Following requests, here is a pix. of the filter capacitor I installed across the battery in my Jag.

The capacitor is 68,000microfarad, at least 16volt working, electrolytic type.
Panasonic is good if you can find them (5000h life). Usually sourced from radio ham-shacks etc.

The fuse, of any type at 10 to 13amp. must be installed because any short will set things on fire and easily write your car off. For the same reason, I covered the terminals of capacitor and fuse with several layers of masking tape (not shown in pix.). Be careful to ensure everything is firm when installing.

Polarity must be observed with the capacitor.

If you have a test-lamp of sidelamp size, you can use it to check the capacitor is charged. Disconnect the +ve clip from the battery, then apply test lamp to capacitor. If it has been charged ok, the lamp will light for a second or two, then go out.

A blob of epoxy fixes the fuse holder firmly to the capacitor.

Common alligator/crocodile clips are used to connect to battery terminals.
Stick to red and black colors!

Checking with an oscilloscope (AC connected) across batt. terminals at engine idle reveals about 50mV p.p. of a band of frequencies from around 5Hz to 500Hz, prob. from the alternator, as the frequencies rose when engine was revved.

When engine switched off, a very small squarewave was observed until the driver's door was shut. Then it disappeared, not to return even when door was opened again.

Leedsman.
p.s. The capacitor appeared to cure a "restricted-performance" warning on my S-type 2.7D and was tested over a period of approx. 8 months. However, because of the nature of possibly spurious error codes appearing on the CAN-BUS network, there is no guarantee that the filter capacitor will cure your partic. problem. It is however so easy and cheap to apply (should be less that $20 or so) the filter capacitor is worth a try. Mechanics might want to make one of these as something to keep as a tester, along with test-lamp, voltmeter, lap-top etc. when faced with baffling spurious error codes on any car. Just clip it onto the battery, then test out the car for a few days. There are incidentally, many other ways of glitch reduction (such as large ferrite cores on suspect wiring, zener supressor diodes etc.) but this procedure takes time and effort.

 

Leedsman, you may find that the capacitor will do a better job of filtering if you place it on the main power lead to the fuse box or even on the power wire to the ECU. In the case of the capacitor, it is looking at the resistance of the circuit and the closer you get to load you want to filter, the more resistance it will see and therefore be able to filter better. Granted, the filter that you have will do an adequate job too. BUt, there are ways to get more out of the same mod.

 

I have just discovered (at nearly midnight) a feature of lead-acid car batteries I had no idea about.

They have a resonance frequency in the 2 to 6Mhz region!

This has been discovered by the people who make pulsed battery de-sulphators, and they use the resonant frequency of the battery itself to greatly assist in desulphation.
This resonant frequency is present in all lead-acid batteries, new or used. It now makes sense that if the fundamental frequency of a glitch appearing on the 12volt battery line can "ring" this resonance, there is a distinct possibility of this energy getting into a microprocessor circuit and corrupting stored data bits.

So it makes perfect sense, if this contingency is at work for the filter capacitor to be placed directly across the battery. Do the research yourself if in doubt, like I did. Just Google "lead-acid battery resonance".
Leedsman.

 

There have been reports for years about X300s and x308s needing new batteries to correct everything from ABS to dodgy idle to probably even jerky sunroof operation. Since I have left my car for months until the 6 year old battery was flat, then cranked it with a jump, and driven the car and never seeing ANY of the reported symptoms, I have wondered about he mental condition of those who advise new battery for everything. But, so many people have reported the effect, there must be something there.

I beleive it is quite possible we are seeing the effect of deteriorating filter capacitors in some, or all of the car's modules. This would solve that, assuming that is the root cause.

 

Yes indeed Sparkenzap, a strongly valid point you raise IMO. If electrolytic capacitors are used in the micros. various, they will deteriorate with time and temperature, as even the best will. The best have a life of around 5000h. as quoted by manufacturers. And let us not forget that the autobox micro. actually lives in the box itself, and therefore can be subject to temperatures in excess of boiling water. I'm talking about the ZF autoboxes for those picky contributors here.
There is a much better capacitor than electrolytic though, but it costs a lot more. Those are the tantalum type which I always fitted as replacements in CCD cameras.
Following my discovery of battery "resonance", which in the big Jag. batteries would be around 2Mhz (due it is said to plasma effects around the lead oxide, PB-O2), the main filter capacitor of 68,000 microfarads needs a tantalum shunted across it of around 33 microfarads at 25volts. If you can't get tantalum ones, plastic foil such as Mylar will do. Take care with tantalum capacitors, they are polarized just like electrolytics.
Leedsman.
p.s. forgot to explain -- electrolytic capacitors aren't too good at filtering high frequencies like 2Mhz, so they need to be shunted with a different type with a better dielectric, e.g. foil or tantalum. Electrolytics have "ESR" or Effective Series Resistance problems -- but they are cheap.
p.s.2 The scope traces to left show a 2Khz repetetive rate of pulse charging with sharp square pulses that set off the "ringing" mentioned --see the trace to the right. It's essential that these pulses are short duration as the peak amplitude across the battery may be 40volt! This is the basic de-sulphation process. It is said that the "ringing" is the piezoelectric type reaction of the growing crystals of lead suphate, and tends to 'shatter' the crystals so the undesireable lead suphate falls off the plates, revealing fresh lead to be charged by the accompanying trickle charger. The full process takes a couple of weeks or so, and can rejuvenate poor, sulphated batteries, and is apparently used by the US Navy.
NOW HERE'S THE CLINCHER! ---- Those sharp repetitive pulses look just like a "glitch" excepting they are on purpose, and continual. So if we get a "glitch" on an older but still serviceable battery, it will excite a resonance producing unwanted frequencies on the 12volt supply in the car. This could explain why a part used battery can be the cause of random problems to do with microprocessors on our Jaguars. It's really a certain amount of sulphation that's not bad enough to stop it cranking the engine, but enough of a problem to cause havoc with some of the microprocessors.

 

Those of you wanting more will find some interesting stuff here:--
Lead acid battery maximiser

It deals with lead-acid batteries primarily, outling the principles behind sulphation and how to deal with it, possibly saving hundreds of dollars on a new battery. If that's not an incentive, I don't know what is! The article is very practical, illustrating how to build pulse chargers that will de-suphate lead-acid batteries, plus the reverse-pulse discharging technique to shorten charging time. Of note in the disclaimer is a portion about what pulse chargers might do to microprocessors in a car. (If of course the battery is not disconnected before charging in such a manner!)

Leedsman.

 

Leedsman:
An argument has gone on for years about pulse charging and sulphation in particular. I know because I was the electrical design engineer for the largest electic golf car company and kept up with the state of the art. Some studies "proved" the value and some indicated no gain and some claimed pulsing hurt battery life.
So, it is an interesting debate, but the best way to prevent sulphation is to keep the battery charged, especially when it will be subjected to cold.

 

Just a note about the original subject of the post.

If you rummage around JTIS long enough, there is a picture(line drawing) of a device that is installed in the power system much like the capacitor pictured by Leedsman. It apparently has about the same purpose and is about the same size. So, the capacitor installed by Leedsman may be substituting for such a unit gone bad. It may also be why some others note no difference with and without.

The picture was found in the JTIS section specific to X308.

 

plumsauce:
Any idea where such a device is to be found on the car? I assume it would be somewhere that large wires could connect it to the main power circuit.

 

From memory, it is somewhere where it can be junctioned by a heavy gauge lead to the main wiring of the alternator output, the engine bay power stud, and the starter. Right fender liner area, down low? It is a vertically mounted cylinder about the same size as shown by Leedsman and involves a sizable fusible link. The notes said the fusible link is to protect the rest of the system in the event of a short in the component. If that link blows, or the component fails, it could explain why *another* capacitor between +ve and -ve would have improved things for Leedsman. It is doing the same job.

As you know, there are many things in JTIS that only get found while in the midst of a search for something entirely unrelated. Maybe the search was for the engine mount procedure because it is close by.

 

yea- I remember seeing something on my x300 a while ago- I willlook again. Too may folks have reported ailments from "low battery: for there not to be some connection, but I and others I have spoken to have never seen the effect.

 

Most cars have a capacitor (or two) mounted in them inside the engine bay. If you follow the wires, normally it is associated with the ignition coil(s) of the vehicle. This helps to minimize the noise created from the ignition coil(s) pulsing the electrical system. But, anything connected inside the electrical system can have an effect on other circuits. So, putting in more capacitance (ie, more capacitors), you are going to filter out more of the noise that is generated and help make for a better flat line DC in the car.

I wonder just what sort of "advantage" installing something like a 1 or 2 farad capacitor would do when connected across the battery. Won't do much for your car amp, but I'm sure that would filter the DC to almost a flat line.

 

Yes, but this one is one big honkin' mother. Looks to be around the size of one of those 6oz. tomato juice cans at a minimum.

Besides filtering, a really big capacitor can mask voltage sags. And that is probably the prime benefit to all the computers. Don't forget that they run on DC rails, likely with local solid state voltage regulators. If those do not see sufficient voltage the operation of the computer is disrupted. Digital devices also do not like to see noise. And the analog transducers are read in a voltage range. Therefore, the base reference voltage must be reliable. Or you get the classic gigo. Garbage in, garbage out. (non-sequiter: that is an amazing resemblance to twatter).

As far as audio amps, it probably would help. Home audiophiles do this all the time. It is almost mandatory for clean bass. Don't car audio people do this to prevent sags?

 

Re. a supercapacitor as filter in a 12volt car system:--

This is a good idea, BUT there are practical problems. The only supercapacitor I could find when I researched a few months ago at 16volt working (battery under charge is 14.4volt) cost £75, say well over $100. this was less than one farad. (Of course, others may find them a bit cheaper somewhere). For those not au fey with these capacitors, they have a low working voltage of something like 2.5volt (but don't quote me) so they have to be stacked with balancing zenerdiodes to make higher working voltages possible. Of course then the effective capacitance goes down in ratio -- two 1-farad capacitors in series gets you 0.5farad effective and so on.

Another problem with supercapacitors is that the battery will see one as a dead-short for a few seconds if you try to connect it, until the capacitor charges to full. This will blow any conventional fuse immediately, and a fuse is MANDATORY for safety from fire. So some clever arrangements would be needed to circumvent this. Even if you pre-charged the supercapacitor prior to connecting, the volts-drop during cranking may well blow a fuse in series with the supercapacitor.

Not so straightforward...

Leedsman.

p.s. The 16volt supercapacitor I saw was the size and shape of a freeview tuner. It didn't look like a conventional can-capacitor at all. Also when considering cost, you could likely get the same capacitance with conventional quality electrolytics at less money -- but a lot of soldering work! I don't know the life of supercapacitors, but conv. quality electrolytics have around 5000 hours life; so maybe older cars are likely to get microprocessor gremlins?

 

Who mentioned "supercapacitor"? The first occurence of "super" in a search of the whole page was in your most recent post.

The statement was simply that a can very much similar in size and shape to the one in your photograph was illustrated in JTIS with some relevant notes. No information was given in JTIS as to its specification. Whether it is "super" or not remains to be seen by whoever finally lays their hands on it.

Now, as far as cost is concerned, it is a factory installed part. A lug nut over the counter might be $30 but might cost Jaguar only 10 cents on the assembly line. The same would hold true to a greater or lesser degree for the device found in JTIS.

The supposition is that if you connected a capacitor across the leads of the positive and negative rails, an existing capacitor across the positive and negative rails would have a similar effect and purpose. It would in fact be a vindication of your claim.

 

Plumsauce, yes, car audio guys do this to mask power unsufficiencies in the way of too small of wiring or too small of an alternator. Proper planning and knowledge of what is required to run high power amps will save money in the long run and prevent issues with a vehicle.

As for high capacitance capacitors, you have a few choices out there. THe standard "coiled" capacitors (the round cans most are used to) can be had up into the 2 farad range and they are rated for 16 VDC and are about the size of a 1 liter/32 ounce sports bottle. I have one in my house that I picked up for like $15 and it is rated at 1 farad. You can also find what are called "batcaps". These are ultra high capacitance "capacitors". They are ultimately a battery that allows for high current charging and discharging (acting much like a capacitor). These are normally on the scale of 40 farads or so.

As for charging up these high farad devices, what you end up doing is using say a 10 ohm/10 watt resistor and putting that in series with the capacitor for the initial charge. Once the voltage on the capacitor is stable, then you can then short around the resistor without a high current surge. Even if you break contact for a few seconds, the capacitor is going to hold the voltage (it has no path to discharge). I have also been known to use a small DC power supply (rated at a max of say 400 mA or so). Then what you do is you max out the power supply and it will cause the power supply voltage to drop significantly and then as the capacitor charges, the current draw will remain constant (due to the power supply being maxed out), but the output voltage will rise until where the capacitor is near full charge.

 

Beg pardon about the supercapacitor -- it was in a different thread of mine -- must be gettin' old!

I think further discussion re. electrolytic capacitors is in order; in fact it was the most oft. replaced component in my electronics days.

Electrolytic capacitors, as the name suggests, use an electrolyte as dielectric; so these capacitors are moist inside. These capacitors give you a lot of microfarads for your money, but not for ever, and not as efficiently as other more expensive types.

Electrolytics have a finite life, good ones about 5000 hours, cheap ones as little as 1000 hours. This is due to "drying out" of the moist electrolyte gradually over the service life of the capacitor.

There is another problem -- because of the style of operation, there is an internal, very finite resistance known as "ESR" -- effective series resistance. This gets gradually worse and worse until the capacitor mal-functions in its circuit. This effect is very well known indeed to electronics engineers, both design and maintenance.

Because of this ESR, the capacitor doesn't 'de-couple' or protect the intended circuit too well, ESPECIALLY where high frequencies are involved -- hence the method of shunting the electrolytic with a better but smaller kind, e.g. foil dielectric, usually plastic, or tantalum which have lots of microfarads but are a bit pricey.

ELECTROLYTICS DON'T LIKE HIGH TEMPERATURES! In every case in my experience, electrolytics which were near hot resistors etc. failed far quicker than in any other position. Also, high power plus high frequencies plus heat as in switch-mode power supplies running at 150KHz., were conditions resulting in the kiss of death for electrolytics, in some cases, less than a year.

One of the contributors here is suggesting that filter capacitors are fitted by the car manufacturer -- I wouldn't be surprized, as this knowlege about electrolytic deficiencies is pretty common. It may indeed be the case that unserviceable electrolytic capacitors fitted by the maker is the root cause of all these random errorcodes popping up.

So can you test a capacitor like this? Probably yes! Even in your driveway with minimal tools. Remove the capacitor from the car, and using youir crocodile clip leads, charge it up from your 12volt battery, observing polarity. Then get your test-lamp comprizing a sidelamp bulb with leads soldered on and croc.-clips on the end, and touch it across the capacitor. You should get illumination for a second or so, showing the capacitor will hold a charge. It will have to be more than about 10,000 microfarads for the lamp to light. Alternatively, instead of a test-lamp, you could clip your voltmeter across it and see how long it will hold its charge. Comparing your old capacitor with a new one should then determine if the old one has had it. HOWEVER -- if the capacitor is more than 5 years old and lives in a hot area, CHANGE IT ANYHOW!
Be wary that the maker may have fitted more than one of these filter capacitors. Don't pay too much for a OEM capacitor, if it's more than a few dollars, buy one from a radio hobbyist/computer store and transfer the leads from the old one. If you can get a tantalum to shunt it, so much the better, it will last longer and be more effective. CHECK ANY PLUGS/SOCKETS AROUND THE CAPACITOR FOR CORROSION! Poor contact in a plug will render the capacitor useless.

Leedsman.

p.s. Checkout "supercapacitors" by Googling. They are a relatively new kind of capacitor where the dielectric is only nanometres thick, hence the low operating voltage. Not to be confused with a gigantic electrolytic.