Well, shoot. Your car battery went dead. Fortunately, a knight of the road came by and gave you a jump, so now the engine’s back on again. That means your alternator is on, too, which should be charging the battery, right? So how long do you need to sit there while it does?
Unfortunately, it’s not like the old days anymore, when all an alternator did was charge the battery and run the lights. These days the alternator also needs to handle all the electrics in the vehicle, from the air conditioning to the power windows to the infotainment display to the radars and — on some cars — even the digital dashboard. It’s got a lot to do, in other words, leaving precious little capacity left over for actually charging the battery.
In fact, if the engine is idling, the alternator is basically not charging the battery at all. To increase its amp output, you’ll need to increase the engine’s RPMs. That is, you have to drive the car, not just leave it parked.
Recharging a car battery
To maximize your alternator’s charging, you’ll want your engine RPMs to be high and steady, which is to say, highway-speed driving. How long do you have to do that for? Conventional wisdom says around 30 minutes will give your battery enough charge to start the engine again, but that’s cutting it pretty thin. To be safe, best to stay on that highway as long as you can. If you really want to top up the battery, you’re going to be out there for as much as eight hours — although just to be clear, it is not possible to fully charge the battery while driving.
If driving through highway traffic for hours on end is not your idea of fun, your next best bet is to buy a dedicated battery charger. These are designed to safely bring your battery back up without damaging it. That said, this process can take many hours, so you’ll likely leave it connected overnight at least. If that’s not possible for some reason, there are some more far-flung options you might consider such as hooking it up to a solar panel.
If you’re curious, hybrid cars actually do have a 12-volt battery, just like an ICE car. So if you need to jump start a hybrid, the process is basically the same. It will obviously take a lot longer to charge the main hybrid battery, though.
