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A Mechanic's Life - Tales From Under the Hood

3515 messages,  Last post on Jun 18, 2013 at 2:06 PM

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Messages Page 49 of 352

#477 of 3515 Re: What's the big deal? [Mr_Shiftright] by srs_49

Nov 06, 2012 (5:44 am)

Replying to: MrShift@Edmunds (Nov 05, 2012 1:34 pm)
Oh that's a point---venting---best to leave it open.
 
Don't those trunk mounted batteries like in the BMWs have a vent tube that passes through the fender to the outside? I think the one for my '87 '325 was that way.
 
Ooops, didn't see roadburner's reply .

#478 of 3515 Re: What's the big deal? [srs_49] by thecardoc3

Nov 06, 2012 (5:45 am)

Replying to: srs_49 (Nov 06, 2012 5:44 am)
Don't those trunk mounted batteries like in the BMWs have a vent tube that passes through the fender to the outside? I think the one for my '87 '325 was that way.
 
 
They are supposed to, however many people improperly choose to install a cheaper battery and ignore the vent hose.

#479 of 3515 Re: What's the big deal? [thecardoc3] by roadburner

Nov 06, 2012 (8:31 am)

Replying to: thecardoc3 (Nov 06, 2012 5:45 am)
They are supposed to, however many people improperly choose to install a cheaper battery and ignore the vent hose.
 
I know that Advance Auto, Autozone, and Interstate batteries come with a vent tube adapter if you are buying a battery size that is designed to fit a BMW(91, 48, etc...). The problem is -like you said- the nitwits who buy the cheapest battery that will physically fit in the tray.

#480 of 3515 Re: What's the big deal? [isellhondas] by MrShift@Edmunds HOST

Nov 06, 2012 (9:17 am)

Replying to: isellhondas (Nov 06, 2012 2:39 am)
My only concern with using the engine-bay jump start connection (I thought of that, since my MINI has one, too), is I'm wondering if the long road back to the battery will steal a fair amount of current away from that dinky charger. Sure, a heavy jumper cable, no problem, but pushing 1 amp through a 12 foot long connector cable might result in the charge taking a very long time.
 
On the other hand, the slower the charge the better---so maybe it's not such a bad idea.

#481 of 3515 Re: What's the big deal? [Mr_Shiftright] by thecardoc3

Nov 06, 2012 (9:25 am)

Replying to: MrShift@Edmunds (Nov 06, 2012 9:17 am)
Sure, a heavy jumper cable, no problem, but pushing 1 amp through a 12 foot long connector cable might result in the charge taking a very long time
 
You might want to re-think this.
 
Would it take a long time tryin to charge a battery at one amp of current flow, why of course. But the resistance of that cable isn't the reason.

#482 of 3515 Re: What's the big deal? [thecardoc3] by MrShift@Edmunds HOST

Nov 06, 2012 (9:58 am)

Replying to: thecardoc3 (Nov 06, 2012 9:25 am)
Well it's tricky----the LENGTH of the wire most certainly causes MORE resistance, but the WIDTH of the wire causes LESS resistance.
 
So how to calculate what's at play in this case?
 
My only credible response is that I do know that the longer the cable, the more the resistance.
 
I also know that he'd be much better off with silver battery cables than copper ones!
 
So let's see here.....that would be R = P L
                                                           A
 
so what's your trick question?

#483 of 3515 Re: What's the big deal? [thecardoc3] by isellhondas

Nov 06, 2012 (10:04 am)

Replying to: thecardoc3 (Nov 06, 2012 5:45 am)
They are supposed to, however many people improperly choose to install a cheaper battery and ignore the vent hose.
 
I think they guy who sold me the BMW did just that. I sure don't see a vent hose!

#484 of 3515 Re: What's the big deal? [isellhondas] by MrShift@Edmunds HOST

Nov 06, 2012 (10:58 am)

Replying to: isellhondas (Nov 06, 2012 10:04 am)
hydrogen gas---remember the Hindenburg!
 
Another good way to blow your face off is to apply a hi-amp fast charge to a completely discharged battery.

#485 of 3515 Re: What's the big deal? [Mr_Shiftright] by srs_49

Nov 06, 2012 (2:33 pm)

Replying to: MrShift@Edmunds (Nov 06, 2012 9:17 am)
I'm wondering if the long road back to the battery will steal a fair amount of current away from that dinky charger. Sure, a heavy jumper cable, no problem, but pushing 1 amp through a 12 foot long connector cable might result in the charge taking a very long time
 
Couple of things to consider. That "long road back to battery" has to be able to supply whatever the cranking current that's needed for the starter motor, with acceptable voltage drop, right? Whats typical cranking current? 50 amps? 100 amps? 200 amps? So that 12 foot cable from the battery to the engine compartment has to be able to pass that amount current with only a volt or two (or three) of drop. So the resistance has to be pretty small, say less than 20 milliohms for a 2V drop at 100 amps (from V=I*R).
 
Now, what is the charging current fromr your charger? Say it's 10 amps. Through that same cable, you would drop 0.2V (10 amps * 20 milliohms), probably not enough to affect the charging of the battery.
 
Just for reference, 10 ft of 8 gauge cable only has a resistance of .006 ohms (6 milliohms), less than the 20 milliohms I used in the above example.

#486 of 3515 Re: What's the big deal? [Mr_Shiftright] by thecardoc3

Nov 06, 2012 (1:36 pm)

Replying to: MrShift@Edmunds (Nov 06, 2012 9:58 am)
Lets start with the statement about pushing one amp of current through the cables. Whether they are 12", 12', or 12 miles, your statement set the condition that one amp of current is flowing. As the resistance of the conductor increases, the voltage applied to the circuit would also have to increase to maintain that one amp of current. The question then becomes how much more potential would be required for the extreme length of the cable? We could figure that out if we wanted to.
 
Now back to the reality of the BMW. At 1 amp of current, that conductor's resistance would only create a voltage drop measurable at the most in the thousandths of a volt (it may be in the millionths, but we will use thousandths to keep the math simple). Easily negligible. Raise the current to ten amps, and your voltage drop is now ten times larger. Go up to 100 amps, and your voltage drop is another ten times larger, and is only now into the tenths of a volt range.

Messages Page 49 of 352

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