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Old 03-03-2012, 15:51   #1
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Battery Isolator Diode and Alternator

hello
i have remouved the isolator diode
the on who have one input "alternator" and two ouput "batery 1" and "batery 2"

because i just have a batery bank and an emergency starting batery
that i put in paralell with two swith on in the neg and on in the positif
to charge it every month and to use it in case the main batery bank fail

that the way i choose to manage the energy (Two motor , two alternator, 1 batery bank, 1 emergency batery)

so the main bank is used for everything, and the two alternator charge the main bank only

so by this way not problem, with overload or underload batery

As i remouved the isolator diode, i think i have remouved an alternator protection , all both alternator output go directly to the batery

and i don't loose the drop voltage from the diode, and the changing voltage output is good now

What do you think
beter to add again an isolation diode, to have an extra protection for the alternator

or it is usefull to make this

thank you again
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Old 03-03-2012, 16:02   #2
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Re: batery isolator diode and alternator

That's fine. You don't lose any alternator protection by removing the isolator diode and connecting directly to the house batteries. In fact, that's the recommended way of doing things these days.

However, it is a good idea to have a fuse in line on the alternator output cables, located close to the batteries. It should be sized 120-150% of the alternator's maximum output. ANL or MRBF fuses are the best to use, as they have the necessary ampere interrupt capacity (AIC) and are "slow-blow".

Bill
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Old 03-03-2012, 16:14   #3
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Re: batery isolator diode and alternator

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Originally Posted by btrayfors View Post
You don't lose any alternator protection by removing the isolator diode and connecting directly to the house batteries. In fact, that's the recommended way of doing things these days.
That is right but not the whole story. The batteries are not likely to harm the alternator, unless someone disconnects them while the engine is running (a very good reason to not have a switch between the alternator and batteries). OTOH the alternator can fail and short, which can cause a fire or at a minimum damage the batteries, and is prevented by the isolator diodes. That said, a fuse is always a good idea.

Some alternators have the voltage sense for the internal regulator available on the outside, such that the voltage can be sensed on the battery side of the diodes. This is the case on the new Volvo I just bought. Otherwise, installing external multi-stage regulators will improve charging performance and will allow voltage sensing at the battery.

Personally, I dislike having any switches in the charging circuit for the simple reason that it is too easy to make a mistake, resulting in incomplete charging or even a damaged alternator. Diodes are inexpensive, simple, and very reliable.
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Old 03-03-2012, 16:42   #4
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Re: batery isolator diode and alternator

about the switch story
I have put a switch to put the emergency batery in paralell
i am not using a swith in the output of the alternator
what will be a big mistake a think

I will add a fuse at the output never know
i alway forget to put a fuse

thank you
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Old 03-03-2012, 17:49   #5
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Re: batery isolator diode and alternator

Yes, that's what I understood from your original post. It's OK as is, but it could be better to use, e.g., a voltage follower device like the Xantrex EchoCharge or the Balmar DuoCharge to automatically keep the reserve/starting battery charged. This is totally automatic, and would not necessitate switching anything.

And, you're right, it would be a mistake to put a switch in the positive output cable between the alternator and the house batteries. Only a fuse of the appropriate type and size should be put in that circuit, as mentioned above.

As for switching the negative, that's a big NO-NO for boats on this side of the Atlantic. While I understand that many European boats have such a switch, the belief over here is that it is a very bad idea to switch the negative because for safety reasons there should ALWAYS be a good path to ground.

Bill
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Old 03-03-2012, 17:55   #6
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Re: batery isolator diode and alternator

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Originally Posted by CarinaPDX View Post
That is right but not the whole story. The batteries are not likely to harm the alternator, unless someone disconnects them while the engine is running (a very good reason to not have a switch between the alternator and batteries). OTOH the alternator can fail and short, which can cause a fire or at a minimum damage the batteries, and is prevented by the isolator diodes. That said, a fuse is always a good idea.
.......
A diode isolator is unlikely to save your bacon if there is a dead short...it doesn't have anywhere near enough AIC (ampere interrupt capacity).

Per ABYC recommendations, any CPD (fusable device or breaker) hooked directly to the house batteries should have, at a minimum, 5,000 amps interrupt capacity. Only 3 fuses in common use have that rating or higher: ANL, MRBF, and Class-T. And, only the new Blue Sea Systems high AIC breakers meet the ABYC requirement.

Bill
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Old 03-03-2012, 18:26   #7
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Re: batery isolator diode and alternator

I think you are confusing an overcurrent device with a diode. A diode does not limit forward conduction (from alternator to battery) - the alternator output or battery absorption rate will do that (and MUST be less than the rated diode forward current). A short in the alternator results in the diode seeing approx. 12V in the reverse direction - this results in essentially no current as the 12V will be much lower than the reverse breakdown voltage. Of course fuses and breakers are completely different.
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Old 03-03-2012, 19:01   #8
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Re: batery isolator diode and alternator

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Originally Posted by CarinaPDX View Post
I think you are confusing an overcurrent device with a diode. A diode does not limit forward conduction (from alternator to battery) - the alternator output or battery absorption rate will do that (and MUST be less than the rated diode forward current). A short in the alternator results in the diode seeing approx. 12V in the reverse direction - this results in essentially no current as the 12V will be much lower than the reverse breakdown voltage. Of course fuses and breakers are completely different.
No, not confusing the two (CPD and diode isolators).

Diode isolator devices as used on boats have several potential modes of failure. They may fail OPEN, CLOSED, or RESISTIVE. In the second mode of failure, e.g., Diode 1, Diode 2, or both Diodes 1 and 2 may fail in the closed condition, thereby effectively creating a two-way bridge from the battery to the alternator.

Now, if in this condition there should subsequently be a direct short circuit, a very large current could attempt to cross this bridge, perhaps sufficient to create massive overheating and a fire. Yes, I know that this would be a rare occurence, but I believe it's possible. And, as we know, Murphy is ever present on a boat!

In any case, I believe we completely agree that the best solution is to place an appropriate (high AIC) fuse or breaker in the alternator positive cable, located near the batteries, and forget about the isolator.

Bill
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Old 03-03-2012, 19:47   #9
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Re: batery isolator diode and alternator

Part of the point I was making is that a short in the alternator does not cause any stress to the diode and would (almost) never result in the diode failure. Of course if the diode failed shorted (or close to it) then we are back to the disaster scenario. The question you have me thinking about is whether I would know if a diode had failed short at any time prior to an alternator failure (perhaps operating in high heat and high charge rate). I think the answer is no, I wouldn't likely notice. So while an isolator diode pack improves greatly on most installations as far as safety, a fuse is the only (near) guarantee.

I still think the isolator has value, beyond the safety issue. But back to the OP's situation: having the house bank permanently attached with a switch to the starter battery should work fine, particularly after fusing. Since we are all just human, and batteries do die even without our help, I think a means should be available to start the engine from the house bank.

I have to admit that I have a low opinion of the ABYC standards on electrical issues - their track record has been less than stellar. In the past they have been late to extend (or even correct) standards. Until at least fairly recently the default situation for production boats was to tie the alternator and starter together and connect them to the batteries through a selector switch. No fuses in that circuit. Turn the switch off while the engine is running and watch the alternator self-destruct. Crazy design.
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Old 03-03-2012, 19:55   #10
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Re: batery isolator diode and alternator

It does seem like it would be extremely bad luck to have the isolator diodes fail around the same time you had a short in the alternator, but stranger things have happened? I think you might notice eventually that something was amiss if one or both of the isolator diodes failed because the batteries would then be connected and they would always be at the same voltage, which would be unlikely if they were isolated. Also, you might find out one morning that you had drained all of your batteries and the engine wouldn't start!
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Old 04-03-2012, 08:56   #11
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Re: Battery Isolator Diode and Alternator

Thank you to all of you
cruisersforum always the best and faster place a found to get good info

i am starting a new thread i will be happy if you have a look

http://www.cruisersforum.com/forums/...tml#post901208
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Old 04-03-2012, 09:44   #12
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Re: batery isolator diode and alternator

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Originally Posted by btrayfors View Post
A diode isolator is unlikely to save your bacon if there is a dead short...it doesn't have anywhere near enough AIC (ampere interrupt capacity).

Per ABYC recommendations, any CPD (fusable device or breaker) hooked directly to the house batteries should have, at a minimum, 5,000 amps interrupt capacity. Only 3 fuses in common use have that rating or higher: ANL, MRBF, and Class-T. And, only the new Blue Sea Systems high AIC breakers meet the ABYC requirement.

Bill
That doesn't make any sense. I see no reason for a fuse interrupting capacity greater than the CCA rating of the battery plus a safety factor. If the maximum output the battery can produce into a short circuit is 1,000 amps you don't need to be spending extra $$ on 5,000 interruping capacity.
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Old 04-03-2012, 10:08   #13
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Re: Battery Isolator Diode and Alternator

I think Bill is assuming the alternator goes direct to the house bank of batteries, which are very likely to have much more than 1000 CCA, hence the need for a fuse with a 5000 AIC per ABYC.
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