Car battery that is. It's a fairly dependable gizmo, except in winter when your car fails to crank. They last four winters on average. A battery works like a bank account, you put juice in, and later you can take juice out. If you fail to put enough juice back in after a hard winter start, the battery may not be there next time. In winter you need to run the car engine for as much as a half hour after a cold weather start.
Batteries are temperature dependent, they work much better when at room temperature than they do at - 40 F. A cold winter morning might have the entire car chilled down to -40 F. Wait until early afternoon and things might have warmed up to a mere 0 F. This might not help if you gotta get to work at 8 AM, but if you just need to go the store, wait til things get warmer. Brush the snow off the car and it will soak up sunshine and get surprisingly warm. Keeping the car in even an unheated garage will keep it 20-30F warmer than parking it outdoors. Starting is a lot easier at 0 F than at -40 F. And after getting her started, be sure to run her long enough to charge the battery up.
Lotta new cars now come with a battery voltage gauge or indicator. A new fully charged battery might show 13.2 volts. This "sulfation charge" will go away, dropping the battery voltage down to say 12.5 volts after just a whisper of discharge, say running the head lamps for 10 minutes. Call 12.5 volts normal full charge. As the battery discharges, the voltage drops. By 11 volts, you have trouble, your car may not start next time. At 10 volts it surely won't start.
When the engine is running, the alternator will maintain 14-15 volts on the electrical system, it has too, the battery won't accept charge unless the alternator voltage is a volt or two greater than battery voltage. If you don't have 14-15 volts with the engine running, you have alternator trouble, and shortly you will have a discharged (flat) battery and the car won't start. If the alternator has been doing it's job, and the car won't crank, you have battery trouble. They only last four winters, and maybe yours is just shot and needs replacement. Last new battery I bought set me back $50.
Naturally, you need the engine off, to see the battery voltage. With the engine running, you are seeing alternator voltage.