# advice on simple alternator excitation circuit?

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• 10-28-2010, 12:46 AM
Simbalage22
Quote:

Originally Posted by therussman2002
here's my problem:

i have a 3 battery 160amp isolator under my hood such that my three batteries do not draw off each other (1 for car, 1 for amps, 1 for 'puter/etc)

the alternator has this wire coming off of it that reads the voltage of the battery and then decides how much voltage to spit out, called te exciter wire i suppose

it seems that when i connect a small wire from my main car battery to this wire, that battery becomes fully charged and causes the alternator to quit charging, but one of the other batteries may NOT be fully charged

so i suppose i need some circuit that will input 3 voltages, and output the lowest of those values out the other end... this sounds pretty simple, but the few EE classes i've had (and gotten c's in) arnt much help at this point (twas a while ago)

this will give that exciter wire the lowest voltage, and ensure all of my batteries are sufficiently charged

can anyone offer some design advice/insights?

This is what I am after... My alternator has this sense wire (Ford Aspire) I have a 3 leg Vanner isolator installed. The big lug is connected to the center pole, and bat 1 and 2 respectivly.. My problem is what therussman2002 describes. As the alternator has the regulator built in and controls when its on between the S & L Wire... how would you wire it up to see the voltage of both batteries? I have it the way it is stock.. and at one point had JUST the starter on the front battery, and EVERYTHING else on the back battery.. But because the front battery is only drained for a second starting.. The alternator is only sensing that battery and ignorning the state of the second battery..

I too though diodes, but wouldnt that create a 24V current to the sense? if not, still would sense that 1 battery is 100% thus ignoring the second one and shutting down (IE: still say hmmmm i see 14.4v kick off)

Someone said there is some smart switch that can do this, but I've yet to find it.

I got this isolator and the reece lavelle alternator from an ambulance.. But the alt is toooo big for my little ford... So if anyone knows how to properly install this so it will correctly charge both batterys without connecting them ++ and -- because i want them separated (IE ISOLATED) from each other!
• 08-16-2011, 10:37 PM
OldSpark
Alas this is a long dead thread - long before my time.

Alas the solution is to reduce the inter-battery resistance. Isolators should only add milli-Ohms to the interconnection.

For the OP and other suggestors, and readers - the alternator does not stop charging, it continues to put out its 13.8V to 14.4V set voltage. The main battery merely stops absorbing current (except for its "float" current - typically up to an Amp or two).
The other batteries are still being supplied with that same (say) 14.2V, but their is voltage drop along the distribution because if the IR losses (current I through the interconnection resistance R, where the current I is the sum of the battery's recharge current plus its load current.
It is not a mere case if averaging or sampling voltages - that is inappropriate without some form of regulator between the alternator and the "higher voltage" battery(s) (eg - a PWM chopper).

Summary: Improve your inter-battery distribution. Ensure there are minimal voltage drops to the battery +12V and along the ground path.

PS - Simbalage, do you have a charge light?
• 08-16-2011, 11:00 PM
Simbalage22
Yes I have a charge light, as well as a sense wire connected to a 3 pole switch as well as a digital voltage display.. This way in the even one battery is drained flat, flipping the switch will not only display the charge in each battery but will trip the sense wire to acuratly measure the weaker battery so it doesnt stop on account that the other battery is full. Thats the only flaw with an isolator but you are correct to an extent.. the isolator will give the charged battery a float and concentrate on the battery with most resistance.. only factor is most alternators "Do" have a sense wire that tells it to kick back charging when the battery wired to it is full.. SO large wire goes to alt term on the isolator and my switch has 6 legs.. so the top 3 work the led digital display, and the other 3 at the bottom direct which battery will feed the sense wire. Havent had any problems with overcharging/undercharging of these 2 yellow top optimas.. Likewise relays are better when you want to increase the overall capacity of a system, but arent really meant to isolate batterys as people use them... because imagine a dead battery being thrown at a completly charged one.. that good battery had to work twices as hard to feed that dead battery.. would be like starting your car everytime and throwing a dead battery at the alternator every time... like jump starting someone dead car everytime.. Isolators just even out the load and charging and do what they were ment to... Separate hence why they cost more then solid state relays .... Just my 2 cents on the matter
• 08-17-2011, 03:40 AM
OldSpark
Sorry, but you do not understand how a car's electrical system works.
There is no "kick in", the sensing is simply to regulate the systems voltage to (say) 14.2V.

12V battery systems are akin to a "constant voltage" circuit.

If the alternator stopped charging, the system would drop immediately, eventually going under the ~12.7V of a fully charged battery (with surface charge dissipated).

Of course all alternators have a sense wire - how else do they regulate? With D+ types, the sense is internal (hence their traditional SS isolator problems).

Alas we don't have any problems. We don't seem to have surge problems.
We charge both and all batteries with the same source voltage, automatically, and without any switching.

I think I have sufficiently addressed the failed battery issue. And it has no effect on cranking the car!

I'm posting free.
• 08-17-2011, 09:01 AM
Simbalage22
Quote:

Originally Posted by OldSpark
Sorry, but you do not understand how a car's electrical system works.
There is no "kick in", the sensing is simply to regulate the systems voltage to (say) 14.2V.

12V battery systems are akin to a "constant voltage" circuit.

If the alternator stopped charging, the system would drop immediately, eventually going under the ~12.7V of a fully charged battery (with surface charge dissipated).

Of course all alternators have a sense wire - how else do they regulate? With D+ types, the sense is internal (hence their traditional SS isolator problems).

Alas we don't have any problems. We don't seem to have surge problems.
We charge both and all batteries with the same source voltage, automatically, and without any switching.

I think I have sufficiently addressed the failed battery issue. And it has no effect on cranking the car!

I'm posting free.

Sense wire hence my way of explaining.. I know about 12v.The sense wire also tells the alternator to stop charging in the event the battery is charged, until the system places a demand on the alternator, Likewise its a matter of preference. You wont see your setup on any large 260amp or higher application unless of course battery separation isnt the goal.. Putting batteries together sure your relay will do just fine.. but they both have flaws.. Relays have moving parts hence fail.. for the 0.2v loss my diodes cause i aint complaining. Least I know my battery's life isnt shorted using the type of setup u are talking about
• 08-17-2011, 09:10 AM
Simbalage22
to therussman2002

I will take pictures of the switch is speak of in above post. Its not necessary to have, but does address the issue of under/ overcharging..Still would rather have to flip a switch and know my batts are equal charge then have a full charged one in parallel and it have to work hard when you decide to put in back in the electrical system.. who would want to keep throwing a dead battery in the equation.. Isolators keep them connected all the time even when charging yet still does its job to keep them separate... its common sense really.. Relays are mainly switches that are controlled by an electrical source. A true isolator is a one way valve simply to charge 2 batteries so they NEVER see each other
• 08-17-2011, 11:35 AM
OldSpark
LOL! It's called the Sense wire - that's what S in SL & SIL stands for. But just so you are clear, that's alternator jargon.

You still have not still answered my original question.

And you have never had my relay set up - that is obvious.

And sorry, but I have seen the alt-S set up for several hundred Amps - some set-ups involving several isolators of 400A rating, one for each battery.
The diode isolator you have is unsuitable in those setups because the aux batteries are required to be paralleled during use (when charging; also when not charging, but that's a separate issue).

The alt-S isolators do the same as yours - they "keep them connected ... when charging yet still does its job to keep them separate" - that too is common sense and readily understood by most.

I can only repeat, the Sense wires DOES NOT tell the alternator to stop charging. Look at any alternator and show me one that does. But that should be common sense after the description I gave earlier. Or does you vehicle drop to ~13V once the battery is charged?
(Trick question - the alternator has no way of knowing the SOC of a battery.)

I agree too that's it is often a matter of preference.
There are those that like to spend over \$200 on isolators. Admittedly they are usually voltage-sensing types (probably the bulk of the market), but since the alt-S type is DIY with off-the-shelf components, it is difficult to gauge their market penetration. (A colleague that has recently been enlightened to alternator-controlled isolators found a 4WD colleague that has the same arrangement - not that the owner knew much about it - it was simply there when he bought the 4WD and it was still working.)

But there are others that like have similarly available components (though 200A - 400A relays may be less widely available, but certainly more available than diode and even voltage-sensing isolators) and like the price of typically ~\$30-\$40 for a 400A rating, which can also be easily manually controlled, or fitted with added features. (Now you have ME repeating myself!)
But the normal operation is automatic. Connect when charging. Disconnect when not.
[ Noting that voltage-sensing isolators often have a connect delay, and a disconnect delay, as well usually staying connected until the rail voltage drops below 13.5V (max) to 12.8V - and I've even seen 12.5V as a disconnect voltage (~20% discharge on an otherwise unloaded battery!) yet people still buy them! But 13.5V and even 13V can still mean several minutes to several hours of connection. They then wonder why they blow their isolators or distribution (fuses etc) when they get in and crank the car! LOL. ]

I would like to know how your isolator "gives one the float and concentrates on the other". That is rubbish. It is merely a diode. If it were a MOSFET type, yes - it could throttle back the current, but why? (And few if any do.)
If it's a faulty battery, then replace it - the isolator should not be compensating for that.
But I do detect a lack of understanding of vehicle charging systems and "constant voltage" concepts etc, and that is understandable (I knew, yet it still took a while for the ramifications to really set).

A true isolator is a device that isolates devices. Look it up. It does not mean they never see each other.
But why should 2 batteries not see each other whilst they are charging?
Some arguments seem as incomprehensible as the desire for priority charging! (With a few exceptions where it is desirable or warranted, but they are exceptions to typical and general dual/multi-battery use.)

And I worry that your isolator keeps them connected all the time! I thought is was an isolator! (It's diodic so you have their one-way behaviour, hence that are NOT interconnected per se. That is why you MUST have an separate switch (relay) to bridge out the diode isolation.)
I know what you mean, but is is a senseless comment - that interconnection means nothing without the charging source.
And as I wrote before, Venner even have that special add-on "excitation circuit"(sic) for cars that won't charge without it! So much trouble to go to, and all because of that diodic isolation! (Diodes are old tech. Schottky is merely new fabrication. FETs replaced diode isolators ages ago and they can cater for those "alternator excitation" problems.)

Now please, you are repeating the same over and over, and somewhat elementary stuff at that. Yes, I think we all know what a relay is.
And I presume most readers would assume you have a 0.1 to 0.3V drop across your isolator (whereas I have way under 10% of that).
And many might ask what their (voltage) transient protect is. A not uncommon 200V - 400V spike will do nothing to a typical relay, but as I recall, those Schottkys only have a PIV or skin isolation of 100-200V, and can scare many off semi-conductor isolators. FETs are now ok to 600V and above. (Bring back SCRs!!)

I expect not to reply unless you have something useful and new to add, else answer my original surge-damage question (ie, your "problematic surge" statement), or what original setup you had that "damaged" your batteries, or explain or (usefully) link instead of parenthood "common sense" type statements, .
(Others should feel free to query - especially those that reckon it is "common sense" to use the alt-S aka UIBI that I oft describe! Though the latter may already understand what I am writing, or see the irrelevance or insignificance of various comments... )

I should state that I was privy to a team that brainstormed and analysed voltage-sensing isolators against alternator (S) controlled isolators for rotor controlled chargers (alternators).
The upshot was that the voltage sensing had several failings. (That varies with specific "smart" isolator characteristics, but the plethora of different voltage thresholds and switching delays marketed was {quote} a testament to the disarray of opinions etc.) Compare that to the alt-S control that essentially only had one downfall, and that was considered an insignificant rare event compared to the various voltage sensing problematic situations that typically occur (both theoretical, and empirical data).

But in that evaluation, diode & FET types were written off very early due to their expense, size, heat, virtually nil expandabilty and inability to be augmented with manual controls (ie, bypass, paralleling with no charge, etc) though FET types can be capacity expanded (expensively and with compromise) and provision made for added triggering, but voltage sensing an alt-S activated isolators can incorporate that very cheaply and simply.
In summary, diode isolation was considered ideal for signal switching (circuits generally less than an Amp) and small isolator requirements (up to 10 Amps, maybe more with heatsunk alternator-rated diodes (but they are generally only up to 40A), but unsuitable for the majority of dual-battery user intentions (max voltage, big audio setups, winches). Then there is the aforementioned expense and limited supply. (How many carry a spare diode isolator in case the original fails? Few even carry spare voltage sensing isolators, though their relay can usually still be used if the electronics fail.)

But you'll have also to convince the many other isolator users out there,
By far the majority use relays (voltage else isolator controlled; I/we don't consider manual or (IGN) switch activation as part of that group), so there are far more people switching these alleged surges etc than those that use diode-type isolators. I wonder why if they are so unreliable or problematic - not that a majority means it's the correct practice (eg, previous practices of fixed parallel-battery wiring; oil-pressure to control electric fuel pumps; brushing teeth AFTER meals; or Halley's comet being shiny; etc - just some of the typical things I have mentioned on this site).

But don't restate basics.
And you'll need something better than the Vanner literature. As far as they are concerned, all isolators are solid-state, and they only compare to older typical silicon-diode isolators (>0.6V drop), and conveniently ignore FETs, and applications other than basic "separation of power sources" (as for ambulances and emergency vehicles). Alas we on here are not fitting isolators to such vehicles. And if diodes are used, they are usually only up to a 10A rating.
Like I wrote, at least you understand the need for battery isolation - not that that is majorly contested on this site. (But they are a clever bunch!)
And likewise, your arguments merely augment mine against voltage-sensing isolators - eg, that with delays or (cough cough) "priority charging", any switching surge is made worse!

We like something that is simple and reliable, preferably cheap and small, and adaptable to differing requirements and scaling. Solid-state isolators generally fail on those grounds.

And I think that most of us expect to find basic specs - like the voltage drop of the Vanners. Why don't they publish them - is it because they are an order of magnitude or more than relays?

Can you could provide the MTBF or similar for your Schottkys (again, no Vanner data, nor transient tolerance etc) as well as their voltage drop? Isn't that stated in its manual?

Relays typically have over 1 million contact operations and over 100,000 electrical (coil) actuations. (And I know they tend to be (or used to be!) conservative - typically lower percentile figures rather than the majority.)
So, lets see - 100 car starts per day, that's 3 years minimum Not bad for \$3 upwards.
Or 30 years for 10 daily starts.....

If only alternators lasted that long! And batteries.

And may you overcome your voltage difference problems. Alas that is something we do not experience - another advantage of NOT having diode isolators!

Over and out. (New posters and value-added info excepted.)

PS - don't worry about pictures. I fail to see what possible use they can be. Like I say, I am quite familiar with isolators and vehicular installations and applications.
Find those specs instead, or provide a typical voltage drop, or explain your "original" isolator type and wiring, or describe your "surge failure" else problems caused for others (and why if any difference to jump starting and the majority of isolators increasingly being used)....
Pictures... - why?
• 08-17-2011, 03:07 PM
Simbalage22