Check your Bat voltage with the key off. All lights and power drains off. A fully charged battery is 12.65V. you will likely get 12.3 -12.6V. If it is lower something is dragging it down or your battery needs charging. The quetion is what and why? You may have a parasitic drain. there are lots of posts on this issue. Aftermarket stereos, alarms etc. are notorious for being the issue.
Here are some simple things I have learned to do with just a cheap DVM and some leads with spring clips or aligator clips. It may help you to find an issue or rule many out.
With the car idleing check the Bat voltage. Should be around 13.7-14.7 volts.
With the engine on, Turn on all of the accessories. Turn on high beams, blower on high ect
. Voltage should be .5 volts over the Bat voltage. When I have done this on my Sable I get around 13.5 volts. The ECU
can influence voltage but I don't know enough to tell you if that is an issue. Maybe someone knowledgeable can weigh in here.
To check the voltage drop on the neg side. Again, with the engine running, Turn on all accessories, EXCEPT the Rear Window Defroster. Have a helper hold the engine to around 2000 RPM
and check voltage from Neg battery terminal to the alternator case. If it is More that .2 Volts you have a bad grounding connection. Check all grounding straps and ground connections.
To check the voltage drop between the alternator and the battery put your voltmeter between the POS battery terminal and the Positive lead on the alternator. With the engine at idle it should be less than .5 volts. If it is more check the wire for weakspots and all the connections in between.
Temperature does effect these tests. It should be done between 60-100 F.
If it is colder you will get higher voltage drops as the resistance of everything goes up as it gets colder.
Best of luck.