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| Under what circumstances can a car battery be too dead allow a jump start? |
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| SilverSolder:
--- Quote from: scootley on June 16, 2020, 05:43:47 am --- [...] Now assume the car's battery can only do 10A at 12V due to being previously discharged too much for too long without a charge. I guess this causes a difficult-to-reverse damaging chemical state (sulfation, the crystallization of lead sulfate) and high internal resistance. [...] --- End quote --- The battery is toast if it has been fully discharged for more than a few days, even if it was a brand new battery in excellent condition. (Ask me how I know...) Even if you get it back up to 12V, it is a shadow of its former self in terms of capacity. |
| Rick Law:
--- Quote from: SparkyFX on June 16, 2020, 11:21:15 am ---... There is also an airport parking lot phenomenon, when plenty of remote controls are used around the car that constantly wake up receivers. Not really a day to day use case. ... --- End quote --- Interesting. I heard from a friend his reasonable new car (~6 months) was parked at the airport and when he was back in just 5 days, the battery was stone dead. Can't even be jumped and had to be towed back to the dealer... That it could be jumped puzzled me a bit. At the dealer, he just needed a new battery, and according to him, something about ECU that the dealer has to do in order for it to understood how to make this car works again. |
| SilverSolder:
--- Quote from: Berni on June 17, 2020, 05:17:29 am ---Yeah for a short period of time the starter might draw many >500A to get itself going from a standstill. When the motor is not moving the only thing limiting the current is the resistance and with a high power motor such as a starter this resistance must be very very low by design. Here is a typical automotive cold cranking test waveform. It is used to test automotive electronics, so that they don't crash or malfunction during cranking ( https://www.ti.com/lit/ug/tidub49/tidub49.pdf ). (Attachment Link) So they expect the voltages on the battery to really dip low. --- End quote --- 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.) |
| Seekonk:
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. |
| SilverSolder:
--- 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. --- End quote --- That will give you 250A for a couple of seconds - enough to start a car! |
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