Engine sensed alternator additional diode

[catmandoo]

Got two alternators at present .Both mechanically installed ie turn with engine . Newer one is Motorola 70 amp with integral regulator domestic battery sensed via Sterling booster recently fitted . Old Bosch 30 amp alternator has field wiring disconnected but has separate electro mechanical regulator machine sensed mounted on back of engine .Output of both alternators feed 3 battery banks(starter, lighting and domestic) via split diode isolators. Field circuit setup was to use both alternators or either by on off switches in both field circuits . Want to reinstate system . However looks like additional diode missing on old regulator sensor line to regulator to ensure correct operation of the complete system .

Anyone know where I can get a suitable diode presumably with voltage drop of 0.7V to replace the missing one . Not sure that Maplin can oblige .Thinking of spade connections . Location close to alternator

 

[William_H]

Maplin RS etc. should be able to provide a diode. You might consider a diode bridge rated at 35 amps or 50 amp. They have a single bolt mount to go onto a heatsink (large mass of metal ) and spade terminals. Now the confusing part they have 4 diodes all interconnected. You could use 2 diodes or one. The terminal marked + has 2 cathodes connected to it with the anodes connected to the 2 AC terminals,while the terminal marked - has two anodes connected to it.with the 2 cathodes connected to the 2 AC terminals. For a normal battery isolation set up you would connect the alternator output to the - negative terminal and a battery to each AC terminal.
I suppose it is the way you describe but obviously if the old alternator is to work at all it must have the field wiring connected. If you go to an aircraft electrics workshop they have a balancing regulator which will ensure equal load on each alternator on twin engined aircraft. Go to a light aircraft field and ask, unfortunately they are mostly 24volt systems now. (You might be interested to find big alternators available ie 200 amp) I am not sure how the Sterling booster would fit with this but it is a thought otherwise I would use the old alternator connected in the standard manner on the engine battery and the big alternator with Sterling on the other 2 batteries. Just a few thoughts to confuse you. PM me if you are confused about usibng the diode bridge. (diodes are all .7volt drop unless you get a special lower voltage drop type) regards will

 

[pvb]

If you have a Sterling booster on the 70A alternator, your old 30A alternator won't be contributing much, if any, charge. Better solution would be to wire the output of the 30A alternator direct to the starter battery, and have the 70A alternator charging the other batteries. With no split diode isolator in the starter battery circuit, you won't need that extra diode in the 30A alternator's sense line.

Although it's not necessary, you would gain some overall improvement in efficiency by combining the lighting and domestic batteries into one bank. You could then dispense with the diode splitter (one less thing to go wrong!) and your batteries might last longer.

 

[catmandoo]

Got 2 field on off switches on engine control panel .Had thought of dedicating old to starter battery but was concerned that if I inadvertantly put field switch to off then would not know till too late that starter battery was flat . I suppose I could then use battery switch to start off no 2 . If I linked 2 to 3 could get rid of split diode hardware but then this would not give me a separate unique alternative to the starter battery. With 2 alternators to feed all batteries then would have supply if Sterling booster failed andalternatively both to give me 100 amps should I need it all at once

need to do a risk and load analysis perhaps /forums/images/graemlins/grin.gif

 

[pvb]

Some more thoughts...

You should try to make it all as fail-safe as possible, so that inadvertently flicking a switch won't ruin your day! Might be an idea to disconnect those field switches.

If your starter battery is completely separate, in good condition and independently charged, you shouldn't have a situation in which it gets flat.

Sterling booster has "fail-safe" mode in that it works in parallel with the alternator's own regulator. If the Sterling booster dies, the alternator's own regulator will continue to work.

Both alternators won't charge the batteries if one of the alternators is being boosted. If you imagine what happens at the battery - the Sterling booster increases the 70A alternator's output until it achieves say 14.8v at the battery terminals; meanwhile, the unboosted 30A alternator is merrily working at say 14.0v. Because the voltage at the battery is higher than the voltage at the 30A alternator, no current will flow from that alternator.

Incidentally, you need a big battery bank to absorb 100A. I've got 660Ah in my domestic bank, and it's hard to push more than about 90A into it.

 

[catmandoo]

Re: Some more thoughts...

got 400 Ah but thought of adding a winch and using both alternators to backup domestic supply( 200 Ah ) when operating . 30 amp alternator acting as spare in normal duty so if 70 amp failed 30 amp would come in to play