Good gawd

https://www.instagram.com/reel/C5ZmAcCv9j7/?igsh=aHY3MGFtYmxid2Ju

 

using a power tool battery is a great way to fubar every single computer in the vehicle.

power tool batteries average 20v. most vehicle systems are rated to 15v max.

 

That’s the most California thing I’ve seen all day.

 

I've seen a lot of customers that are willing to boost their car every day, inflate their tire every day or top up fluids every day.

Flashy clothes, big house, no common sense.

I'd do this in a desperate pinch, but ideally with a 12-16v

20v will start it, but a lot of ECU's will shut down right away unless you take it off quick.

Nothing will be damaged if the polarity is correct.

 

If the battery in the vehicle is at a fairly low state-of-charge, say, at 8 volts or so, when you apply an 18-20V power-tool pack across the vehicle battery terminals there is only a very brief moment when it is actually supplying 18-20V. Maybe on the order of 10s-100s of microseconds before the 18-20V power pack sags under the extremely low resistance offered by the flat/low vehicle battery.

That is, if the pack is at 19V, the vehicle battery at say, 8V, the difference is 11 volts, then assume the internal resistance of the vehicle battery is 0.02 ohms (typical for a car battery,) the 11V differential will result in a current flow of 550 amps from the 18-20V pack. (About 6 kW.) Never tested an 18-20V 6.0Ah power tool battery but I would guess it sags down several volts (or more) instantly when subjected to that kind of current demand, which would take it conveniently into the safe range for automotive electrical systems.

Separately: Is it reasonable to assert that ECUs start to just keel over and die at 15V, when the normal charging voltage from an alternator can easily be in the 14 to 14.5V range? Doesn't seem legit. I'd think they're able to withstand voltage spikes/overloads to at least 20V or so?

 

Had a guy come in one time and told us that he tried to charge his car battery by wiring an extension cord to his battery and plugging it in and after that his headlights wouldn't turn off and the car would no longer start.
He never brought us the car but I'm sure all the modules were toast.

 

They shut down after 16-18 volts, all modern ECU's have voltage protection. Older 80s and 90s vehicles would get damaged from alternator surges.

Even if the ECU could handle high volts, it makes sense to shut the vehicle down to protect the battery.

 

Welcome to Just Rolled In... customer states...

 

No kidding, I had a hard time containing myself when I heard that story.
He apparently didn't have a battery charger and figured he could just plug the car in to charge the battery