Start with low 12v battery

If the pump is draining your battery, you need a new battery.

 

You are absolutely right. If you come to your car in the morning and if after opening the drivers door the pump manages to get your battery so low, that the car does not start - you definitely need a new 12V battery.

But if you left your courtesy light on the day before (or two), you see it before you enter the car, you could want to know how to try to start your car without the pump running _unnecessarily_ as you are getting in

I just tried it - only the drivers door triggers the pump right away. Of course will the pump start pumping if you step on the brake (I think), but the more it pumps, the more the battery gets down.

If you manage to start the car, you are good. If not, you can prevent the pump starting by opening the drivers door and draining the battery even more.

I usually do manage to start a prius with a small (a pound or so) 12V battery and really small cables, if the battery is not drained to the bottom.
But it happened to me that my small cables melted the isolation on them, even before I tried to push the start button on a Prius with the courtesy light on for a week

This is of course an academic discourse about situation, which should not happen with a well maintained car, but, you know, ..it happens

Mat

 

Cliff notes version:

Take battery to big chain auto shop, they’ll charge it and test it free. They ll give you a slip of battery condition.

If you have that only vehicle for transportation, get a battery charger and top off battery. If battery still has same symptoms after a week or two, get a new one.

You shouldn’t be pulling relays abs disabling pumps if you don’t have the expertise to troubleshoot 12v battery issues.

 

The 12-volt battery can be fit as a fiddle and pretty much anybody can walk away from the car one time with an interior light on and come back to a deeply discharged battery. I've done it myself—including just last summer, when I left the car in a parking garage a block away from a conference and didn't return to it till the conference was over.

I definitely disconnected the white cable before calling on my small jump pack to handle that situation.

I succeeded without fussing with the brake pump at all, but I was feeling fairly confident (summer, fair weather, jump pack recently charged). If I had been in more doubt of any of those things, I might have pulled the brake pump fuse too, just to further boost the odds of a successful outcome.

It only takes a few dozen amps for a second or so to make the car READY, and the jump pack doesn't have to turn a starter, but it does have to power all the car's 12-volt loads that happen to be turned on for that interval, until the car is READY and the DC/DC converter takes over. The brake pump is one of the heavier of those loads.

That's a pretty good illustration. May only need a few dozen amps for a second or so to READY the car, but if your jump source has more capacity than that (and some advertise really crazy peak current numbers) and the drained battery in the back of the car is hungry enough, it'll scarf down whatever the jump source has.