Under what circumstances can a car battery be too dead allow a jump start? - Page 1
Forum section: electronics / general

author: scootley
author_rank: Contributor
timestamp: June 16, 2020, 05:43:47 am
content: 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 200A2) 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 examplehttps://www.wirebarn.com/Wire-Calculator-_ep_41.htmlclaims 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 currentNotes:a) Please correct any of my assumptions or numbers if they are offb) 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

author: Berni
author_rank: Super Contributor
timestamp: June 16, 2020, 06:38:37 am
content: 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.

author: duckduck
author_rank: Frequent Contributor
timestamp: June 16, 2020, 07:26:02 am
content: Quote from: scootley on June 16, 2020, 05:43:47 amI 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.

author: tom66
author_rank: Super Contributor
timestamp: June 16, 2020, 08:20:50 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: June 16, 2020, 08:39:16 am
content: Quote from: scootley on June 16, 2020, 05:43:47 amCurrent 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.25mm2is 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2020, 08:40:04 am
content: Right, as it's discharged the ESR goes up. You could even model it as the battery voltage not really changing at all, but the ESR goes up so high that any little leakage drags it down, hence the 8V OC.In reality, the voltage does of course change, and it corresponds to the difference in log concentrations of reactants -- namely acid, lead sulfate and lead dioxide. With even just a little PbO2 left, the voltage is still there, it's not down by orders of magnitude -- but it's not representing any significant change.Likewise, over the course of a useful charge cycle, say 10-90%, the voltage changes little, hence why battery gauges need precise thresholds to do their job.So there's that, plus the PbSO4 is an insulator. A fully discharged lead-acid battery has very high resistance. You can apply 14.4V to a discharged car battery and get very little current, then after some minutes, it recovers and starts taking real charge current (which if the charger hits a limit, will pull down to a lower voltage, then eventually go back up as it reaches a normal state of charge).If it's not been fully discharged, the PbSO4 thickness is still slight, and it almost all converts back on charging. If it's over-discharged, a full thickness forms, or crystals grow, and it will never completely go away, no matter how long you float it for. Every complete discharge cycle loses a lot of capacity, and must be avoided.Tim

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 16, 2020, 09:10:12 am
content: a) a short in one of the cells (a plate broke)b) generator malfunction shorts out the systemIn both cases it depends how much current the donor vehicle will supply without releasing the magic smoke itself. My bet would usually be case a).

author: Berni
author_rank: Super Contributor
timestamp: June 16, 2020, 09:21:45 am
content: Quote from: tom66 on June 16, 2020, 08:20:50 amI did hear about a little research project at my uni about a particular failure mode on the starter motor. They found that overtighting or mishandling could cause a crack in the plastic around the big battery terminal. So they did a experiment of exposing a starter motor to dirt and salt spray, it went well for a while but then one day they noticed that the car battery they left connected to its terminals was dead. They found a low resistance path between the terminals, so they replaced the battery with a full one and this resulted in an arc flash at the motor connections followed by the plastic catching fire. Turns out dirt and salt spray could collect in the crack, cause a small breakdown that chars the plastic into carbon that eventually shorts out the terminals (Btw this includes the starter solenoid as most starters do, so the battery is always connected in normal use).To make things even more scary a friend from Norway with his Mercedes had something similar. He also found it with a mysteriously dead battery, but ran fine once the battery was charged again, but some time later the car caught fire(Likely due to whatever short drained the battery the first time)

author: nctnico
author_rank: Super Contributor
timestamp: June 16, 2020, 10:33:41 am
content: Quote from: tom66 on June 16, 2020, 08:20:50 amMore 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.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 16, 2020, 11:21:15 am
content: Quote from: tom66 on June 16, 2020, 08:20:50 amCars can have very specific bugs regarding parasitic current draw... or especially regarding automatically switching off the loads you mentioned. E.g. mine does run empty (< 10.5V, then it will really switch off), if i forget to switch the dome lamp to auto/off, lock, later unlock and only open the trunk and lock again. Ajar doors are also a classic with certain models.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.Other possible causes are simply misuse. My neighbours car starts wildly blinking the turn indicators and headlamps at night when Mrs. Neighbour repeatedly engaged the parking lights while leaving the car (some models require the stalk to be pushed or tipped in one direction with ignition key out). Parking light being the one load that for safety reasons should not automatically deactivate - as it is legally required to be on depending on parking situation.Quote from: Berni on June 16, 2020, 09:21:45 amThe starter solenoid usually provides a good amount of clearance and thermal stability, should activate only during start and is exposed to the elements in practically every car since decades, so i stumble a bit that arcing at the motor connector should even occur... There would need to be a lot more cases for this to be a significant finding.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 16, 2020, 11:48:49 am
content: Quote from: scootley on June 16, 2020, 05:43:47 amAt 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.QuoteThis contact surfaceareahas only a very shortlength, 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.

author: duckduck
author_rank: Frequent Contributor
timestamp: June 16, 2020, 03:15:07 pm
content: Quote from: nctnico on June 16, 2020, 10:33:41 amI 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:

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2020, 04:12:03 pm
content: 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 say2) 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.

author: duak
author_rank: Super Contributor
timestamp: June 16, 2020, 07:18:03 pm
content: 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.

author: Berni
author_rank: Super Contributor
timestamp: June 17, 2020, 05:17:29 am
content: 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.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 17, 2020, 03:43:26 pm
content: Quote from: scootley on June 16, 2020, 05:43:47 amThe 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.

author: Rick Law
author_rank: Super Contributor
timestamp: June 19, 2020, 05:28:32 pm
content: Quote from: SparkyFX on June 16, 2020, 11:21:15 amInteresting. 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.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 19, 2020, 05:46:02 pm
content: Quote from: Berni on June 17, 2020, 05:17:29 amNot 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.)

author: Seekonk
author_rank: Super Contributor
timestamp: June 22, 2020, 01:09:56 am
content: 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.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 22, 2020, 01:14:33 am
content: Quote from: Seekonk on June 22, 2020, 01:09:56 amThat will give you 250A for a couple of seconds - enough to start a car!

author: Berni
author_rank: Super Contributor
timestamp: June 22, 2020, 05:18:51 am
content: Quote from: Seekonk on June 22, 2020, 01:09:56 amYep 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.

author: Rerouter
author_rank: Super Contributor
timestamp: June 22, 2020, 07:08:00 am
content: 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)

author: tom66
author_rank: Super Contributor
timestamp: June 22, 2020, 09:13:44 am
content: Quote from: SilverSolder on June 19, 2020, 05:46:02 pmPretty 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!

author: Berni
author_rank: Super Contributor
timestamp: June 22, 2020, 09:37:28 am
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: June 22, 2020, 11:02:20 am
content: Quote from: nctnico on June 16, 2020, 10:33:41 am+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.

author: tom66
author_rank: Super Contributor
timestamp: June 22, 2020, 11:26:45 am
content: In my GTE I had a worse failure, the 12V fuse on the battery failed. So it cut out while driving because the DC/DC converter can only supply the average current and as soon as you exceed the limits the DC-DC shuts off immediately. Since the power steering is electric from the 12V supply you only need to demand a lot of torque from that and it'll die. Then you're stuck steering a car which coasts but with no power brakes or power steering, and no hazard lights, or way to wind the electric windows down to flag other motorists ...Bad design IMO - would be extremely unsafe on a motorway! Luckily the road this was on was not too busy. The car had been warning me to service the 12V system before the failure but I thought I could drive it to the mechanics safely enough. Little did I know...

author: SilverSolder
author_rank: Super Contributor
timestamp: June 22, 2020, 12:55:52 pm
content: Quote from: tom66 on June 22, 2020, 09:13:44 amI have an older hybrid car in a climate that on rare occasions can approach -30C - and it is noticeably unhappy until the battery warms up!