completely charged.

Not exactly sure how this is measured, but I assume that it is based on the voltage of the cells. Is this drop in voltage linear? If it is linear, then how do iPhones and iPods still run at the end of the charge with so little voltage?

Edit: Bonus points for useful graphs.

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  • \$\begingroup\$ For common chemistries, it is not linear, though portions of the discharge curve may be approximately linear. Voltage usually plummets precipitously as the battery nears the end of it's useful capacity. Devices measuring "remaining capacity" have to take the shape of the discharge curve (and ESR) into account. \$\endgroup\$ – RedGrittyBrick Aug 19 '13 at 14:25
  • \$\begingroup\$ People who care about %energy remaining use "gas guage" ICs + sensors to measure current inb and out. | harge terminatin depends on chemistry. Lithium chemistries usually charge at constant current to a preset voltage (eg 4.2V/cell LiIon) and then let current tail down at constant voltage to a % of Imax. | NimH and NiCd use negative voltage change under constant current OR cell temperature OR cell voltage if desperate OR max time to detect endpoint. | Lead acid variants vary and use a complex dance of CC CV and boost and float and double Mason's handshake. \$\endgroup\$ – Russell McMahon Aug 20 '13 at 23:31
  • \$\begingroup\$ Temperature and history and charge and discharge rate and construction style and sub material (eg Lead calcium) and electrolyte type and concentration and much more + magic all matter. | Large amounts of informed experience is the main component of a good battery management system. || Read all & assimilate :-) : batteryuniversity.com \$\endgroup\$ – Russell McMahon Aug 20 '13 at 23:35
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It's better to measure charge in and out to determine charge state. You can get ICs that do this for you, e.g. http://www.ti.com/product/bq27000 : it works by having a tiny ammeter connected on the output of the battery pack.

It's usually possible to determine "fully charged" from the EoC "end of charge" conditions of the battery, and the charging logic must make this decision to stop charging.

Lithium battery chemistries do not like being completely discharged; usually the battery pack has extra circuitry that disconnects it at a particular voltage drop. The ones I've been using do this at 2.8V.

The variation in voltage from "charging" to "fully discharged" is just something that you have to design for. Usually there will be a buck/boost converter that turns the actual battery voltage into a steady 3.3V or whatever voltage is needed for the logic.

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