- Labs project
Cheap, accurate 12V battery checker for solar banks 
August 20, 2014
This little idea came about when I was thinking about an accurate way to constantly measure the voltage of each battery in my solar battery bank. A lead acid battery's voltage remains fairly constant as it discharges. As the attached chart indicates, a no-load voltage of 12.6 volts means 100% charge but 12.2 volts means you're down to 60%. So it's desirable to have a cheap method of high enough accuracy so you can reliably keep a tab on each battery. "Cheap" suggests the many panel displays available on eBay for under $2, but the accuracy of these is usually not better than 1-2% which is not good enough. Ask for 0.1% accurate panel meters and your budget goes up considerably. The attached circuit allows you to stay cheap and accurate. These panel meters typically have three terminals - Gnd (-) Vcc (+) and a measurement input (T). The LM4040-10 drops exactly 10 volts with better than 0.1% accuracy, and what is left is fed to the measurement terminal of a 0-10V panel meter which then shows how much above 10V your battery is at. If the meter reads 2.5V, your battery is at 12.5V. The meter even if really poor, contributes an error of 2% of 2.5V or 0.05V, the LM4040 contributes a max of 0.01V, so your error is not more than 0.06V. Good enough for the purpose. The 47K resistor ensures that the LM4040 has more than the minimum of 60 microamps required for it to work reliably.
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