Jump starting wager


#1

A few days ago a coworker asked me to help jump start their car after they had left the headlights on all day at work. We connected the cables and started his engine easily enough but then he asked me to help quick charge his battery by leaving the cables on and both engines running at idle for about ten minutes. He felt this would give him a better chance of getting his car started next morning after his fairly short drive home (<5 miles). I countered with the thought that his battery would be better off with ten minutes on the highway then with two idling cars working together in the parking lot. I disconnected immediately and went home. Today he told me that his car would not start the next day and he’s blaming me for not hanging around and helping charge up his battery. I say bull. Who’s right?



Just to muddy the waters further, a third party said I did the right thing because leaving two running engines connected by jumper cables can fry one or both alternators in short order. What damage, if any will this cause?


#2

Hmmm, lets see. The output of an alternator is always regulated, as is the current, otherwise you could overcharge a batt if it were left to charge to infinatum or if the charge current were much too high for the windings.

Charging off two alternators would suppy extra current than a single alternator. However I’m not sure how the regulator of the alternator of the dead car might respond. REmember it is sensing V and Current, so it my be the case with both connected, it senses the voltage of your good system rather than the voltage of the discharged battre. Also remember your batt will also get in the frey and try charge the dead battry also.
Given the effect of one on the other and the fact that the settings on each is differnt, it is virtually impossible have two regulators with exactly the exact same settings, so one alternator may be dong all the work. A bit like a weak horse and a strong horse pulling a plow, frankly I’m not sure whose battery might be setting setting the pace.

I think your suggstion of a 10~20 min U turn on the highway was a much better idea. And anyway, if it coudl be charged at idle, let him drive home and let it idle in his own driveway, and if he wished, stick a shim under the throttle return and let it fast idle for 10~20 min or do it right there in the parking lot once started, put a foot on the pedal, hold and listen to re-runs of C&C for 30 min…, Gas is expensive enough without having to charge other peoples cars as well. He should be prepared to burn his own gas and time, or buy a mains charger, only $30 in most catalogs. You got him started, so what more should he need?.

also discharging a battery like that, ie completely flat can damage them beyond repair, the plates can warp, the fluid evaporate and they never fully recover afterwards. (tell him go check the electrolyte level also).

I had a batt in the truck for 7 years till I loaned to a friend who let the lights on, it dies completely 3 weeks thereafter.

So on the face of it, I’ll say Double Bull, You are right, on a social scale if not also on an electrical scale.


#3

Let’s start with your second question, the one in which a third party insisted there would be quick alternator damage. We recently had a lengthy discussion over this issue. Some folks claimed that it is a myth, that current and voltage regulation easily protects each alternator from damage. Others backed the claim. The final outcome of our debate seemed a compromise for the sake of peace, that a car’s charging system is protected when charging a good but low battery (your case) but not a defective one. So we conclude that 3rd guy is well-intentioned but in error. No matter; you should quote him. His statements support your actions.

Now what about your coworker’s claim that his failure to start in the morning is all your fault? Baloney! It’s his car, his problem! He had the means to solve it himself. He knew that. He chose not to. Cheapness? Laziness?

He knows better. You know better. Don’t let him burden you with guilt.


#4

he asked me to help quick charge his battery by leaving the cables on and both engines running at idle for about ten minutes. He felt this would give him a better chance of getting his car started next morning after his fairly short drive home (<5 miles). I countered with the thought that his battery would be better off with ten minutes on the highway then with two idling cars working together in the parking lot.

You are correct. Here’s a challenge back to him- if he felt 10 minutes of idling would accomplish the task, why didn’t he just drive around for 10 minutes on the highway instead of going directly home? The solution was very simple and within his control so he has no one to blame but himself.

Only a fool would allow a fully discharge lead-acid battery to remain low on charge for extended periods. He intended to restore just enough charge to get it started the next morning and then repeat this process over many days to hopefully restore the battery to full charge? Better start saving for a new battery. The appropriate action would have been to immediately place the battery on a trickle charger until it was fully charged before driving it again- especially given his usage pattern.

He sounds like the type that is never at fault for the bad things that happen in his life- that are directly related to his choices.


#5

To be fair, that recent discussion was about a single charging system charging a really dead battery. The claim here is slightly different. The idea being that the two alternators and their voltage regulators would “fight” each other causing damage. I’d say that’s possible, but unlikely. If the voltage regulators were set very close to each other, it probably would not hurt anything, but more importantly to the OP it would not help anything either. Each alternator would supply half as much current as normal. If the regulators were not set close together, then the one set higher would tend to supply all the current and the other would be doing nothing.

Unless someone has actual test data showing something else.


#6

A low battery, one which will engage the starter, but hasn’t the strength to crank the engine, will supply 100 amps to the effort. When the jumper battery is connected, it adds, only, a few more amps, but several move volts to the effort…and, then, it starts!
The jumpee knew he had a problem, he should have used the opportunity to go to an auto parts store to get the battery and alternator checked (for free!).


#7
  1. “If the regulators were not set close together, then the one set higher would tend to supply all the current and the other would be doing nothing.” This is the correct answer.

  2. It takes several hours to fully charge a deeply discharged car battery. Two alternators for ten minutes won’t do it. Ten minutes on the highway won’t do it. Your coworker could have gotten an inexpensive ~4A charger and charged his battery overnight.

  3. A deep discharge is very stressful on a lead-acid battery. If he’s run it down once or a few times before (he sounds like he’s experienced at this) this last run-down could have been the straw that broke the camel’s back.

  4. If he drives <5 miles at a time each day his battery may not ever get a full charge anyway. If he put his battery on a charger for a couple hours once a month it would be happier and healthier. We can save discussion about what such short trips do to his oil and exhaust system for another thread.


#8

A short drive will not be enough to charge a battery. Completely dead batteries can take 15 minutes of applied voltage before they begin to accept a charge. A deep discharge on a battery several yeas old can destroy the plates which are very thin. An extra 20 minutes of both cars connected together would have done nothing to help him start the next day. Damage only occurs when the cables are not connected properly or one of the charging systems are out of order.


#9

I don’t know what the response of this Dual Alt/Regulator setup might be,
In either case I suspect something similar, the battery will only accept so much charge current for a given regulator voltage, and as it is pretty much flat the delta between the Fat battery and the alternators will be great, hence a high current would result. However the regulator will again limit that.

So I might think each regulator would allow the maximum current to flow, for the given voltage.

A much easier way, and simpler and illiminates the risk of any damage by coupling between charging systems is connect teh cars, but let the doner car at a high idle for a few minutes, maybe up to ten, time permitting. Then proceed to start the victim.
This also reduces the draw across the cables, as the flat bettery would be in a position to supply more of the currentand the cables less.

Alos, any car that is driven only 5x2 miles per day needs to be taken out for a brisk drive on the WE to undo all the damage short journies have done during the week, ie, gunking up the engine and oil and exhaust system… not to mention charging the batt.