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Deeply discharged car battery - basic questions (no charger around).
Deeply discharged car battery - basic questions (no charger around).
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Old 13th November 2018, 11:34 PM   #11
jplesset is offline jplesset  United States
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Another thought... Years ago, I left lights on overnight, and did indeed come out to a dead battery. So dead that rolling downhill for 1/4 mile didn't get enough juice to run the fuel pump enough to get it started. Had to call AAA.... after walking back up the hill.
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Old 15th November 2018, 06:17 PM   #12
martin clark is offline martin clark  Europe
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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.

Last edited by martin clark; 15th November 2018 at 06:22 PM.
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Old 15th November 2018, 06:28 PM   #13
oreo382 is offline oreo382  Canada
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Quote:
Originally Posted by martin clark View Post
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).
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Old 15th November 2018, 06:50 PM   #14
martin clark is offline martin clark  Europe
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No, Q1 I suggest is not wrong. It serves both purposes. Stick a DVM across the block-battery neg while cranking and watch the Vdrop. Avoiding that when jump-starting is useful - quite apart from also addressing ignition risk.
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Old 15th November 2018, 10:14 PM   #15
academia50 is offline academia50  Argentina
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Quote:
Originally Posted by martin clark View Post

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


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Old 15th November 2018, 10:40 PM   #16
scott wurcer is offline scott wurcer  United States
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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.
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Old 16th November 2018, 03:03 PM   #17
leadbelly is offline leadbelly  Canada
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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).
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Old 16th November 2018, 11:17 PM   #18
academia50 is offline academia50  Argentina
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Originally Posted by leadbelly View Post
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).
How do you know ?
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Old 16th November 2018, 11:20 PM   #19
leadbelly is offline leadbelly  Canada
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Originally Posted by academia50 View Post
How do you know ?
That was rhetorical. Still hurts to talk about it.
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Old 16th November 2018, 11:20 PM   #20
academia50 is offline academia50  Argentina
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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 .........

Last edited by academia50; 16th November 2018 at 11:23 PM.
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