Hello forum, I want to measure the voltage of lead-acid batteries (as a quick and dirty state-of-charge calculation) and came across these threads:http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1239635198http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1142283743
Now, battery voltage can't go much higher than a few volts above 24V so I think a simple voltage divider will do the trick just fine although if you believe I should go the opamp route please let me know.
Now, I have two questions:
1) I don't want to drain the battery with the tiny but non zero current drain of the divider. One obvious solution is to use some pretty enormous resistors but of course it is inelegant. So, I was thinking, can I do something clever with a transistor? Can't I dump a transistor in series with my resistors and switch it on with one of the Arduino's pins whenever I need to take a measurement (it won't be that often, we are talking time of the order of minutes or hours between measurements)?
If that is possible, what sort of transistor should I use?
2) Any simple way I can I "zoom" into the interesting bit of the voltage? This means the upper 4-5 Volts of the measurement. One very dirty solution I can think of is to make a voltage some 18, say, volts higher than the batteries' GND and connect the Arduino's ground there. However, this is unbelievably hideous as it will introduce a current drain for this new divider as well as making life a living hell if I want to connect other sensors which require a common ground between Arduino and batteries.
Any and all ideas welcome.