Deeply discharged car battery - basic questions (no charger around).

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Hi all,

I simply cannot find a correct answer from physics side for my questions so I thought I might ask this here. Maybe some good electricians can answer me these questions or think about it a bit.

Situation1: for whatever reason, the car's battery is deeply discharged. Let's assume the car's charging system is in good shape.

Q1: can I jump start the car from another car, batteries connected directly ?
Meant here: direct connection. Lot of people say we shall directly connect battery positive leads but on the charged car the negative shall go onto the body instead of the negative battery lead. Why ?

Q2: shall the charging car's engine run meanwhile or not ? To compensate for quick discharge of the charging car's battery itself.
Meant here: won't there be a quick current rush onto the bad battery at the moment I connect the cables ? Which can then destroy the diode bridge on the providing car's charging system.

Q3: some people suggest a 50W bulb in series with the positive lead (a non-bulb load, big resistor is even better) so this way at the beginning when the discharged battery cannot pick up much current (due to high internal resistance) the current gets dissipated on the bulb but there's still some little current flowing towards the battery. When it starts to fill up slowly, its internal resistance gets lower which again contributes to even more charging current (because the giving side is a voltage generator and constant charging voltage makes current flow depending of resistance).. so there's sooner or later a point where the charging car's charging system might be overloaded.

Q4: OR not ? If not, why ? Are there any kind of charging circuit protections employed in modern cars ? ( = cars of the last ca. 20 years).

Q5: does a direct battery-battery connection quickly discharge the giving battery while charging the dead battery too fast ? -> Cable overload, too much current. If that's the case, why does it happen, when we talk about discharged battery's big internal resistance ? Can it be that at the beginning nothing strange happens but then as after couple of minutes the internal resistance of the dead battery lowers, currents begin to rush and be too much for a cheapo normal cable ? (Is there an internal resistance / charging curve for normal car batteries ? I haven't seen some yet). I know too much current within a given time can make the receiving side battery "boil" so there's definitely an upper limit of charging speed for the receiver part. (And also a discharging speed of the giving side if that's another battery).

Q5: can a car with a discharged battery be push-started ? What will happen in the charging circuit side ? So when a voltage generator meets a big internal resistance and only a minimimum of current flows. I assume after couple of minutes as "R" lowers, it starts to charge more, more, more - is there a maximum of it and does some kind of protection kick in to apply less charging and be gentle with the battery AND with the charging circuit itself, diode bridge, etc. ?


Besides explanation of basic physics and the happenings (with respect to Ohm's law) this might also clarify how to quick-start cars and how don't do it.

Winter is coming. (At least here on the northern hemisphere). :eek:
 

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Proper technique is to start the good battery car. Attach the red positive lead between the batteries first. The attach the black lead to the good battery terminal. Second attach the black lead to the dead car on the frame away from the battery. That way any spark will not ignite any possible hydrogen gas from the battery. If you just jump the two batteries there is a good chance you will have two dead cars.

The dead battery must charge for a bit before you can start the car as the jumper cables have high enough resistance to prevent a good start.

That resistance is also why you can just jump the batteries.

Yes you can roll start a car with a manual transmission. Best is to roll it downhill and then once moving nicely pop it into second gear.

If you have a good charging system and the battery has died without a cause such as leaving a light on or let it run dry, you will need a new battery.
 
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Q1: can I jump start the car from another car, batteries connected directly ?
Meant here: direct connection. Lot of people say we shall directly connect battery positive leads but on the charged car the negative shall go onto the body instead of the negative battery lead. Why ?
A1: I always connect direct. It's easier than finding a good ground connection. I think the idea is that since flooded lead acid cells can discharge hydrogen, and since there is usually a spark when you connect the batteries in parallel, there could be an explosion.

Q2: shall the charging car's engine run meanwhile or not ? To compensate for quick discharge of the charging car's battery itself.
Meant here: won't there be a quick current rush onto the bad battery at the moment I connect the cables ? Which can then destroy the diode bridge on the providing car's charging system.

A2: A fully charged lead acid battery is 13.2VDC, but to charge a battery up to that voltage you need to present it with an even higher voltage, which your alternator will do. So the charging car's engine shall be running. I don't know anything about destroying the diode bridge.

Q3: some people suggest a 50W bulb in series with the positive lead (a non-bulb load, big resistor is even better) so this way at the beginning when the discharged battery cannot pick up much current (due to high internal resistance) the current gets dissipated on the bulb but there's still some little current flowing towards the battery. When it starts to fill up slowly, its internal resistance gets lower which again contributes to even more charging current (because the giving side is a voltage generator and constant charging voltage makes current flow depending of resistance).. so there's sooner or later a point where the charging car's charging system might be overloaded.

A3: Never heard that.

Q4: OR not ? If not, why ? Are there any kind of charging circuit protections employed in modern cars ? ( = cars of the last ca. 20 years).

Q5: does a direct battery-battery connection quickly discharge the giving battery while charging the dead battery too fast ? -> Cable overload, too much current. If that's the case, why does it happen, when we talk about discharged battery's big internal resistance ? Can it be that at the beginning nothing strange happens but then as after couple of minutes the internal resistance of the dead battery lowers, currents begin to rush and be too much for a cheapo normal cable ? (Is there an internal resistance / charging curve for normal car batteries ? I haven't seen some yet). I know too much current within a given time can make the receiving side battery "boil" so there's definitely an upper limit of charging speed for the receiver part. (And also a discharging speed of the giving side if that's another battery).
A5: It'll discharge the "donor" battery, but will not do it all that fast and definitely not too fast for the discharged battery.
Q5: can a car with a discharged battery be push-started ? What will happen in the charging circuit side ? So when a voltage generator meets a big internal resistance and only a minimimum of current flows. I assume after couple of minutes as "R" lowers, it starts to charge more, more, more - is there a maximum of it and does some kind of protection kick in to apply less charging and be gentle with the battery AND with the charging circuit itself, diode bridge, etc. ?
A5: I have no idea.

Besides explanation of basic physics and the happenings (with respect to Ohm's law) this might also clarify how to quick-start cars and how don't do it.

Winter is coming. (At least here on the northern hemisphere). :eek:


The alternator includes a voltage regulator and that combined with the battery's internal resistance is what regulates charging current.

When I need a jump I find the largest vehicle I can. They usually have larger alternators and can supply enough power for the starter motor on the disabled vehicle. Thus very little actual battery charging by the donor vehicle is needed, it just supplies the starter motor with the power needed to get the engine running again.
Oh, also: buy and carry really thick jumper cables. Lots of power just gets turned into heat with thin cables.
 
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Beyond the jumping queries there is the question of why the battery is dead in the first place. A battery in good condition should easily last more than a month in a parked car and still be able to start it. If this is not a "maintenance free" battery the obvious first step is to check the electrolyte level in each cell...it should be filled to the bottom of the fill tube so that a meniscus forms and should at least show all plates covered when checked.

If the battery itself is not shot then the reason for a dead battery is either a charging system malfunction or a parasitic load on the electrical system when parked. Modern vehicles go through a sequenced shutdown of the various controllers in the vehicle that typically takes some minutes after shutdown to reach the full sleep state. Even in full sleep there will be a current draw of a modest number of milliamps so that the vehicle can respond to a keyfob command to unlock.

If you find that the current draw 15 minutes after shutdown is still substantial then something is interfering with the shutdown sequence. You can pull fuses one at a time to isolate which circuit(s) is responsible for the high quiescent current draw and take it from there to identify the controller that is the insomniac.
 

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Don't fret over-much. Unless you spent a fortune on jumper cables, their resistance will limit peak current to much less than infinity. They typically will not pass starting current (100A) without many volts of drop. Peaks to 300A, once or twice, do no real harm to car batteries.

I used to "push"-start my Willys for weeks on end. The available rebuilt starters were crap, and never in stock. It rolled easy. At home and at work I parked on a slight grade. Traveling though south Delaware (DEAD flat), two friends at the back were able to gain start-speed.

It is "possible" to pop-start a car with another car and a rope. Very Dangerous! When the towed car catches it tends to ram the towing car.
 
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If a car battery has suffered a constant, low drain, I will charge it back again slowly, over the course of a day or so, or at least long enough to get it part way back before hitting it with a bunch of amps.
When they are totally dead, you can charge them up in reverse polarity!

There are battery terminals that have a feature that allows one to isolate the battery from the car’s electrical system, and help prevent draining when the car is being stored.
 
Q1


The reason you connect the black lead to target car chassis ground (ideally, the engine block somewhere) is becasue this is the most direct return to the target starter motor ie minimum impedance - does not involve the target car's Negative battery terminal to body, in series with the body-engine ground links (either of which add a further smidge of impedance)

Q5

A manual-transmission car can be push-started but it it has been built in the last 15yrs or so don't do it. It won't start, and it won't help.

Long answer - these days the engine management unit won't even instruct the injectors to run and the ignition circuit to fire until the engine is turning over a certain min rpm (often 120-150rpm) to guarantee a rapid start (min unburnt fuelling). This is to protect the catalytic converter(s) and lambda sensors, and it is not enough to get your engine up to that once at a clutch-dump downhill - needs to be sustained, and battery terminal voltage also above say 11.5v (also checked by the DME as part of the firing cycle...) - and a push won't be enough for the alternator to bring the dead battery system voltage up far enough for all 3 conditions to be sustained. Don't even bother.

Long story short - not likely to be useful. Hook the battery to a known -good car or charger, let the known-good car idle to supply some bulk charge in for 5-10mins, try a start on the target then.

HTH
 
That's another very good point, modern EMUs often draw c.15A; and the fuel pump on an injected engine 10A or so alone to prime/presurise at turn-on - and maybe half that to sustain. The there's the 'body electronics' boxes. Quite easily 30+A draw at the run position, fundamentally not doing much except being 'live to run' *

The cure for a flat battery these days is - charge it; don't push it...

* I have a DC clamp meter and have used it on my cars. It was instructive when the old alpina (B10 3.3/e39 shape), with a fully charged, recently-new battery, shows the alternator still shove 50A+ back into the battery for a minute or so after startup! And that's a 15yr old car, with nowhere near as many 'toys' as people now expect.
 
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Q1


The reason you connect the black lead to target car chassis ground (ideally, the engine block somewhere) is becasue this is the most direct return to the target starter motor ie minimum impedance - does not involve the target car's Negative battery terminal to body, in series with the body-engine ground links (either of which add a further smidge of impedance)

Q5

A manual-transmission car can be push-started but it it has been built in the last 15yrs or so don't do it. It won't start, and it won't help.

Long answer - these days the engine management unit won't even instruct the injectors to run and the ignition circuit to fire until the engine is turning over a certain min rpm (often 120-150rpm) to guarantee a rapid start (min unburnt fuelling). This is to protect the catalytic converter(s) and lambda sensors, and it is not enough to get your engine up to that once at a clutch-dump downhill - needs to be sustained, and battery terminal voltage also above say 11.5v (also checked by the DME as part of the firing cycle...) - and a push won't be enough for the alternator to bring the dead battery system voltage up far enough for all 3 conditions to be sustained. Don't even bother.

Long story short - not likely to be useful. Hook the battery to a known -good car or charger, let the known-good car idle to supply some bulk charge in for 5-10mins, try a start on the target then.

HTH

Regarding Q1 answer.That answer is wrong and possibly a safety concern.The reason to connect the neg. lead to the bad battery engine block (last connection) is so you don't create sparks near the dead battery.There could be hydrogen gas.I've seen first hand what happens. My buddy went to jump a dead battery the wrong way and the battery exploded,blew the side right out of it.He was lucky his hand was not in the way.It was later found the dead battery was almost dry.In addition if it blows up there may be active acid sprayed everywnere(eyes).
 
.................The cure for a flat battery these days is - charge it; don't push it...

..............

Thus, modern cars should not be ripped off by pushing them the old way, it is perfectly clarified in user manuals. Among other drawbacks, raw naphtha spoils the catalyst, which is not exactly cheap to replace, which makes the atmosphere continue to contaminate because users do not understand the operation / importance of it or have no ecological conscience and do not replace it by high cost. .
Did you know that if we unroll (they are built in a similar way to a film condenser) a catalyst has an approximate surface to a football stadium?
A comment about something that many people did not know. Vehicles with conventional ignition engines, that is with a distributor and the classic platinum and condenser, could / could be improved by installing transistorized electronic ignition modules.
The car always starts, the plates are not damaged, the capacitor is no longer necessary, and a voltage of only 9 volts in the accumulator is enough to start the vehicle without problems.
The main cause that makes it necessary to push the vehicle to start is the poor state of the starter motors, the wear of the rotor hubs causes a short circuit between the rotor and the stator, which causes that consumes all the current that should be available for the ignition coil to supply the spark plugs. Many people think that their vehicle does not start because the battery is in poor condition, they buy a new one and after a while the problem returns, the more stuck the starter motor, the sooner it will be discharged, and the continuous charge and discharge cycle ( especially if it is loaded quickly) will spoil it again ....
Only my two cents.


http://www.aficionadosalamecanica.net/encend_convencional.htm
 

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Automotive batteries have thinner plates than deep discharge batteries like the ones used for solar backup. If discharged to the point that there is no more lead they will not hold a charge even though you can always jump them and keep running (they will just not start the next time). Like old magneto engines, you get them going and they keep going until turned off.
 
One thing to watch for is that if you attempt to charge a deeply discharged battery, you use a device that can deal with large draw for an extended period of time. Modern cars have all kinds of complex systems that introduce failure modes when system power stays at some intermediate voltage for an extended period (ask me how I know).
 
No one has asked why conventional cars with conventional batteries (worth the redundancy, no electric cars or hybrids here) do not yet incorporate lithium-ion batteries ?

I have to admit that I do not know the exact answer, but I imagine .........
 
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