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What is the most energy efficient charge indicator for a 12v battery? how many mW does it use for every state of charge?

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    \$\begingroup\$ Start by asking yourself what measurement is necessary to indicate charge level of your battery and how precise do you need to be in this measurement. \$\endgroup\$ – Andy aka Oct 28 '18 at 10:14
  • \$\begingroup\$ A very vague precision is fine, i.e. 4-5 LED's, 0.2v precision. \$\endgroup\$ – com.prehensible Oct 28 '18 at 12:52
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    \$\begingroup\$ Pick any kind of indicator and have it powered through a push button. There are many charge estimation chips by benchmarq and others, they are intended to draw very little current. \$\endgroup\$ – KalleMP Oct 28 '18 at 18:04
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"State of charge" is not a simple measurement. A DC voltage measurement will give some clues but will show different readings for the same battery when switched between charging, no load, and load. If that is satisfactory then LCD is probably the best technology.

enter image description here

Figure 1. LCD displays are low powered.

A random look at the Lascar EM32-1 panel meter which can be configured for 9 to 14 V DC measurement shows a current consumption of 150 μA in that configuration.

$$ P = VI = 12 \times 150\mu = 1.8\ \text{mW} $$

Any LCD voltmeter without a permanent backlight should give similar(ish) results.

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  • \$\begingroup\$ Is there a way to switch it off to zero mW if the voltage drops to less than 9v? \$\endgroup\$ – com.prehensible Oct 28 '18 at 12:13
  • \$\begingroup\$ There are various means of doing this but the monitoring circuit is going to draw some power. Your question is missing too many details such as battery chemistry, capacity, and the purpose and use of the monitor. \$\endgroup\$ – Transistor Oct 28 '18 at 14:29

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