360modena_cs:
Sensors of the arduino, an LCD 16x2, and the Arduino.
Just so you know, The official stance on supplying power directly to the 5V pin on the Arduino Uno is thus:
5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
I myself have tried it with mixed results. Indeed, sometimes it works, and sometimes it fries the 5V regulator. I know others here have done it successfully with a regulated supply. But if you're going through so much trouble to get the voltages right, and get them monitored, you might reconsider this, and just drive the Arduino from the 6V battery to VIN. Try it. Whatever the result is, worst thing that will happen is the battery voltage not run the system as long as you hoped, but it just may do better than you expect too.
Just so you know, I've recently completed a fairly extensive project where I'm feeding the Arduino about 12VDC, and am using the arduino's 5V out to power a 16 x 2 LCD (with I2C backpack), a DS3231 RTC module, and at least 1/2 dozen LED at about 5mA each. Maybe this might work for you too? I think someone pointed out that the Lead Acid cell will deliver about 6.4V, and will be about dead when it gets to 5.4, remember?