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BMW 5 Series, Electrical
#29 of 77 Battery and Starting
Jun 04, 2003 (9:19 am)
Not sure where other people bought there batteries, but I just bought one for my 1998 528i from the dealer for $218.00. Checked Sears and other stores first but none carried the battery that’s in the car (840 volts). The largest I could find was 805 volts.
See my post “528i Starting Problems”. The new battery, so far seems to have resolved my 6 month random no start problem which several BMW dealers could not and did not even thing of the battery. The battery was the original that came with the car.
#30 of 77 Alternator power...
Jun 04, 2003 (9:37 am)
I've actually done that a few times with my Mopars...pull the battery out while the car's running, and then drive it. Don't ask me why, exactly, I did it...looking back I can't remember! I never tried it with my DeSoto though. That car uses a generator. Would it not make it without the battery?
Were Chrysler alternators better than GM alternators, back in the day? I know that Mopar went to alternators before GM, but what about when GM finally switched to them?
#31 of 77 batteries, alternators, and bears, oh my
Jun 04, 2003 (10:43 am)
that's cold cranking amps, not volts, on the batteries. not even the hybrids use 800-volt stacks.
a generator car would stop dead because you have to turn the armature (basically the same thing as an alternator rotor) within a magnetic field to generate electricity. both alternators and generators use a field coil to set up that magnetic field instead of pounds of permanent magnets; cheaper, lighter, smaller, and lasts longer.
generator fields were always energized from the battery, thus, take the battery off, no magnetism, no voltage... nothing to commutate for spark coil primaries, so no spark... you walk.
old-style alternators, like the original Motorola ones that Chrysler introduced, had a mechanical relay voltage regulator that allowed the battery to energize the fields through a resistor, or allowed the rotor's wye-coil to energize the fields through a diode module... with the higher output to the battery going through larger diodes. so the original Motorola alternators could indeed self-excite just enough at idle to keep operating once started.
doesn't work that way today, the battery is at minimum a reference voltage for solid-state regulators that control output by adjusting the current in the field coil. my big Ford alternator even has a lead from the engine computer to cut alternator output if you whack the accelerator hard and the engine needs to shed some auxiliary power beyond the a/c compressor to try and deliver. you pull the battery cable on a modern charging system and you will get an ugly and dangerous spark along with hundreds of dollars of charging system damage.
Jun 04, 2003 (11:15 am)
You've already bought the extremely expensive dealership-supplied battery, so it will be little solace to tell you, you could've bought a decent aftermarket brand for say 20-25% of what you paid the dealer. That 805 ampere hour battery you found was arguably quite large enough for the job at hand.
#33 of 77 One thing that impressed me...
Jun 04, 2003 (11:35 am)
a few months ago, I changed the battery on my Granddad's '94 Taurus. When I put the new one in, everything kept its memory, even the clock and the station presets! Last time he had a new battery before that, he had the dealer put it in, and I think they charged something ridiculous like $150! Said they had to "reprogram the computer"!
Swschrad, thanks for the rundown on alternators, batteries, (and bears Interesting info. Good to know, too...I wouldn't have known that you can't pull a battery out of a modern car without doing some real damage. Although in the case of my Intrepid, the thing's so buried it's not like it's a 30-second yank-job, anyway!
Jun 04, 2003 (11:54 am)
"you pull the battery cable on a modern charging system and you will get an ugly and dangerous spark along with hundreds of dollars of charging system damage."
But that is only on a running engine/active charging system, right ? If the alternator is not running, there would not be enough energy in a static field to cause that kind of damage, from my point of view.
Otherwise all those manuals asking me to disconnect my battery before doing any electrical work on my car would have been seriously wrong.
#35 of 77 haspelbein, absolutely
Jun 05, 2003 (8:20 pm)
I read the poster's question as, "I can do this with old stuff when running, how about the new stuff?" answer to that is good luck, friend, be ready for a parts run in the beater.
you are completely correct that if the system is not in use, pulling the battery cable(s) off only affects things like computer eraseable memory and so on, and should not be breaking flows of current and removing reference voltages/currents from operating systems that need 'em.
Jun 05, 2003 (8:34 pm)
I have heard that when a battery goes out, and you pretty much know you need to go get a new one, it could be wise (in terms of saving potential damage from occurring, I guess) to leave the vehicle and not run it on the (say) partially shorted or otherwise almost dead and gone battery. For clarification: Often you can use cables and get a vehicle running via jumping from a good car battery, then pull off the jumper cables and keep the idle high in the car and drive it some distance to get a new battery. Is that a no-no?
#37 of 77 I suspect so
Jun 06, 2003 (8:43 pm)
what's happening, in a nutshell, is that in the old days, the regulators were chatter-down relays... the alternator generated something like 16 or 18 volts. 12 volt batteries don't like this, they evaporate out all the water, crystallize the plates, and become hot paperweights. the extra hydrogen and oxygen disassociated from water become an explosion hazard. all these pieces could come together to cause an explosion or battery fire if you haven't been living right. both are evil, evil.
the regulators let through a tad bit of the 16 volts, then click open their relay contacts as coils heated up, and then the battery was fed with something like 10 volts until the coils cooled down again, and higher voltage hit. the battery as a voltage damper masked this action, and you appeared to have anything from 12 to 16 volts, depending on how you bent the relay contacts to introduce a mechanical delay in the process.
those regulators could stick and either overcharge or undercharge the battery, but with nothing to damage, really, it was possible to run on 'em if you didn't have large current loads with the battery off. remember, we didn't have computer control on the 1961 mopars for anything, and computers tend to do things like melt and die instantly if they are fed overvoltage.
what happens now in a regulator is you have a solid-state IC and transistor system -- you have a reference voltage generated off the battery by an IC that biases the operation of a current amplifier stage (pass transistor) or equivalent output circuit in a triac or IGFET. the reference voltage is part of a comparator, in which this is checked against the output of the alternator. if the reference voltage is higher, that means the alternator is low, and more current is allowed to the field, creating more voltage. if Vr is lower than the alternator, the alternator is cut back by reducing the current in the field winding.
and if Vr from the battery suddenly disappears, you go out of range. what happens depends now on what choices the designer made. if the designer of this car's regulator decided that there would be no extraordinary measures to try and recover from what might be a dirty connection, the field will be cut below the point at which you generate electricity, and you stop dead.
if the designer decided they would try and "punch through" a spot of corrosion at a battery connection that could have caused loss of Vr, the bias on the pass transistor (triac, IGFET, whatever) would be radically modified, and the last gasp of power from the collapsing magnetic field in the alternator would basically be cut through. you "spike the field," which is probably already spiking, to try and weld through the problem in the battery leads.
physics predicts that as the magnetic field dies in a coil, and the steady (saturated) field collapses, this magnetic field collapsing will in itself cause a spike voltage. the collapsing field cuts across the coil wires and provides a magnetic analog to the engine's rotation usually used to cut the field with the windings of the rotor, generating voltage. the spike from a collapsing field is often higher than the steady voltage the rotor makes cutting a steady field.
/ late edit / oh, yes, the little detail about the field "should already be spiking" due to collapsing voltage. this means if the variables are aligned in the wrong way, even if your design of the regulator doesn't cut the spike through, you can still generate an overvoltage spike in a "coast to stop" system that can damage electronics. almost forgot that point. / end edit /
there should be "snubber" diodes to short out the spike in the wrong, anti-polarity direction, which is why you are advised to never try and "polarize" an alternator by shorting the thing briefly on installation... something you had to do on a generator. whether or not the snubbers are damaged by peak reverse voltage over their limits, there will be a big spike positive.
big spikes positive with solid-state electronics punch through the silicon's structures and kill stuff.
so there is quite enough likelihood that pulling the battery from a running alternator system nowadays is going to be ugly that I am not going there on a bet. the Belchfire Motors alternator for 2001 might have been designed to just quit if you do this, but the 2002 supplier might have had another idea altogether. and I'm not going to be the test agency for these parts, particularly as the costs keep rising for the things
lest anybody say it's idiotic to put a zap into an open circuit and anybody who does it is a thug on drugs... the telephone system, from DC to daylight, on every service, is built with a "sealing current" flow that does the exact same thing. this is done because corrosion at the connection points has shut down lines since the 3-wire days. "sealing current" flows as the 48 volt battery is interrupted, and that usually has an arc-welder effect to cut through the crud at one or many points of the wire/connector mating point and restores the circuit. the end effect to a voice user is crackle or hiss, and to a data circuit is a shot of spikes that causes a loss of nail-up information between ends, taking the circuit down for transmission until the two ends nail-up a new logical connection on the wire.
MORAL: if your battery is a paperweight, copy the numbers off the label or out of the manual, and whistle up a cab, bus, or neighbor to ride down to Parts Is We and get another one that way.
Jun 06, 2003 (10:04 pm)
These little devices that keep all your computer memory and radio codes intact while you switch batteries?