Hi, before I buy a new alternator I just wanted to double check with you guys. When I start the car and for ~5-10 mins the voltage is 14.8 (I replaced my cigarette lighter with voltage display and 2 usb ports), after that it drops to 12.8 - 13.2. Even with everything ON (heater, lights, etc.) the voltage is not going up. Could this be that once the alternator is warm enough something happens and it just cant produce the voltage? The weird thing is that if I switch off the car and turn it on again, the voltage reads 14.8 again for a period of time. Any ideas?

 

https://www.jaguarforums.com/forum/x...nation-202162/

 

my low voltage setting drops way below 13.6 so this means I should get a new alternator, right?

 

Hi RADas-

Just reading your thread and can I share some initial thoughts please.....
Before you go and buy an expensive alternator you probably should verify the voltages you are seeing with a quality meter measuring directly on the battery terminals.
Two things that might be tricking you into assuming a fault......

• You do not know the quality and precision of the monitoring gadget you have plugged into your cigarette lighter. Just because it has a digital display, doesn't mean it is accurate.
• Also the cigarette lighter is separately fused and is some length of wiring from the battery itself, so you could be seeing a false reading there anyway.
• It seems you are reading about 0.5V lower for both the boost voltage and for the standard charge voltage, hence my suspicion of your measuring device or connection point.
• If your battery was not being sufficiently charged then you would be having lazy starting and other issues with the internal systems of the car as they can be a bit fussy on a good battery voltage state.

From the link Dell pointed you to, it is evident that the Alternator is commanded by the ECU and has two charge levels that it can apply from the alternator's internal regulator.
The high level (15.3 volts) is requested initially by the ECU straight after a start sequence to replace the bulk of the heavy discharge the battery sustained. The length of time that high level is applied to the battery will vary depending on the ECU command to the alternator.
The ECU is looking at the relevant ECT (Engine coolant Temperature) and AIT (Air Intake Temperature) to help it decide how long the High charge state is to be requested.
If the air or coolant is under 5 degrees the ECU will prolong the high charge state.
If the Air and coolant sensors are giving a similar temperature reading (within 3 degrees of each other) the ECU will deem the vehicle has been sitting for an extended period and expect the battery to need a longer boost period.

If you had an oddity with either sensor your charging stage could be affected, however you would also be getting either false readings of your engine temp on you dash gauge or ECU fuel mapping would be skewed if intake temperature was being misread, leading you to perhaps rough running engine or poor fuel economy.

Please check with good meter on the battery terminals before you spend any serious time or money chasing a problem that might not exist.

Automotive batteries like to work out of their top 20% of their available capacity....they are not generally engineered for repeated deep discharge, they lose capacity and life expectancy when operated in lower capacity states.