What is this ?

[sy_gilana]

OOps , I just saw the drawing. Its not for the fridge at all (slap on my head)
The only reason that I can think of is to be a general surge suppressor on the DC mains system. It cant react fast enough for RF noise, it reacts much faster than a battery, and its connected at the combiner. Perhaps it is to stop arcing as the combiner switches in. I think that if the combiner switches in with batt 3 down a bit, there is going to be a short sharp drop on the 12v side, recovered slowly as the charger catches up and the voltages equalize between the bat1/2 and 3. Thats all I can think of...

[arjand]

@ hpeer
this is what I made of it. Not sure how I could rationalize it further....

[s/v 'Faith']

The prior replies are correct in saying that it could be there to filter 'noise' to ground, but it would have another use also.

If you switch your battery switch to off, and then on again you would not loose your radio pre-set's or (possibly) make your GPS have to get a new fix on power up...

Such a large cap is more likely intended for such a use, (IMHO) then as a filter.

[arjand]

Very clear. It is not really in the way either so... Thanks!!

[arjand]

Can it be connected to the ground on the engine or would that be incorrect? (I assume that is where it came from....)

[Reefmagnet]

It can be connected anywhere a good ground is present, and the engine is normally a good ground with suitable preparation at the connection point.

But....

Logic would dictate that the capacitor is/was physically installed in the circuit at the point where the problem was perceived to be. How good is the installation? Does it look professional? If it doesn't, don't underestimate the ability of a clueless PO to have installed it in a totally ineffective spot for a totally ineffective solution to a problem. If it looks amateurish it could have been intended to solve any of a multitude of issues, even RF noise (it is the wrong type of capacitor for that). If it appears professionally installed it would be more likely, I think, intended to act as a surge suppressor (stopping spikes in the DC supply), load balancer (e.g. for the fridge start up, especially if the wires are on the light side) or temporary power source while the batteries where switched over (e.g. to stop the radio loosing station memory).

[arjand]

The original set-up (drawing from 1996...) looks professional and done by someone who knew what he was doing. With the capacitor connected to the ground and the battery combiner (that combines the batteries when one hits a certain voltage when charging) it would indeed be efficient as a surge suppressor correct? I will read the manual of the battery combiner if that is indeed recommended. Thank you!!

[stevensuf]

it is a 12,00 micro farad 40v dc rated electrolytic capacitor, the can be used like a current source for surge loads eg helping to start motors, large momentary current draws, think like a very low resistance battery to a dc load, on ac they can act like short circuits, they have what is called reactance, kind of like a variable resistor whos resistance changes with the frequency of the ac voltage, quite often they are used to filter ac ripple on dc power supplies, eg noise from an alternator, someone most likely had a noisy alternator in the past and fitted the capacitor to remove the whine, think old car radios where you would get a background whine which was related to the engines revs.

[flemmingsveen]

I have seen this sizes mounted in yacht with very much equipment. They do as Reefmagnet says. It have some adventages and some disantvantage. They make the dc voltage more stable and some other good side effects. But very important to understand that it can give very much current in no time in case of shortening and damage.

So be careful to mount it properly and protect it from any physical abuse it scary to jam it somewhere. (I dont know how to put it in good english)

"Edit: Normal is to mount a supressor on the alternator to reduce noise, this is a 1.2 farad one, some people use capacitor inline between alternator and battery"

Have fun with it!

[arjand]

hmmm... that is a totally new point. So: if there would be a short on the boat, the amount of 'available' current is a multifold of a situation without the capacitator? Does this also mean that the fire risk is multiplied ? (Batteries: 2 x 198Ah and 1 x 100Ah, all AGM)

[stevensuf]

not really, they can only supply large amounts of current for very short periods of time, think micro seconds.

[stevensuf]

nb it is 0.012 farad not 1.2 that would be a fairly large capacitor! on 12v they are not especially dangerous but the high voltage ones can be lethal!

[arjand]

ok, sorry, just good to know what is on board I guess. thanks!!

[Rapanui]

One use for such a large capacitor is to prevent voltage dips when switching loads which can reset/upset some sensitive equipment. Most stuff designed for cars and boats these days copes ok but stuff brought from home running on DC or DC/DC conversion tends to want more stable and regular power supplies, particularly things with programmable settings eg satellite receivers, DVD players etc. I did the same thing myself when my original DECCA receiver used to get reset by spikes/dips. One reason why these herberts with big CB transeivers in cars use 1 farad or bigger capacitors across the 12V. Not a bad thing to do now such capacitors are very cheap as it reduces significantly the voltage spikes/dips which are the main reason for electronics failing early. It will particularly help if you have a relatively small battery capacity. The only issue is to ensure that the capacitor is fed from the main fuse before the smaller consumer fuses as the inrush current when switching the DC on will be very large for a short time.