General > General Technical Chat
Under what circumstances can a car battery be too dead allow a jump start?
SparkyFX:
--- Quote from: scootley on June 16, 2020, 05:43:47 am ---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)
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At a max length of 5.05 (feet...?), which is also part of the table. Remind that you need double the distance between contact points, if not already considered in the 22 feet. The formula for resistance/power dissipation in a conductor always includes a length, the ballpark gauge figures found elsewhere are mostly picked based on the fuse/electrical code.
I heard something about aluminum jumper cables advertised in the market with a sketchy copper conductivity rating. So maybe this is something to look out for as well.
--- Quote ---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)
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This contact surface area has only a very short length, therefore does not concentrate as much power dissipation as most people would guess. You can get away with little. Reductions in cross sectional area are a problem when it allows oxidation to form and isolate longer stretches of stranded wire from each other so that it exceeds the current-carrying capacity (exceeds the thermal limit of the material), but the clamps will not start to weld to anything.
Actually starter current draw is (very) high only for a couple of tenths of a second (given there is no mechanical block)... once the starter turns it drops significantly or (if there is a mechanical block) will drop the voltage to a level that disengages the starter solenoid.
duckduck:
--- 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. These do tend to corrode due to the acid from the battery or become lose due to thermal cycling. The clamp-on style battery terminals aren't the greatest to make a good contact to. The first thing I do when a car gets problems with cranking over is clean the battery contacts. That has fixed about 50% of the cases for me. In your case clamping the jump cable on re-made the contact on the battery. Always remember that 99% of electrical problems are caused by poor contact.
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I strongly second this. At the first hint of battery problems, clean the terminals and then put some vaseline/grease/whatever on them to prevent corrosion. If you own a car with a car battery, you should own one of these:
bdunham7:
Generally speaking, you should not attempt to start any modern car until the battery has at least enough charge to light the headlights on it's own. The reason is that a very discharged battery has low internal resistance, especially at low temperatures, and this can cause major damage to electronic components once the vehicle starts because the battery is a vital part of the charging regulation system. Also, if it is cold out the battery may be frozen, in which case you are done for the day unless you like excitement.
As for whether your car will start, it depends. Large jumper cables and a robust donor battery will easily supply enough current for a modern small-to-mid size car with a permanent magnet starter motor. These typically do not have tremendous current draws, 200A wil cover it for the average econo 4-banger. An old big-block V8 with a series-wound starter with bad bearings, OTOH, typically draws between 5 and 10 million amperes initially. I've literally seen jumper cables 'jump' when the starter tries to engage.
To answer your specific questions:
1) yes, it should start if everything is as you say
2) it is unlikely that a dead battery would sink enough current to make the difference (typically 30 to 40 amps at the most) and if it did, that would actually be a good sign that the battery is OK and the charge will slow down in a few minutes. If the battery is sinking that much current due to damage, which would be unusual even with a shorted cell, you are again done for the day unless you like excitement.
3) a decent grip with the jaws is usually sufficient--nowhere near the area of the cable is needed. If it is insufficient, it will tell you.
duak:
I think the intial starter motor current is being underestimated by a factor of 5 to 10X. The Locked Rotor current (LRA) is a term used for AC induction motors and I expect something similar applies to DC series motors. ie., the nomimal applied voltage divided by the series resistance of the motor while not turning. To get to the cranking RPM the motor has to first overcome all the friction in itself, in the engine and accessories and then accelerate it. If it can't generate the torque to overcome stiction it's not going anywhere. The starter motor can't start turning until it's engaged the ring gear so its kinetic energy can't be used. (Didn't some aircraft start their engines with inertia starters? Imagine the stiction from eighteen 6" diameter pistons with a 6" stroke.)
Let's assume each jumper cable contact has 1 mohm resistance so there's 4 mohms there. AWG 4 wire has about 0.25 mohm per foor so eight feet is 2 mohms. Assuming the assist battery maintains 12 V, then in the limit, 2000 A is possible. If the starter motor needs 500 A to break free, we're off to the races. Practically though, the resistances will be numerous and higher and the assist battery's voltage will likely drop lower.
Berni:
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 ).
So they expect the voltages on the battery to really dip low.
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