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Blown fuse to isolator


BobBeery

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I found a 30A fuse in the wire from the engine battery to the isolator.  This fuse is blown which explains why the house battery did not charge when I was driving.

The question is why did it blow?  What would have caused it?    I have only had this Toy for a few weeks.  Note--the isolator is bright shiny metal and looks brand new.

I will put in a new fuse as soon as I get one, today or tomorrow.

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20 minutes ago, BobBeery said:

I found a 30A fuse in the wire from the engine battery to the isolator.  This fuse is blown which explains why the house battery did not charge when I was driving.

The question is why did it blow?  What would have caused it?    I have only had this Toy for a few weeks.  Note--the isolator is bright shiny metal and looks brand new.

I will put in a new fuse as soon as I get one, today or tomorrow.

I don't know what happened, but I do know this. You have a 50 or 60 amp alternator.  How in heck can a 30 amp fuse handle amperage like that?  The alternator has no way of "knowing" you only have a 30 amp fuse in the line. If voltage is low in the "house" battery - it can run full charge to try to charge the battery and can easily exceed 30 amps.  I have a 80 amp fuse on my charge-wire to the back. Note - do NOT increase the fuse size if you only have 10 gauge wire.  Fuse size much match the wire-size or you create a fire hazard.

If you DO only have 10 gauge wire - get rid of the fuse and put in a self-resetting 30 amp circuit breaker. It cost less then $10.   

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29 minutes ago, fred heath said:

I use the 30A circuit breaker. It works fine.

Yes, but you only have a 45 amp alternator (unless you've upgraded).  The poster here should have a 60 amp alternator.   A 60 amp alternator can send 60 amps of current down any wire it is hooked to, when called for. Pretty simple match.  60 amps will blow a 30 amp fuse;  Now - some isolators max out and will blow before passing 60 amps. I have no idea what model isolator the poster had, nor do I know what you have.

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Sorry JD, didn't see the before isolator comment. I have the 30A circuit breaker protecting the 10ga. wire going to the coach battery.

I was agreeing with your comment that a CB is much better than a fuse for this application.

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33 minutes ago, jdemaris said:

Fred - maybe a little off-topic - but do you realize you have a "sexy" motorhome?

cover.jpg

Haha. I'll have to use that in the future. Do you have the narrative of the article?

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A 35 amp breaker is sufficient there is little chance of the coach battery drawing that kind of current or the #10 being able to supply it. It's more about setting fire to the wire with a dead short.

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I had forgot that alternators put out that many amps.  There is no writing of any kind visible on the isolator as it is mounted.so don't know its rate.  But all the wires on the isolator are small, #10 at best and maybe #12 but I don't think National RV would have stooped to #12.  So I am glad the 30A fuse blew.  I will try replacing it with a breaker. 

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A coach battery that is 50% discharged could easily ask for, and get over 30 amps of charge from a 60 amp alternator. I've seen it happen many times.  It is one good reason that when first starting RV with a worn-down "house" battery - start the engine and let it idle for awhile before revving it up and driving.  That 60 amp alternator only puts out around 20 amps when the engine is at low speed and it only takes a few minutes for the high-surge demand of the battery to get lower.  It kind of baffles me why anyone finds this concept "unlikely."  That is what an alternator/regulator does. It senses low voltage and attempts to correct it by upping the amperage.  A 60 amp alternator is called just that because it can send 60 amps to your battery if it is low.   

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15 minutes ago, BobBeery said:

I had forgot that alternators put out that many amps.  There is no writing of any kind visible on the isolator as it is mounted.so don't know its rate.  But all the wires on the isolator are small, #10 at best and maybe #12 but I don't think National RV would have stooped to #12.  So I am glad the 30A fuse blew.  I will try replacing it with a breaker. 

OOps.  Correction--This one is from Winnebago not National RV.

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55 minutes ago, BobBeery said:

I had forgot that alternators put out that many amps.  There is no writing of any kind visible on the isolator as it is mounted.so don't know its rate.  But all the wires on the isolator are small, #10 at best and maybe #12 but I don't think National RV would have stooped to #12.  So I am glad the 30A fuse blew.  I will try replacing it with a breaker. 

Nobody in their right mind is going to install an isolator rated for less then the alternator max output.  So at least with OEM, any older Toyota RV is going to have an isolator rated for at least 50 amps and a newer one at least 60 amps. Now adays I doubt you can find a new one rated for less then 70 amps.  Also note that although 10 gauge wire is usually protected with a 30 amp fuse, it is approved to carry up to 55 amps for short periods of time. 12 gauge is approved for 41 amps, again for short periods of time in short runs.  Thus why it is much better to have a slow-blow circuit-breaker or a slow-blow 30 amp fuse and not an "instant blow" fuse.  It is the same reasoning used in standard household wiring.  A 20 amp circuit breaker is engineered to allow up to near 50 amps for short periods of time and not trip.

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Just a side-note. I've got a Denso alternator on my farm-tractor that came off a Toyota truck.  I have a amp-gauge hooked into it instead of a voltmeter with a 60-0-60 amp scale. Every time I start the engine that meter gets pinned all the way to 60 amps and it feels like the needle is going to bend.  Only happens for a few seconds and only if I've got the engine revved up. The belt also tends to slip a little unless I lower the RPMs until the battery catches up.  Same sort of thing to some extent has to happen with our Toyota RVs. More good reason to have a relay for an isolator instead of a box of rectifiers.

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You can only get so much current down a #10 yes the demand maybe be higher with a weak battery but going through a #10 wire being the bottle neck it's not going to deliver 60 amps. The alt regulation is voltage dependent so if the truck battery is up to near full charge it's not going to make the amps and even if it's weak it will be the first one to get charged because it has the least resistance then the voltage comes up and the current drops back.

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9 hours ago, Maineah said:

You can only get so much current down a #10 yes the demand maybe be higher with a weak battery but going through a #10 wire being the bottle neck it's not going to deliver 60 amps. The alt regulation is voltage dependent so if the truck battery is up to near full charge it's not going to make the amps and even if it's weak it will be the first one to get charged because it has the least resistance then the voltage comes up and the current drops back.

Nonsense. I watch 60 amps on my amp-gauge every time I start my tractor and it is only wired with 10 gauge copper.   Technically, 10 gauge is rated at a max of 55 amps and is supposed to be fused at 30 amps for safety.   Note my tractor draws no more amps when I start it then a Toyota pickup.   If I begin with a full charged battery, then crank the engine for 5 seconds - that lowers battery voltage enough to created a 60 amp surge of charge as soon as it starts.  Usually only for a few seconds and then it tapers right down.   Do that with a Toyota RV with two batteries calling for charge and you expect a lower amp demand? I don't get it.   Not unless you are "tricking" the alternator's voltage sensing circuit somehow.    I will note that if someone has a rectifier-based isolator that simply cannot pass 60 amps safely (or even 40 amps) - it's  a different story. Some just blow out and go dead.  Been plenty of people on this forum with dead isolators.  Note that a 10 gauge copper wire, 2 feet  long, with 60 amps @ 14 volts - only has a voltage drop of 1.7% which is pretty low.

ampacity.jpg

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The #10 is more like 15-20' long by the time it gets to the battery. The chart's listed amp rating is free air, the current carrying capacity is more like 30 amps in real life due to heating that is why it is protected with a 30 fuse/breaker and not a 55 amp. I just checked my 2011 Tacoma with a 150 amp tow package alternator it's inrush alternator current checked with a Fuke 374 $350 meter showed 22.8  amps that included every thing in a modern vehicle would load down an alternator, lighting, ign /fuel injection,day time running lights etc. plus an instant battery charge it had not been started in 3 days. I don't find that to be excessive. The lowest rated solid  state isolator I found was rated 75 amps the average was around 150.Your results will probably vary.

 

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1 hour ago, Maineah said:

The #10 is more like 15-20' long by the time it gets to the battery. 

 

I don't know what vehicle you or I are talking about.  Thus why I gave a length.  Some of these Toyota RVs have the cranking battery and the "house" battery both under the hood and the charge wire is sometimes only 2 feet long.     If we are talking about a rig with the "house" battery in the rear - it is a whole different situation.  I have two "house" batteries in the rear of my Minicruiser.  Actual length of the charge wire is 16 1/2 feet and I have #2 copper with an 80 amp circuit-breaker.  

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1 hour ago, Maineah said:

The #10 is more like 15-20' long by the time it gets to the battery.

Just an added comment.   #10 cannot even carry 30 amps for a 20 foot run.   If 14.2 volts on the alternator end, and around 13.8 once it leaves the isolator - voltage by the time it travelled 20 feet would be 12.6 volts and that is less then a charged 12 volt battery. I.e. no charge possible.

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I have no idea what the actual circuit length is in the Winnebago diagram. I DO see that it uses a relay and not a rectifier-based isolator.   So that is more voltage going to the 10 gauge charge wire because there is no voltage drop across the rectifiers.  Also, Winnebago has slow-blow, self-resetting circuit breakers in the line and not fuses.  30 amp as I recall. So when the "house" battery calls for more then 30 amps of charge - the system says "no" after a second or two.  I'd rather have full charge capability in my system.  I also have two "house" batteries in back, not one; 
 

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An added comment.  Winnebago having that relay instead of a rectifier makes the difference between " no charge at 30 amps" to "some charge at 30 amps."  I have not found Ohm's Law to lie yet.   If that setup shown (by Derek) used a rectifier instead of that relay shown and has 15 feet of wire - at 30 amps - 14 volts at the alternator, drops to 12.3 volts (approx) by the time it tries to get to the "house" battery.  12.3 volts cannot charge a battery that requires at least 12.7 volts to be fully charged.   By using that relay, at 30 amps - voltage drops to 12.9 volts.  Still dismal, but at least high enough to charge. A lot of loss to heat and poor circuit design.  Just because Winnebago did it, does not mean it is optimal.

Not sure what the point of all this issue.  The post - as I recall - was about a blown 30 amp fuse hooked to an alternator capable of sending 60 amps.   How is that confusing?  How anyone can claim that the alternator is not capable of sending more then 30 amps down a 10 gauge wire is beyond me.  Very easy to prove.   

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OK prove it with out setting fire to it. It will not carry 60 amps for more than a few seconds. I don't own a Toy home any more but if you like I can hook my camper to the trucks 150 amp alt tow package that runs a # 10 to the camper's 180 amp battery bank and check the current it's fused yeah real fuse at 30 amps never blown it yet. Bottom line it;s not much more that a trickle charger but that's OK most people don't go camping next door.

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Prove what?   I cannot make such an attempt unless you focus on something specific.  Generalizations don't work well in technical discussions.  I have no idea what your point is - nor do I understand what it is exactly you do not agree with? Ohm's Law? The Hole Theory?   That a 30 amp fuse hooked to a power source with 60 amp capability via a 10 gauge wire cannot blow a 30 amp fuse when hooked to a run-down battery?? If that is what you are trying to argue - it tells me you lack the level of understanding to the point that any answer will not make sense to you.  Or you are just arguing because you enjoy it.

As I said earlier - one case-in-point is my IH tractor with a Denso 60 amp alternator.  Output runs through  an amp-gauge on to the battery.  5 feet of 10 gauge wire total run. It pegs at 60 amps every time I start the tractor and does that for a second or two.  Tractor has just one small 80 AH battery (not the correct battery in there).  If I had a 30 amp fuse in the circuit it would blow. If I had a 30 amp slow-blow circuit-breaker in the circuit it would be fine since the surge does not last long.

If you feel the need to use your truck as  proof that all what I say is "wrong" - some specifics might help.  "150" amp alternator is not useful. Not unless you are saying it is rated 150 amps at 3000 engine RPM and you rev your engine to 3000  RPM every time you start it.   Does your factory wiring use a relay?  Does it have a current limiter?  Does it have a fuse or a circuit breaker?  What is the total length of the wire run?  What is the usual state of discharge of the trailer-battery at first engine start?

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57 minutes ago, Maineah said:

I don't own a Toy home any more

Et Tu Brute?  Dropping like flies they are. 

Hey I don't know crap about this argument but if adding an inline diode to prevent reverse current is used you will get a .7 volt drop there.  Add the DC resistance of the wire and terminations you are going to get a pretty good voltage drop combined.  Laws of physics and all that.  The battery has electrical resistance based on charge state.  Dead, dead, it is pretty low.  Fully charged, kind of high.  Ohms law is hard to escape.  The resistance hence the charging rate is affected by the entire circuit.

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9 hours ago, Back East Don said:

Et Tu Brute?  Dropping like flies they are. 

Hey I don't know crap about this argument but if adding an inline diode to prevent reverse current is used you will get a .7 volt drop there.  Add the DC resistance of the wire and terminations you are going to get a pretty good voltage drop combined.  Laws of physics and all that.  The battery has electrical resistance based on charge state.  Dead, dead, it is pretty low.  Fully charged, kind of high.  Ohms law is hard to escape.  The resistance hence the charging rate is affected by the entire circuit.

Thus my point about the Winnebago system working better with a relay instead of a couple of rectifiers. No voltage-drop across the relay.

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On the subject of 10 gauge wire being the main charge-lead to a rear battery.  Let's say it's something like a 17 foot run.  And let's say a rectifier-based isolator is used.  Alternator sends out 14.2 volts and sticks with that since it is what it senses.    By the time that gets through the isolator and 17 feet of wire, that 14.2 volts drops to 13.2 volts and that if only charging at 5 amps. That is a p*ss-poor setup in my opinion. Who, in their right mind - would design such a system on purpose?  The one variable here is where the alternator senses voltage.  I guess there are ways to "trick" it and sense voltage at the output of the isolator.  Then alternator output would rise to 14.9 volts and voltage leaving the isolator would at least be 14.2 volts.  But now we are getting complicated.  Lot easier just to wire it all correctly.

I am dealing with this sort of issue right now.  I built a gas-driven DC battery charger.  It has 15 feet of cable but it is #2 copper, not # 10.  Even so, the voltage difference between the alternator output and the batteries being charged is kind of amazing.  A full volt drop and high charge rate (maybe 60-70 amps).  Math said the drop should of been less. I ended up running a third wire the full 15 feet as a remote "sensing" wire. Now - the alternator charges at over 15 volts and the batteries at 14 volts.

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13 hours ago, jdemaris said:

Prove what?   I cannot make such an attempt unless you focus on something specific.  Generalizations don't work well in technical discussions.  I have no idea what your point is - nor do I understand what it is exactly you do not agree with? Ohm's Law? The Hole Theory?   That a 30 amp fuse hooked to a power source with 60 amp capability via a 10 gauge wire cannot blow a 30 amp fuse when hooked to a run-down battery?? If that is what you are trying to argue - it tells me you lack the level of understanding to the point that any answer will not make sense to you.  Or you are just arguing because you enjoy it.

As I said earlier - one case-in-point is my IH tractor with a Denso 60 amp alternator.  Output runs through  an amp-gauge on to the battery.  5 feet of 10 gauge wire total run. It pegs at 60 amps every time I start the tractor and does that for a second or two.  Tractor has just one small 80 AH battery (not the correct battery in there).  If I had a 30 amp fuse in the circuit it would blow. If I had a 30 amp slow-blow circuit-breaker in the circuit it would be fine since the surge does not last long.

If you feel the need to use your truck as  proof that all what I say is "wrong" - some specifics might help.  "150" amp alternator is not useful. Not unless you are saying it is rated 150 amps at 3000 engine RPM and you rev your engine to 3000  RPM every time you start it.   Does your factory wiring use a relay?  Does it have a current limiter?  Does it have a fuse or a circuit breaker?  What is the total length of the wire run?  What is the usual state of discharge of the trailer-battery at first engine start?

You are the one that said you could prove it so have at it my point you can't run 55 amps through a #10 wire with out smoking it for more than a few second it will get too hot. If it is free air you may be able to run 55 amps but once it is bundled or in a compartment it drops to 30 amp. Ever notice you power co. drop its 2/0 but your 200 amp house wire is 4/0 it's all about the heat. You said you can see a 60 amp spike I said I have a 150 amp alt that means I could put 150 amp across the 30 amp wire right? Your's apparently can do max smoke at cold idle so mine should do 150 using that reasoning but of course neither of them do full output. Alternators limit current by field voltage they do not have current limiting via a regulator as old DC generators did. It's current reating is by design max being full field voltage. Yes my truck has an isolator relay imbedded  in the fuse panel and a 30 amp lead fuse. The run to the battery is at least the length  of the truck so we'll say close to 20 feet The camper sits a lot as do most and generally it's not some thing people think of when they start what ever charges the battery. My Toy Home had the same setup with a 60 amp alt it never blew a fuse or tripped the breaker it had two coach batteries. They have been using a #10 or more so a #8 to make up for voltage drop for a very long time to charge camper batteries and it is far from a common problem to trip breakers or blow fuses when the engine is started.

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1 hour ago, Maineah said:

You are the one that said you could prove it so have at it my point you can't run 55 amps through a #10 wire with out smoking it for more than a few second it will get too hot. If it is free air you may be able to run 55 amps but once it is bundled or in a compartment it drops to 30 amp. Ever notice you power co. drop its 2/0 but your 200 amp house wire is 4/0 it's all about the heat. You said you can see a 60 amp spike I said I have a 150 amp alt that means I could put 150 amp across the 30 amp wire right? Your's apparently can do max smoke at cold idle so mine should do 150 using that reasoning but of course neither of them do full output. Alternators limit current by field voltage they do not have current limiting via a regulator as old DC generators did. It's current reating is by design max being full field voltage. Yes my truck has an isolator relay imbedded  in the fuse panel and a 30 amp lead fuse. The run to the battery is at least the length  of the truck so we'll say close to 20 feet The camper sits a lot as do most and generally it's not some thing people think of when they start what ever charges the battery. My Toy Home had the same setup with a 60 amp alt it never blew a fuse or tripped the breaker it had two coach batteries. They have been using a #10 or more so a #8 to make up for voltage drop for a very long time to charge camper batteries and it is far from a common problem to trip breakers or blow fuses when the engine is started.

Hey, I'm pretty close.  You want me to come up with an amp meter and a low ohm/high current shunt?  I could wire it between your Alt out/wire and ground.  Do you think I will smoke it?  I'm all for experimenting with your stuff.  Problem with getting 150 amps out of your alternator is, it would require the load be .096 ohms at 14.5 volts.  Hard to do with the resistance of the 10 gauge wire over any distance even if you connected the wire directly to ground.  Even taking out relay contacts or connector losses.  1 ohm of resistance in the run will limit the current to 14.5 amps at 14.5 volts.  Can I get 30 volts out of the alternator/motor combination to even double current output?  That is the whole point here.  Context much anyone?  So unless your battery is close to completely dead, the total circuit load is not going to get the alternator output anywhere close to full output.  Stupid arguments are just that, stupid.

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Rule 1 The MFG used the bare minimum (cheap) needed to show a + charge on the house battery, if you are doing any rework on the charging system USE bigger wire for better results. I use #6 for house batteries.

Rule 2 Solid state isolators cost you .7v period, its the nature of the beast. Solenoid isolator do much better when new and should be replace when they start to show .5v drop. Better quality solenoids (silver contacts) will last longer and should be worth the price if you plan on using your RV a lot.  

Rule 3 LONG wire runs should be 1 piece. My poor Escaper's charging system was a perfect example of PO abuse. 5 pieces of wire between the isolator and the house battery. Poor battery never reached full charge as it was only getting 12.3v.

Something that I've done and others might consider if you boondock a lot, is to connect the house battery to the alternator post on a solenoid isolator and the truck battery to the isolated post. The truck battery still gets a full charge, short big cables, and the house battery gets charged directly from the alternator with no v drop. 

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6 hours ago, Maineah said:

You are the one that said you could prove it

????   Hey, maybe my memory is getting like Hillary Clinton's.  Just exactly where did I use the word "prove?  Either you are having fantasies, or I am getting senile.

Even the Electron Theory has never been "proved."

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5 hours ago, Back East Don said:

So unless your battery is close to completely dead, the total circuit load is not going to get the alternator output anywhere close to full output.  Stupid arguments are just that, stupid.

I am not even sure where this discussion is at anymore.  My comment to the original poster had little to do with full alternator output however.   It was about how half the output could blow his 30 amp fuse and it very easy to send more then 30 amps down a long 10 gauge wire.

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You are certainly correct that I used the word "prove." I certainly did not say it was something I could prove via some silly Internet forum, did I?  And yes, with equipment in hand, it IS very easy to prove.  Again - to do so on some forum to someone like you? Not something I'd try to attempt.   All you need is a Denso 60 amp alternator in a Toyota RV. 15-20 feet of 10 gauge wire, a  30 amp fuse and a run down "house" battery.  Sorry, but other then the way Captain Kirk might do it by having Scotty beam the stuff up - I cannot do it via the Internet.  If you do not understand Ohm's Law, it not my problem.  If you do not believe Ohm's Law, it also is not my problem.

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