Nominal voltages for the battery and alternator

Tangent: There are a few cigarette lighter devices that display the vehicle's voltage. These devices are really useful. [1] [2]

• Voltage when car has been off for days, or the battery is new   (this measures the battery's long-term 'health')
  • a lead-acid battery
    • < 12.4 — The battery's lifespan is being shortened (due to sulfation).
    • < 11.8 — The charge is extremely low. You might take some take some steps to try to "recondition" or "rejuvinate" it. (see bottom section)
    • < 10.5V — The battery is severely damaged.
    • references: [1] [2]
  • specific types of lead-acid batteries:   Flooded battery, AGM battery, and GEL battery [1] [2]
    • references for voltage: [1]
• Battery voltage when engine is off   (this measures the battery's current charge level)
  • See the 'float' voltage here. The float voltage is the ideal voltage if the battery were fully charged.
• Battery voltage when engine is on   (this measures the alternator's health)
  • It should be ≥ 14.0 V, though 13.8 to 14.5 V is probably acceptable.
  • Jiffy Lube says the acceptable range is 13.5 - 14.5 V.

Additionally, you can measure the parasitic current [2] to see if the battery is being slowly drained while parked.

What kind of maintenance can I do, as a consumer?

• If the vehicle is going to sit unused for a long period of time, attach a battery maintainer to the battery. [2]
• List of suggestions.
• There are a variety of suggested ways to "recondition", "rejuvinate", or "desulfate" a lead-acid battery. As far as I can tell, none of them are actually helpful.
  • Pulse charging. This graduate thesis concludes that it doesn't provide any benefits over a normal charger.
  • Signs that some reconditioning might actually work? [1] [2] [▶3] [4]