Isolation diodes for battery charging

[William_H]

Being a compulsive puller apart of junk specifically in this case old computer power supplies I have found that they contain Schotky Diodes in a high powered pack. The specific one I found data for is rated at 30 amps and has a forward volt drop measured of .15 volts. This means that you could uuse it for isolating yet charging your second battery or for ensuring safety when using 1,2,both switches. Thi9s volt drop will detract form your charging voltage but obviously not as much as a silicon diode at .7 or more volts will.
The diodes come as a pair in one package unfortunately the cathode is common so you would need 2 packages. Connect the outside 2 anodes together and connect to the alternator out and the middle terminal to your service battery. Connect another package in the same way outer 2 terminals to your alternator out terminal or wire and inner terminal to your engine battery.
Of course the wiring of your system is never that simple so these diodes should be used as any other diode isolator.
The body of the diode pack has a metal surface which should be firmly bolted to a piece of aluminium which becomes a heat sink. You will need 2 pieces each at least 10cms by 10cms or similar area. If the diodes are bolted to the aluminium then the Al becomes electrically connected to the positive of each battery. You will need to isolate it from any thing at negative earth potential and fuses at the battery would add to the safety.
The other alternative is to bolt the diode to the aluminium with a thin mica or special plastic washer between the body and the Al. These washers are designed to conduct the heat but electricaly insulate. You may need also plastic insulating washer to insulate the attaching bolt. These insulators will come with the diode from the computer power supply. Check with a meter after tightening the bolt to ensure insulation. and use fuses.
the only tricky part is to identify the diodes about 1.5cm square 4mm thick with 3 wire legs. They have a symbol of 2 arrows pointing toward one another and the centre lead. There may only be one large one in each power supply. There will be 2 transistors in there that look similar but are no good to you. This is especially for those of us who love to recycle and go cheap. PM any questions olewill PS no I havn't tried it. Wouldn't have a motor on my boat so let me know if it works out well.

 

[jerryat]

Hi Will,

That's brilliant! I'm afraid I'm pretty basic with electronics but I'm was just about to ditch an old computer, so will strip it down and have a look tosee if I can identify the diodes you mention.

Only got one computer so won't be able to make the full Monty, but there are loads of old computers out there, so another should be available soon.

My Adverc controller does take account of the 0.7 - 0.9 voltage drop across the blocking diode, but thi sis a better solution.

Cheers Jerry

 

[savageseadog]

Of course you can use scottky diodes, but do remember that there is more to current ratings. There is the average current rating and peak current rating and also something you touched on to do with temperatures. You will not find that any designer will use a diode at anywhere near a working current equal to it's rated I max. A reasonable rule of thumb would be between 1/10th to 1/3 depending on the situation. There is the problem of temperature, the diode will heat up and will have to be cooled as you say, the point is that the diode will still be higher than the ambient temp whatever you do. The higher temp willl limit the current you can put through it. Then there is the problem of peak or surge currents, the current will not always be a nice smooth waveform you will have to allow for that. The current you quote is low for most boat charging systems anyway many of which are at leat 40 Amps going up to 100 amps.
Please remember when you put stuff like this on your boat you are potentially reducing the reliablity of it.
It's good though to see people willing to experiment and think about these things.

 

[Phoenix of Hamble]

How about finding a second one, and placing that in the sensing circuit, and then the volt drop will be compensated for and corrected.... this is a principle that I use on my boat to good effect.....

 

[Anonymous]

I don't follow that...the whole point of a sensing circuit is to measure the actual battery terminal voltage. The sensing wire and circuit is designed to compensate for the volts drop of the splitter as well as the volts drop due to the charge current and there is no need to add more circuitry. If you fitted a diode in series with the sensing wire, then the sensed voltage would be under-measured by an amount equal to the forward volts drop of the diode and the battery would possibly be overcharged.

 

[William_H]

Had another brainstorm for these diodes. And yes Savageseadog is right in that the 30 amp rating may be marginal but OK for smaller battery installations. I have an old Johnson O/B (despite what I said about never having a motor) though it mostly sits at home. I put a silicon bridge rectifier on the lighting output to see what it would do. I got about .7 amp into a 12volt ( read 13 volt) battery It occureed to me that these schotky diodes may improve the charging current by reducing the volt drop in the diodes. In a bridge rectifier (4 diodes) you have twice .7 v drop so lose at least 1.5 volts however with the schotky diodes this could be down to .5 volt. Then again the current into the battery may be limited by the internal resistance of the O/B coils pulling the voltage down while actually starting at a higher voltage. ( a bit like a solar panel) so I have yet to try the schotky to see if there is an improvement.
When tearing apart the computer PS you will find a dandy 12 volt fan and also a silicon bridge rectifier over near the fuse. This is usually a black plastic pack 2.5 x1.5 x .4 cm with 4 thickish wires out of the base. and of course there is a fuse so save all these bits. also sometimews a switch.
PS a bridge rectifier is a block of 4 diodes connected in what looks like a diamond shape on a circuit. The cathodes of 2 are connected together as are the anodes of 2. An anode of the former goes to one AC line while the other anode of the former goes to the other AC line. A cathode of one of the latter goes to one of the AC lines while the remaining cathode goes to the other AC line. The effect is that a positive on one AC line gets directed via a diode to the + o/p pin while the neg on the other AC line gets directed to the -ve o/p pin. When the first AC line goes neg another diode directs that neg to the -ve o/p line while at the same time the pos goes via the last diode to the +ve o/p line. All of the AC wave is utilised hence best efficiency. (hope you can follow) ( hope I am not teaching grandma to suck eggs) As you can guess it is bleeding cold and wet here and no way am I going outside hence the key board diarhoea. don't let em talk about a drought in Oz regards olewill