General > General Technical Chat
Under what circumstances can a car battery be too dead allow a jump start?
Berni:
--- Quote from: Seekonk on June 22, 2020, 01:09:56 am ---At my shop mu car battery instantly failed. Had another battery that had be kicking around for a long time. Measured voltage was close to zero volts. Had a charger that could put out 10A at best. First 15 minutes I saw close to zero charging current. Then it slowly increased to about 7A. I was in a rush so I tried that battery anyway afer an hour total. It started the vehicle and I got home. I can only conclude that it takes less than 7AH to start a car.
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Yep it doesn't actually take all that much energy to start a car. You can find videos of people starting cars with supercapacitors no problem. Its the kW level peak powers required to do it that make it difficult. So the low internal resistance is the more important part.
Tho im surprised that a battery that sat at near 0V would get its low internal resistance back after so little charging. Perhaps its the very time dependent nature of lead acid cells that made the battery feel like its fairly full right at the plates because it was just being charged a moment ago.Or maybe it was not sitting empty for long enough to develop the internal resistance problems as badly.
Rerouter:
The more modern a car the more time you will want to give it to get the original battery back in an operating state. If below 6V. Give it a solid 10-15 minutes charging off the donor before you try jumping. Otherwise load dump when you disconnect the donor car can cause issues. The goal is to get the original batteries esr down low enough where it can handle it without spikes or dips.
And where possible use a red texta or similar to make it clear as possible which side is positive so when your heading over to jump your freinds car in the middle of a rainy night you do not have that risk (a stupid number of cars have both battery lugs black or unmarked)
tom66:
--- Quote from: SilverSolder on June 19, 2020, 05:46:02 pm ---Not everything in the car is tested to that standard. A friend had his alternator fail in a newish Ford... on the way to the dealership to get it fixed, things started to fail as the battery got depleted... the radio turned off, the instruments stopped working, by the time he got to the destination, the ONLY thing still working was the engine - and it quit just as he rolled up to the garage door! (At least Ford had the priorities right with that design.)
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Pretty much the same thing happened with my old Peugeot. The alternator would crap out on that car until you could restart it. I was stuck on a motorway with miles to the next exit. First, the rear window demist function is disabled, then the cabin fan and A/C clutch, then the alternator light comes on, then the radio goes off, then all the instruments died. By the time I found somewhere to safely pull over the only thing running was the engine and the single check-engine light on the dashboard.
This is one of the interesting things about owning a plug in hybrid (I have a Golf GTE now.) The car doesn't have a conventional starter. It uses the 400V battery and electric drive motor to spin the engine up. Consequentially, I've got in the car when it was -4C out and started the engine from stone cold. It didn't even cough, because it can put 10kW straight into the engine to get it to turn over. The diagnostics show that the car engine is at 800 rpm before any spark is fired so it starts pretty much at the speed it idles at. This contrasts to most starters which don't get over 200-300rpm because they can't get enough current from the 12V battery. But a disadvantage is that VW tells me that the car cannot be used below -30C as the li-ion battery cannot safely provide that much power when cold. So if I move to Norway I might have trouble!
Berni:
Well good luck getting a diesel engine started at -30°C too. Not only does a lead acid battery have trouble providing a proper amount of power when so cold, but the engine becomes really hard to turn over with the thick oil and trying to compress all that dense cold air. In places that do get so cold people like to install electric coolant heaters.
Tho when it comes to electric cars with a HV Lithium battery you can get a failure of the 12V DC/DC converter. Pretty much everything in those cars still runs off 12V (And besides running 400V DC to everything is very unsafe) so you also get a similar situation where things stop working. In once case i heard of this happening it was much more sudden tho. It started off as a error message on the dash, but a bit of driving later all of a sudden the car turns off, lights go out, power steering goes away, assisted breaks go away all until the car coats to a stop and can't be turned on anymore. The 12V battery is used to switch on the safety disconnect relays in the main HV battery pack, so no 12V is no power anywhere.
Psi:
--- Quote from: nctnico on June 16, 2020, 10:33:41 am ---
--- Quote from: tom66 on June 16, 2020, 08:20:50 am ---I carried a jumpstarter in the car after that, but another 6 months of ownership, it never happened again. So I am really confused as to what happened to cause the battery to die on only one day.
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More likely a poor battery contact.
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+1 to this.
The clip connection to the terminals is where you get most of the voltage drop.
It's one reason why they suggest you clip negative onto the engine block somewhere instead of battery neg.
It's easier to get a good connection and has a lower resistance path to the starter motor.
If you want to jump start cars easier, try super caps.
The lower ESR means you get much less voltage drop under a 600A peak draw and therefore more voltage gets to the end of the cable.
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