Battery voltage fluctuation

[m4rkw]

Sat idle my car's Battery voltage, as read by the custom arduino-based tracker I've got in it, is fluctuating between around 9.6v and 11.5v. Does this suggest a bad Battery? It didn't struggle to start earlier but it seems odd for it to be fluctuating continuously by 2v.

[Britprius]

That Battery voltage is very low suggesting at leased one cell is short circuit. A fully charged good Battery should read 13.8 volts with no load.

It is possible that whatever your using to measure the voltage is not accurate, or the point at which your measuring has some load on it giving a voltage drop. The voltage should be measured at the Battery terminals with everything turned off. There will still be a small load on the Battery from the security systems, but I would expect for a good Battery for the voltage to be at a minimum of 12.6 volts, and preferably over 13 volts.

Measuring the voltage anywhere other than at the Battery terminals will give fauls readings, also if the engine is running the voltages will be higher.

John.

[m4rkw]

Thanks, this is as measured by an arduino-based tracker running custom firmware that I've designed for it. It was lifted and shifted from my previous car, a vw passat, where I never saw this happen. The voltage on that car always just very gradually dropped when the car was idle until it died around 9.6v and the tracker went offline (when it was parked at an airport for a long holiday).

Strangely having inspected the logs from the Lexus it was alternating between about 12.5 and 9.6 for a 10min period between 9pm and 9.10pm last night, and then since then it hasn't dropped once. Very odd.

[Britprius]

2 minutes ago, m4rkw said:

Thanks, this is as measured by an arduino-based tracker running custom firmware that I've designed for it. It was lifted and shifted from my previous car, a vw passat, where I never saw this happen. The voltage on that car always just very gradually dropped when the car was idle until it died around 9.6v and the tracker went offline (when it was parked at an airport for a long holiday).

Strangely having inspected the logs from the Lexus it was alternating between about 12.5 and 9.6 for a 10min period between 9pm and 9.10pm last night, and then since then it hasn't dropped once. Very odd.

The Lexus security system takes at leased 10 minutes to settle, and reduce it's load to a low level. Opening a door, operating the keyless entry system if fitted or the fob is near enough to the car for the car to recognise it, or operating the fob buttons will produce more load for 10 minutes.

Where is the Arduino connected to the car (where is the voltage being measured) ?

John. 

[m4rkw]

Ahhh that's probably what it was then - I had nipped out to check something in the manual. I'm not sure sure where it's connected as my car audio guys did it but it should be a permanent 12V feed.

[Britprius]

To be on the safe side I would check the actual Battery voltage at it's terminals with a multimeter against the Arduino readings. The wiring on all Toyota/Lexus vehicles is very thin, and as such has a fair amount of resistance leading to voltage drop. This is particularly noticeable on the headlamp circuits where voltages at the bulbs often only reach 10 or 11 volts resulting in poor light output from any of the filament type of lighting. HID lighting is fed by inverters that have a regulated voltage output almost irrespective of the supply voltage.

John.

[ColinBarber]

20 hours ago, m4rkw said:

Ahhh that's probably what it was then - I had nipped out to check something in the manual. I'm not sure sure where it's connected as my car audio guys did it but it should be a permanent 12V feed.

Possibly a poor connection to the 12 V feed. As John has said, it would be worth measuring the voltage across the Battery to get a true reading. Leave the vehicle overnight and check in the morning before starting the engine - you should have at least 12.5 V.