12v battery Issues

There is 12V on the OBD port without the car turned on. My Scangauge will read it if I just press the "on" button on it, car off. The Scangauge reads a few things -not- on the digital bitstream with its own A-D converter.

 

That works, but when I do that, even with all reasonable precautions, my Scangauge shows about 0.2 volts below the voltage at the battery terminals.

 

It's the voltage drop between the battery terminal and the OBD2 port due to copper and connector resistance.

 

Connectors, maybe. Voltage drop due to copper resistance should be negligible, with such a tiny current. Another possibility is that the thing is simply not very accurate as a voltmeter.

 

ScanGuage is just reading values through OBD, but yeah, it may be reading inaccurate values. I've noticed some differences, reading with mulitmeter at jump point, vs directly at battery cable. Strange, if I recall correctly: one time voltage readings were slightly lower at the battery.

 

You were reading while the car was in Ready mode, in which an inverter from the HV battery is charging the 12V battery.

Try it again, but with the ignition in Accessory mode instead so that inverter is turned off. This is most meaningful first thing in the morning before the car goes into Ready at all. Or at least when the car has been shut down for several hours.

ScanGauge is measuring the voltage itself, at the OBD port. That is how it can read voltage when the car and ODB data are not running.

 

The power on the OBD port may be going through a diode in the computer (well, -a- computer), in which case there is some drop. I've also noticed the Scangauge reads lower than I would expect, but not enough to worry about. I watch for a loss of a cell, and that's 2 Volts. It's accurate enough to show that.

 

buy a ten dollar dvm.

 

^ ^ ^ What he said! ^ ^ ^

 

good move!

 

Having and knowing how to use a meter is good. Let us know what battery you end up buying too.

 

You might want to get a Solar BA5 tester (or similar) too. It does a quicky test of the battery's capacity, Cold Cranking Amps or several other. Also displays voltage.

Or maybe not: if you're a battery hypochondriac like me, it can be too much information, lol.

 

congrats, all the best!

 

Watch the battery voltage carefully. Check it first thing in the morning, -before- you put the car in "ready". I "fixed" the battery in Pearl, a 2007 Prius, in 2010 by adding water. It lasted till this spring. Unfortunately the add water fix didn't work on Pearl S (a 2012 Prius), it looks like the battery is toast. This morning, after sitting for 6 days of no use, the battery was at 8.8V (as seen on the Scangauge) when I tried to put it in "ready". The car went into "ign on", not "ready". No joy. I had to drive the FJ Cruiser! The battery in that vehicle is also original, and 2007 vintage, but still working fine. The Battery Tender is on Pearl S and I'm shopping for a new battery. Wish I could buy one like the FJ Cruiser has, a Panasonic lead acid with removable caps, but they are not sold in North America. Too expensive I guess to compete.