I am contemplating taking my battery out of the car for several weeks to place it on a special desulfating charge (PulseTech). However, I don't want the car stuck in the garage for all this time! I'm fine with using jumper cables to start the car without the battery, however I read that I'd need to connect a capacitor in place of the battery to avoid destroying the electronics, as the alternator does not provide straight DC, rather it's a 3-phase AC signal at a variable frequency depending the engine RPM. Ok, that's fine, but now I'm just not sure what type of capacitor would be appropriate; I know very little about using capacitors except their basic function of acting like a very brief battery and allowing smoothing of current. What capacitor ratings would be suggested?

 

Sounds iffy to me.
So how will you start the car once you leave the garage?

 

One: the Alternator produces a 3 phase rectified regulated to 14.5v DC supply, and it requires a DC source to start generating initially, and although a alternator generates 12v down to very low RPM, if you had a high load, and were idling, the electronics and other devices may suffer due to brown-out when a to low voltage occurs at that point.
Max.

 

It is very bad practice to run the car without the battery connected. Transients due to door locks, wipers and a host of other things are absorbed by the battery and without it you may damage the electronics that could easily cost ten time that of a replacement new battery. You can buy a clamp on welding suppressor used when bodywork is welded with an arc welder, this will suppress the transients.
But the question by Crutschow is one that needs to be considered, what happens when you are out and need to stop, or you stall the engine?

 

Sounds iffy to me.
So how will you start the car once you leave the garage?

Another jump. Obviously not looking to drive the car daily without the battery!

Actually, I'm now pondering to just try scouring repair shops for their dead batteries, that might be easier and cheaper, and who knows, maybe I can rejuvenate that battery too with this equipment!

 

I'm fine with using jumper cables to start the car without the battery...

Ever try that? Good luck. Jumper cables can help by charging the dead battery and providing an extra boost but I've never been able to start a car through them alone. They drop too much voltage at the high current needed for starting.

A capacitor in place of a battery will do very little unless it's quite large, making it more expensive than another battery. And as noted, it won't help you if/when you need to start again.

 

Jumper cables can help by charging the dead battery and providing an extra boost but I've never been able to start a car through them alone.

That's true for standard car jumper cables (4 gauge).
You would need a set of heavy-duty or truck jumper cables with larger wires (likely 1 gauge).

 

I made mine from welding cable!!
Max.

 

Welding cables work great. My experience was in sub zero weather with cold engines. Jumping without a receiving battery was a no-go. I never tried with a warm, loose engine.

 

Sounds iffy to me.
So how will you start the car once you leave the garage?

Someone had a blog about using a bank of very large lucky find surplus super capacitors.

A full charge was capable of cranking the engine if it wasn't a frosty morning - ISTR: some mention of a solar panel stuck on the roof.

 

I think it would be a bad idea to try/or run your car without a battery. The system needs an electrical cushion.
I wouldn't call a cap a cushion.....it's more like a spring.

 

Dead batteries can kill alternators and their associated devices and circuitry.

 

I think it would be a bad idea to try/or run your car without a battery. The system needs an electrical cushion.
I wouldn't call a cap a cushion.....it's more like a spring.

Exactly. Without a battery there is a high likelihood of a Load_dump event happening as loads (like headlights or blower fans) switch on and off.

 

Earlier this year, I had to drive my truck with a dead battery for a couple of days. It's a standard, so starting wasn't a problem. However, I couldn't run the A/C with it in this condition (not enough juice to fully pull in the clutch plate). I imagine that other things weren't working properly either.

You would be better served by just replacing your battery, instead of the crap-shoot of de-sulferization. Walmart probably has the one you need in the $50 price range.

 

I made mine from welding cable!!
Max.

I believe that's typically 0 gauge.

 

What makes you think your battery is sulfated?

 

Just go and buy a new battery. Even a cheapy will cost you a LOT less that the damage you are likely to do to the whole electrical system without one. If you car has electronic motor control, absolutely don't!
And you car radio, lamps.....
Very false economy I'm afraid.

 

That's true for standard car jumper cables (4 gauge).
You would need a set of heavy-duty or truck jumper cables with larger wires (likely 1 gauge).

Mine are 00 gauge, but maybe 15' long.

 

I am contemplating taking my battery out of the car for several weeks to place it on a special desulfating charge (PulseTech). However, I don't want the car stuck in the garage for all this time! I'm fine with using jumper cables to start the car without the battery, however I read that I'd need to connect a capacitor in place of the battery to avoid destroying the electronics, as the alternator does not provide straight DC, rather it's a 3-phase AC signal at a variable frequency depending the engine RPM. Ok, that's fine, but now I'm just not sure what type of capacitor would be appropriate; I know very little about using capacitors except their basic function of acting like a very brief battery and allowing smoothing of current. What capacitor ratings would be suggested?

I have to agree with many here and don't see this ending well if you do it. The alternator starts out with 3 phase AC which it converts to DC using internal diodes. Pictured below is a typical automotive alternator circuit.

Note the six (two per phase) diodes in the rectifier block. The lower diode trio feed the voltage regulator which drives the slip rings, An old trick when trying to isolate a bad alternator was to momentarily disconnect the battery Positive on a running engine. If the engine continued to run the assumption was the alternator was good.Therefore the battery was likely failing. This was just a quick and simple test in the field. Good shops have actual alternator test stands. My experience with automotive batteries is they have it down to a science. A 60 Month battery last exactly 61 months and dies. Had a smaller lead acid battery on my whole house generator. Had a 5 year battery and at 61 Months I needed to replace it, like Magic. Dozens of batteries have passed through my and my wife's trucks and I never had to do anything to them and got their full life with the exception of one sears die hard which failed in two weeks and they just replaced it. Any battery that survived a few months always made it through its full life cycle. So that leaves me wondering why you seem to want special desulfating charge this battery? A top line Interstate battery cost new about $164 USD and will give me a 30 month full replacement and a pro rated 6 years. Not a very expensive investment. Less than a co-pay on my insurance for a flat bed tow truck.I just do not see running less a battery as turning out well. Less the battery you likely run the risk of a load dump which has been covered.

Load Dump Overview: From our friends at WIKI.

Overview
The windings of an alternator have a large inductance. When the vehicle battery is being charged, the alternator supplies it with a large current, the magnitude of which is controlled by the current in the field winding. If the battery becomes disconnected while it is being charged the alternator load suddenly decreases. However the alternator's regulator cannot quickly cause the field current to decrease sufficiently, so the alternator continues to generate a large current. This large current causes the voltage on the vehicle bus to increase significantly -- well above the normal and regulated level.

All the loads connected to the alternator see this high voltage spike. The strength of the spike depends on many factors including the speed at which the alternator is rotating and the current which was being supplied to the battery before it was disconnected. These spike may peak at as high as 120 V and may take up to 400 ms to decay. This kind of a spike would damage many semiconductor devices, e.g. ECUs, that may be connected to the alternator. Special protection devices, such as TVS diodes, varistors which can withstand and absorb the energy of these spikes may be added to protect such semiconductor devices.

Various automotive standards such as ISO 7637-2 and SAE J1113-11 specify a standard shape of the load dump pulse against which automotive electronic components may be designed.

There can also be a smaller inductive spike due to the inductance of the stator windings. That may have a larger voltage, but it will be for a much shorter duration, as relatively little energy is stored in the inductance of these windings. Load dump can be more damaging because the alternator continues to generate power until the field current can decrease, so much more energy can be released.

My brother played this game once and burned up a few very expensive headlamps. Other electronics were also lost.

If you think you have a bad battery take it to a reputable shop and have the system tested. That's my advice anyway and if you want good jumper cables go to a welding supply shop and buy about 25 feet of premium stranded 2/0 or 3/0 cable and then buy some good quality clamps.

Just My Take so do as you wish....
Ron

 

Buy a new battery. Desulphation does not work. Crystals coming off the plates takes material with it, diminishing the capacity of the battery substantially. Thats another story.
As for running your vehicle on a capacitor, it can be done. Maxwell sells a supercapacitor that will work for your application for a couple of thousand dollars. They are sufficient to carry the load.
As for the rest, plain and simple, one of the batterys jobs is to act as a stabilizer for the electrical system. Take that out and as others have said, transient voltage spikes will go to work on your electronics and cost you a lot more than a battery. Bottom line: replace the battery.