Thingiverse While I was working with CAN scanning, I accidentally drained my car battery to the point where I could not crank the engine. I decided to do a little research on battery capacity.
I found on Amazon a little LCD module which shows the battery capacity in percentages. Capacity is calculated by the measurement of the input voltage. I did a quick check and here is the result:
12.65v – 85%
12.55v – 80%
12.45v – 69%
12.45v – 69%
12.35v – 63%
these percentages slightly off from various sources, but for me, it is important to see the trend of overnight discharge level. The “dark current” 100mA in my car is draining the battery.
To take measurements in a convenient way, I designed and 3D-printed a simple enclosure for this monitor and OBD-II male connector from the Sparkfun store. The OBD-II port provides battery power, which is always on, important for this purpose. Location of the connector also perfect for monitoring.
Design of enclosure shared on Thingiverse.
The monitor consumes 5mA, but after removing LED backlight, it drops to 0.22mA. Because of such a low consumption, this monitor can stay connected for a long time without any harm to the battery.
Initial observations:
After 24 hours of inactivity, the monitor shows 80% of full capacity. Turning ACC without the engine on will drain the battery to the 50% level in 10 minutes. But this is under load and maybe it is not an actual capacity value, because monitor calibrated for the open circuit mode (without load).
Nice article about dark current here.
Recently, I left my car for 15 days with a fully charged battery. After 15 days, the battery monitor shows 81%, which is no bad at all.
Gadgets for my MX-5 Miata (ND) 
Connection: The red wire connected to the pin 16 and black one to the pin 5
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