Thread: P2140; Low Battery Voltage (LO_BAT) setting in PSUMoni

Vehicle voltage will swing normally from the 11's to the 14's based on whats happening. A running vehicle with a properly performing charging system will run low 14's with a low load especially at RPM's above idle. Turn all the junk on (HVAC, wipers, Headlights, radios, heated everything, etc.) and you will likely idle in the mid 13's.
Idle around town with all of this stuff on, roll into your driveway, shut it off, leave it overnight and restart it in the morning on a cold night with the heater running immediately on restart and seeing low 11's or less while cranking is not out of the question.
Having said all of that, if you are concerned that your vehicle seems to run low voltages more than it usually does, take it to a trusted shop for an AVR test. They will check your electrical system and let you know of any issues with battery, starter, alternator and all connections.

Yep - especially PhilG's last "all connections" bit.

Big variations can be due to bad connections.
That's simply a case of varying voltage DROPS as current and resistance changes (from V=IR).
In a stable connection system, voltage (drop) simply varies with current. (Hence reduce cabling & connection resistance, eg, the big-3 or suitably big cables; minimal and clean connectors & fuses.)

If unstable due to intermittent contacts etc - hence resistance varies from zero or low to infinity (open circuit), the voltage drop varies from low to the supply voltage (ie, current drops to zero).
In non-techno-bab, this just means clean, fix & check your distribution from battery +ve terminal back to the battery -ve terminal, and the alternator circuit.


The leaves the battery and charger.
A discharged or old battery will have higher internal resistance (etc) and therefore a big voltage variation with load current.
The charger's voltage can vary with load, and for alternator's - speed as well.

During cranking, a good battery might drop to under 9V.
A fully charged rested and unloaded (open-circuit) 12V battery should be 12.6-12.7V.
An open circuit & rested 12V battery below 11.6V is VERY discharged and needs recharge ASAP, else is faulty.


You may be running into the dilemma faced with voltage-sensing switches (eg, "smart battery isolators") - what is an acceptable voltage dip and for how long?

Perfect, after I installed the carPC my tail light indicator came on. I dont have any burnt out bulbs back there so I figured I must have loosened a connection or pinched it or something. I guess whatever it is, is most likely causing these voltage spikes and drops?

Yep - a bad ground, or bad connections.

Break & re-make all connections involved - that can overcome corrosion etc.
Remove/replace bulbs; check for water; perhaps spray with INOX etc.

Normally the first check to locate problems is to rotate all same-sized fuses - that breaks & makes contact and rotates any intermittent fuses to another circuit.

And don't forget the old Zeroeth check - battery terminals and ground connections.
Do not assume metal to metal is a contact!
(Zeroeth... (C) Asimov???)


PS - I'm assuming a good separate power circuit is being used for the PC.