Alternator Diode Question & SHUNT Catalog Photo

[Mike and Joanne Stimmler]

While preparing to head out of the dock,the Admiral inadvertently turned the battery selector to off while the engine was running.  know this usually fries the diodes but everything seems to be OK. Both voltmeter and ammeter seem to be reading normal or does the problem not show up on the meters. Is it possible that they could survive a short duration of being without a load? Also, I have never physically traced the charging wire from the alternating so it could be bypassing the switch.

Any thoughts?  

Mike

[Stu Jackson]

Mike, try this:  http://c34.org/bbs/index.php?topic=2977.0

You should trace out the wiring.  Most go through the 1-2-B switch, which I discussed in my Alternator Regulator upgrade in the Nov 2006 Mainsheet article.

Here's part of that: 

Alternator Output:  The older boats with only one 1-2-B switches are usually factory wired with the alternator output to the "C" post of the switch.  This means that the switch was actually used for two purposes: (1) which battery bank(s) the alternator charge goes to when the engine is running; (2) which bank is chosen for use for DC power.  ...   Another way to put this is: If your alternator output is wired to the C post of the 1-2-B switch, and you do not have a combiner (or equivalent), then you have to start the engine with the switch on B (ALL) because this would be the only way to charge both of your banks from the alternator.

Don't know what you mean by "normal" operation of the meters.  The standard panel ammeter tells you the momentary draw down FROM the batteries.  I haven't seen one (a standard simple 25 A ammeter with built in shunt, NOT a LINK or other more sophisticated amp measuring device) that measures amps IN. 

The voltmeter is the key, and if your alternator is working you should see over 13 volts on the voltmeter when the engine is running.

[Ron Hill]

Mike : I don't know how your boat is wired.  You may have a Zap Stop on your alternator?  You may have a battery sense wire direct to your batteries?  I've also seem a Perko battery selector that had the capability of being shut "off" and not fry the batteries - you might have one of those? 
The frying occures when the voltage regulator detects "zero" battery voltage !! and tells the alternator to "go get'em" !!!   

[Mike and Joanne Stimmler]

Thanks for your ideas,guys. I'm going to have to dig into this a little deeper. The ammeter I believe was added or moved by the PO and is the type with the needle in the center showing either a + charge on one side or a - discharge on the other side like an automotive ammeter. It is not part of the instrument panel but is mounted over top of the engine access door in the aft cabin (an odd location). My volt meter IS part of the instrument panel but I am missing a fuel gauge so it seems things have been modified.

Mike

[Stu Jackson]

Mike, the voltmeter in the engine instrument panel will show a pretty appreciable voltage drop from what is actually at the batteries.  I recommend measuring the voltage at the house bank, then the switch and compare these two to the instrument voltmeter.  I lose about 0.4 to 0.6 V because of the relatively small wiring between the banks and the instrument voltmeter.  Our voltmeter is in the nav station added by the PO and wired to the 1 and 2 posts on the switch, and shows pretty accurately compared to the batteries, although there is a 0.1 to 0.2 V drop in that wiring alone and it's mostly #4!

You could buy a real ammeter (0 - 25 A with a built in shunt - Blue Seas System Model # 8005) and wire to your common post of the switch.  It'll only tell you what is going out.  We have one of these in our nav station too (neither is part of the original Catalina electrical panel).

You'll need to find the shunt you do have for your ammeter and figure out how it is wired, so you'll know for sure what it is measuring.

This is a description of how shunts work, courtesy of Blue Seas 1997 Catalog.  You can print this out directly and it will be about half size.  If you save it to your computer you should be able to print it out full size 8-1/2 x 11 and be able to read it more easily.

[lazybone]

M&J,
I'm going to assume that when you left the dock that the boat had been plugged into the AC and the charger was doing its job, if that was the case then the altenator was probably putting out very few amps when you set off.  The diode packs will absorb a bit of heat before they barf, so since your altenator is still making a charging current your probably OK.
Its not something you want to do too often though.

[Mike and Joanne Stimmler]

Yes, your assumptions are correct. I think I may have lucked out this time.

A worse problem could have been my tone of voice to the Admiral.