# Calculate current drain from the battery in a drive

Control module voltage is one of the OBD PID. It signifies the car battery voltage. Is there any way to calculate how much current is drawn in a drive (say 7 km from my office to home). The voltage will keep on decreasing, increasing in this drive, but I am interested to calculate how my current (or rate of current) is being drawn during discharging of the car battery. The calculation would involve using OBD PIDs (most preferably the generic Mode 01 PIDs).

• I don't think any production car does this with generic obd2 PIDs and the only manufacturers I know that do measure battery current draw are on GMs and Hondas. – Ben Feb 13 '16 at 22:08

The notion that battery voltage will reduce while driving is incorrect.

All modern production vehicles will boast alternator acting as a electrical generator to maintain battery voltage. The battery cannot sustain sufficient charge for even an hour in the absence of a charging issue.

As such, control module voltage will remain steady during normal operation and cannot be used to perform any kind of electrical energy consumption calculation.

• Yeah, sorry my bad. I meant how will I come to know if my battery is draining faster compared to normal rate while doing operations during driving like listening to the radio, giving indicator lights etc. – Soumya Sen Feb 13 '16 at 11:51
• You can't because the battery is charging while the engine is running. Plus, the OBD-II PID's do not provide you the necessary information. It's like someone trying to assess the condition of a mobile phone 's battery while it is being charged. – Zaid Feb 13 '16 at 11:54

On some vehicles you can monitor duty cycle on time of the alternator control circuit or alternator load. Both are usually an on/off signal that will give you a percentage. Some flaws in this are something like A/C on will cause 100% whether needed or not and hard acceleration ( like going uphill ) the car will disable the alternator 0% to give you more engine horsepower ( the alternator is a load on the engine ).