Does the gas engine have to be running to charge the 12V battery? Say you are driving but the gas engine is off, will the 12V still be charging? I have to charge the 12V if the car sits for 7 days but 3 or 4 it starts OK. I drive short distances once a week is all. I am measuring 14.9V on the plug in port when driving, which seems like excess voltage for a 12V battery even if it is charging The port goes dead if the car is turned off so I wonder if it is really the battery there. Thanks much
And his 14.9 measurement probably means that the battery is VERY low and the charging system is trying desperately to bring it back up. Not common to see the voltage that high but not necessarily a problem either. Unfortunately, the system doesn't have the capacity to do that over short runs. At the very least, his battery needs a manual charging and a load test (or 3). But most likely that his driving habits have caused the battery to age prematurely and a new one is needed. An occasional manual charge should keep the new one healthy a LOT longer.
A well charged battery will often start charging in the high 14's when the car is first put in Ready and will drop to the mid 13's when it is charged up. The inverter's dc to dc converter does the charging. The 12v outlet is wired to the 12v buss but is electronically current limited and turned off with the car. Most likely you have an excessive parasitic amp draw on the battery when the car is off. It can be an aftermarket obd2 device, an aftermarket insurance tracker, third party stereo, lights left on like the hatch light or a number of other things. In some cases it has been a keyfob left too close but thats stretching the issue. Usually these cars draw 15-30ma in off state but we see cars with 90-500ma routinely. The higher the ma draw the faster the battery goes dead. A decent battery with an acceptable parasitic draw should last three or maybe four weeks and still start the Prius. Testing and diagnosis is an electrically advanced skill everyone does not automatically possess. Low amp battery maintainers are a stopgap solution for a car that won't last a week.
Yes, even when the engine is off but you are driving , as @ChapmanF notes. More specifically whenever the green “READY” on the dash is displayed, the 12 V is being charged, irrespective of the engine actually running or on electric propulsion. EDIT- 14.9 V is within spec, I believe the acceptable range is 13 - 15 V.
