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Under what circumstances can a car battery be too dead allow a jump start?
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scootley:
There is a lot of lore online about car batteries and jump starting.  So I am looking for technical clarification from those with expertise in this area.
I believe a typical non-electric car requires an initial peak of a couple hundred amps (at 12V) or more to spin the starter motor at sufficient speed to initiate combustion in the engine.

I am most interested in a scenario where the car's (lead acid) battery is not just weak but dead, meaning that it is incapable of providing anywhere near this amperage.  So for discussion let's take a car that I have tried to jump recently.  The starter motor is rated at 800W (it's a small car), so that would be I guess a steady-state max current of ~67A, but apparently these motors typically peak briefly way above that so let's say 200A peak.

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.  Say it is a generic modern maintenance-free sealed battery (not AGM or Gel) with plates and liquid inside.

Now say one attempts to jump start this car using the following:
1) The healthy & charged battery of another running car which is known to be able to supply more than 200A
2) Jumper cables that are of a gauge and length that would allow 200A to actually make it to the contacts in the dead vehicle at or near full voltage. 
(for example https://www.wirebarn.com/Wire-Calculator-_ep_41.html claims a 4 gauge [5.2mm] cable can send 220A@12V ~22ft/6.7m with less than 10% drop)

So my questions are:
1) Would we expect this jump start scenario to be successful (in general)?  Why or why not?
2) What effect would the presence of the damaged battery in the circuit have on the ability of the starter motor to receive the necessary current?  Could the bad battery somehow actually sink a significant portion of the power due to its resistance (or some other property) and prevent it from getting to the motor? 
3) What is the minimum viable contact surface area needed between the jumper clamps and the battery terminals?  Is it equal to the cross-sectional area of the cable? (assuming it's a good contact without dirt in the way)

I did not realize until recently that most jumper cables are too long and/or thin gauge to provide the full starting current, so the battery in the car being jumped actually has to contribute a significant fraction (or the bulk) of the current

Notes:
a) Please correct any of my assumptions or numbers if they are off
b) Assume of course there is no other electrical or mechanical issue that would prevent jump starting so we can focus just on the core questions.  E.g. the starter motor works fine and when it is able to do so, it can initiate combustion. Etc.
c) This is a US model car
Berni:
Depends on the situation.

The most optimal jump starting procedure is to first start the donor car, connect the dead battery while it is running, wait a few minutes for the dead battery to charge up, then crank it.

The reasons for that is that you make sure you can't drain the donor battery and prevent the donors engine from starting, the engine running produces extra power while raising the voltage up higher to compensate for voltage drops, the higher voltage charges the dead battery, then once you crank the power provided is a sum of both batteries and the engines alternator.

But since starter motors are designed to be able to start the engine in sub-optimal conditions means that they don't need the full 12V. Even a good new battery won't hold anywhere near 12V when starting a big diesel in -20°C. So even if it only gets 9V it will still crank it. So because of this you can still start a car with a dead battery by simply connecting it to a good battery in a car with the engine not running. But if the battery in the donor car is also half way towards dead then you might need that full proper procedure to get it started.
duckduck:

--- Quote from: scootley on June 16, 2020, 05:43:47 am ---So my questions are:
1) Would we expect this jump start scenario to be successful (in general)?  Why or why not?
2) What effect would the presence of the damaged battery in the circuit have on the ability of the starter motor to receive the necessary current?  Could the bad battery somehow actually sink a significant portion of the power due to its resistance (or some other property) and prevent it from getting to the motor? 
3) What is the minimum viable contact surface area needed between the jumper clamps and the battery terminals?  Is it equal to the cross-sectional area of the cable? (assuming it's a good contact without dirt in the way)

--- End quote ---

I know the basics of theory, but my experience is in the practical (many cars and lots of dead batteries from the heat and being young and all that implies :-).
1) Yes. The "helping" battery can spin the starter of the car with the dead battery via the jumper cables.
2) If the amps were coming from another current source in parallel, only if the "dead" battery were acting as a short would it prevent the starter from getting enough amps to spin. I've never seen this in practice, but all of the cells in the "dead" battery could conceivably fail shorted. It's going to get warm.
3) Just nice copper teeth biting into lead (Pb) battery clamps will provide enough surface area. That's just a few mm^2. I don't think it takes the cross section of a 4 AWG copper cable at the connections.
tom66:
I had an oddly-related experience of a car battery failing when I owned my old Ford Focus.

We went on holiday, so the car had not been used for about two weeks, and in the morning I had to drive to the shops to get some supplies.  It had no problem starting and got me there just fine, and drove back as well.   However, the next morning - a relatively mild UK morning at about 8-10C - the vehicle would not start. In fact it was almost completely dead, besides a blinking light on the dashboard.  A multimeter revealed that the battery voltage was below 8 volts.

I am curious what happened to cause this.  Nothing was left on (the car switches off the dome light and other things if you leave them on;  the headlights weren't left on, the doors were all locked.)  But nonetheless I was left with a car that was stuck in my drive with a dead battery and I was already late for work, not great.

So I got out my trusty EA-PS 2042-10 B (42V, 10A, 160W SMPS);  set to 13V and 10A output current and charged the battery. Watched the meter go up, got the battery to 11V.  Tried cranking, got a few sputters but nothing more. Waited another 3-4 minutes and tried again, got it to about 11.5V.   Choked briefly but started after 4-5 seconds slow cranking.  No alternator problem, started charging right away, drove fine after letting the alt charge the battery for another 10 minutes.

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.

Now, I think what my test using the power supply shows is that the battery wasn't failed, it was just over-discharged. Perhaps the little trip to the shops did not top the battery up enough (15 minutes each way).  But I think it shows that even a heavily discharged battery can start an engine; it just needs to be able to keep the voltage high enough for long enough to kick the engine up to speed.
tautech:

--- Quote from: scootley on June 16, 2020, 05:43:47 am ---Now say one attempts to jump start this car using the following:
1) The healthy & charged battery of another running car which is known to be able to supply more than 200A
2) Jumper cables that are of a gauge and length that would allow 200A to actually make it to the contacts in the dead vehicle at or near full voltage. 
(for example https://www.wirebarn.com/Wire-Calculator-_ep_41.html claims a 4 gauge [5.2mm] cable can send 220A@12V ~22ft/6.7m with less than 10% drop)

--- End quote ---
Current is EVERYTHING when jump starting a dead battery and 200A is a fraction of what's really needed to start most vehicles and like Berni said it's wise to let your donor vehicle add some charge into the battery for a few minutes and rev its engine a little to help things along.
25mm2 is the minimum I would consider as adequate for GP jumper cables but I do need that size for tractors and trucks occasionally. Still I sometimes wish I had a heavier set.
Good jumper leads can be fashioned from welding leads and the bigger/heavier the better.
A 400A welding cable is a nice supple thing although there's a bit of weight to them and let's not forget they're 100% duty cycle rated so can stand massive overloads for the duration of normal engine cranking.

Modern vehicles with geared starters offer much lower current draw and hence battery CCA ratings are less these days that for ones with a direct drive starter.
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