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Coach battery doesn't charge

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Back again! Now my project is solving the problem of why my coach battery won't charge from the alternater. Shore power I measure 13.7 volts into the battery. Engine power I measure 12.2 volts. I read in the elctrical forum that it's most likely the isolator. How can I tell and where is it at? I have a 91 Winnie.

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Generally, it's mounted on the inner fender on the passenger side, near the truck battery. There'll be a large wire coming off 1 side going to the truck battery and another large (same size) wire that'll disappear to the coach battery. There'll be a 3rd large wire to the alternator. Finally, there'll be a 4th smaller 'contol' wire that will signal the isolator that the engine is running. Once you locate it, I'd suggest that you undo (1 at a time!) each of the wires and give them a good clean and tighten.

Then you can check voltage at each of the terminals with the engine off and then running.

http://www.winnebagoind.com/diagram/1993/096325.pdf

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Generally, it's mounted on the inner fender on the passenger side, near the truck battery. There'll be a large wire coming off 1 side going to the truck battery and another large (same size) wire that'll disappear to the coach battery. There'll be a 3rd large wire to the alternator. Finally, there'll be a 4th smaller 'contol' wire that will signal the isolator that the engine is running. Once you locate it, I'd suggest that you undo (1 at a time!) each of the wires and give them a good clean and tighten.

Then you can check voltage at each of the terminals with the engine off and then running.

http://www.winnebago...1993/096325.pdf

Thanks Derek. I measured 14 volts going into the isolator from both the alternater and the truck battery , and with the coach terminal removed from it I measured 0 volts of the coach side of the isolator. I went to Kragens and bought a 4 post solid state isolator ($32) which gave me 13 volts on the coach side. I think it works even with the 1 volt drop through the isolator.

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Be careful with the isolator from Kragens. Automotive ones are not designed for full time use.

An RV store will give you a 3 pole for $20. I just replaced mine yesterday. PS mine had a 3 pole in a 92 winnie.

Thanks Derek. I measured 14 volts going into the isolator from both the alternater and the truck battery , and with the coach terminal removed from it I measured 0 volts of the coach side of the isolator. I went to Kragens and bought a 4 post solid state isolator ($32) which gave me 13 volts on the coach side. I think it works even with the 1 volt drop through the isolator.

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mmmmmboy another consideration. 20 bucks you say? The one I installed is also designed for a diesel big rig capable of passing 60 amps. I need an asperin.

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There are 2 types of isolators. One is a mechanical solenoid type ( looks like a Ford start relay) and the other is a solid state diode type, it looks like a brick with fins on it.

The mechanical one MUST be a continuous rated contact type, the starter types will burn sooner or later.

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The Kragens purchase is a solid state isolator with fins. It takes up twice the space of its predecessor. I think I'll install an inside the coach voltmeter to monitor the coach battery.

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A good thing to do. I bought an inexpensive digital volt meter from northern tools, wired it to the coach battery and velcroed it to the wall. A flick of the switch, and I can read my coach battery at a glace.

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mine (mechanical) stopped working in the middle of a trip out west. cleaning all terminals fixed it. make sure to use a good quality electrical contact "goop" forget the correct name for the stuff, but it will keep your terminals happy for years.

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My advice, get rid of the mechanical type if you have one and replace it with solid state (with fins). They mechanical are prone to develop a high resistance through the internal contacts due to arcing when engaged (on) and the solenoid part can fail. For a solid state isolator all our Toyota's require the 4 pole type. 3 large and 1 small. The small is the exciter circuit and connects to a ignition on circuit. There are other reasons to get rid of the mechanical one. Over charging of the coach and or starting battery is the main reason. Bad contacts and failing is the other.

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Back again! Now my project is solving the problem of why my coach battery won't charge from the alternater. Shore power I measure 13.7 volts into the battery. Engine power I measure 12.2 volts. I read in the elctrical forum that it's most likely the isolator. How can I tell and where is it at? I have a 91 Winnie.

that is what I found (had the same problem) bought one at giant rec world- about $27.

with 12v applied to the little terminal you should get the same voltage at both of the larger terminals. Without voltage at the little terminal the 2 batteries are split and should measure accordingly.

post-1354-086047900 1284946407_thumb.jpg

post-1354-015281000 1284946478_thumb.jpg

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My advice, get rid of the mechanical type if you have one and replace it with solid state (with fins). They mechanical are prone to develop a high resistance through the internal contacts due to arcing when engaged (on) and the solenoid part can fail. For a solid state isolator all our Toyota's require the 4 pole type. 3 large and 1 small. The small is the exciter circuit and connects to a ignition on circuit. There are other reasons to get rid of the mechanical one. Over charging of the coach and or starting battery is the main reason. Bad contacts and failing is the other.

I'm, with Greg....a solid state isolator is the way to go.

Since I boondock full time, I have a very high powered setup to charge my (4) 120 AH deep cycle batteries. My Alt is a 160 Amp PowerMaster. The problem with the mechanical isolators are they:

(1). Can be Finicky about when they combine the starter and the coach batteries so that the coach battery can begin charging.

(2). Even when they do combine both, when the smaller battery (usually the starting one) says it's full, the alternator will taper back it's output. This tends to leave the coach battery less than fully charged. The ONLY way to avoid this with mechanical isolators, is to have identical batteries for both starting and coach. If you run down the road with 12V on your fridge, forget about it!. That battery will probably never top off. That will quickly ruin the coach battery.

Since we usually just drive a few hours to our next destination, you want all the alt power going into that battery in the least amount of time. A solid state isolator will do that much more efficiently. I had to put my NOCO 200 Amp isolator behind the seat because the engine compartment was just too hot for the diodes to handle. The main body is a heat sink, and there was just no cooling in there to dissipate the heat. Keep that cooling requirement in mind if you opt for a SS Isolator.

The only downside to the isolator is that you will experience about a 1/2 volt drop from the alt to the battery. No problem if it's putting out a full 14.4 to 14.8 volts. But if it's a marginal 13.8, that drop is going to hurt your charging. The OEM alts that came with most of our Toys were not up for the task of charging a large coach battery, plus supplying all the engine and lighting requirements. I'd say the minimum I'd go with is 60 amps. A worn down coach battery, plus riving at night will use every bit of that, and then some.

If you use your coach battery only very occassionally, and run from hook-up to hook-up, you probably don'n need to worry as much. But if you plan to actually USE the battery alot, Invest in a good charging setup, a group 27 battery at the least,,,and add more if you plan on weekend boondocking or more.

Just make sure they are ALL THE SAME. I see people all the time with different capacity batteries all slung together, wondering why they can't get their system to charge properly! The alt will STOP CHARGING when the smallest battery in the circuit says it's full.\

Here is a pic of my Isolator setup:

The 2 Ga Red wire in the middle comes up from the Alt through the floorboard behind the seat, It connects to a Ammeter Shunt that lets me keep an eye on my output while I'm driving. The red wire at the other end of the shunt leads directly to battery pack (where the old water heater used to be). The black wire leads back to the starter battery.

post-2665-076581700 1287357586_thumb.jpg

Normally, you'd wire each battery from the end terminals, but in my case I direct-wired the coach pack to the Alt. This both eliminated my voltage loss and massive heat build up when pushing 120+ amps through the diode, and gave better voltage sensing to the alt so it knew what it needed to feed the coach pack. That battery is what I live off, so it gets priority. What is left goes to the engine battery through the diode, thereby still providing isolation.

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When two batteries are connected together voltages are the same regardless of the size. An Alternator relies completely on voltage it does not sense amps only voltage. The alternator output is directly proportional to field voltage controlled by the regulator that reads battery voltage not current. A larger alternators output is higher and will keep up with the demand better and charge quicker. Yes a smaller battery will reach the full charge voltage sooner but if the larger battery is not fully charged the voltage will equal out the voltage flows from high to low. When both batteries reach a full voltage charge it does not mean that they are fully charged, batteries need to be charged to the saturation point. Larger batteries need to be charged longer. When an alternator senses a full charged voltage the field voltage drops and the current drops but as long as the output voltage is better then 13.2 volts it does continue to charge but at a much lower rate. Here is the rub with a solid state isolator its voltage sense comes from the key switch (Toyota type alternators) with the solid state isolator there is no voltage at the B+ terminal when the key is off the alternator needs to have voltage at the B+ in order to charge so we add the small “E” terminal (the “E” stands for “exciter”) to the isolator that is connected to the key switch (truck battery). The typical break over voltage of a diode is about .7 volts that voltage is lost to heat not a big deal but its there. What does all this mean? To the average person nothing, we usually drive far enough getting to a camp ground to charge both batteries but with short trips it won’t happen. None of the isolator types are smart they do not control voltage it’s just passing through. I prefer a relay type for the average person because of its simplicity, no extra wiring very easy to replace or for that matter even bypass with a simple jumper and if it goes bad the truck battery will still charge. Both work and I would not change one out for the other. In passing I don't use either one mine is a gadget called a combiner.

post-669-019029800 1287447490_thumb.jpg

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When two batteries are connected together voltages are the same regardless of the size. An Alternator relies completely on voltage it does not sense amps only voltage. The alternator output is directly proportional to field voltage controlled by the regulator that reads battery voltage not current. A larger alternators output is higher and will keep up with the demand better and charge quicker. Yes a smaller battery will reach the full charge voltage sooner but if the larger battery is not fully charged the voltage will equal out the voltage flows from high to low. When both batteries reach a full voltage charge it does not mean that they are fully charged, batteries need to be charged to the saturation point. Larger batteries need to be charged longer. When an alternator senses a full charged voltage the field voltage drops and the current drops but as long as the output voltage is better then 13.2 volts it does continue to charge but at a much lower rate. Here is the rub with a solid state isolator its voltage sense comes from the key switch (Toyota type alternators) with the solid state isolator there is no voltage at the B+ terminal when the key is off the alternator needs to have voltage at the B+ in order to charge so we add the small "E" terminal (the "E" stands for "exciter") to the isolator that is connected to the key switch (truck battery). The typical break over voltage of a diode is about .7 volts that voltage is lost to heat not a big deal but its there. What does all this mean? To the average person nothing, we usually drive far enough getting to a camp ground to charge both batteries but with short trips it won't happen. None of the isolator types are smart they do not control voltage it's just passing through. I prefer a relay type for the average person because of its simplicity, no extra wiring very easy to replace or for that matter even bypass with a simple jumper and if it goes bad the truck battery will still charge. Both work and I would not change one out for the other. In passing I don't use either one mine is a gadget called a combiner.

post-669-019029800 1287447490_thumb.jpg

For most Toy owners who have just one Group 24 coach battery, I would agree that a mechanical isolator would usually do the trick. Their coach battery will rarely get completely filled though, probably only 80-90%, since the alt will stop producing any significant charge once the smaller capacity engine battery tops off first. This is fine if you rarely depend on your battery for any extended camping. But if you are going to spend alot of time out, you will need a bigger coach battery reserve, AND a much more efficient way to charge it.

That is why I took the alt's primary voltage sensing OFF of my starting battery, and ONTO my coach pack. Properly isolated, the starting battery rarely sees even a 1% drop after starting and during the course of driving will easily get charged back up. My coach pack however, which is (4) 120 amp hour deep cycles, will need HUGE amounts of charging after heavy use. That's why I use THAT pack as the main sensing mechanism for the aftermarket alternator. Also, by wiring them directly together, I don't have any voltage/current loss through the diode. That was giving my alt real sensing problems early on.

Until the pack voltage gets charged back enough to equal the starting battery's voltage, almost all the alt's output goes into my coach pack.

I think that for people who rely on their coach batteries more, they should make THAT their primary battery on the isolator (the side that always receives charge) as well as the sensing source for the alternator. The idea that the higher voltage from the starter battery will quickly "flow over" to the coach battery doesn't work very well in practice. The voltage difference between the two isn't high enough to cause a significant current to flow. Additionally, the higher voltage sensing from the full starter battery will 'trick" the alt into thinking it doesn't have to put out that much charge. I have actually measured this with a sensitive ammeter. It will charge the coach pack, but at a trickle charger rate. It is better that the alt's output go first into sensing & charging the depleted coach pack. When I tried both arrangements, my ammeter jumped from about 10 amp output, to 120!

Now if you spend 12 hours a day on the road, this might work. But if you drive just a few hours a day, and deeply use your coach battery at night, it is likely that it isn't getting fully re-charged. ESPECIALLY if you are just using the puny stock Toyota alternator. That is why I did the alternator upgrade here:

http://www.rocketcit...e%20Project.pdf

If you want to add coach battery capacity, I highly recommend you download that PDF file and upgrade. This allowed me to go generator free, as I can just idle my Toy in the morning, and get over 100 amps (1200 watts) charging into my coach battery if necessary.

The stock Toyo alt will burn out long before it can recharge more batteries. In fact, I feel that re-charging just ONE discharged coach battery, as well as running all the other 12 volt demands from our Toys, is why our stock alts burn out so often. I went through two new ones in as many months. And if they aren't burning out, they are blowing regulators or fuses all over the place, causing all sorts of inconvenience. This upgrade should be one of the first for anyone planning to boondock, IMHO.

After a better alternator, the question is, how to isolate the batteries? And for the most part, mechanical isolation for me is just too problematic, especially when handling the higher currents of a high output alt. The higher current creates alot of arcing inside the isolator that leads to early failure.

In conclusion, I think that the electrical demands of our coaches today are just too great for the original alt-isolator packages that came with our Toys. Both needed to be upgraded.

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Nice setup wetboy I'm sure it works well for you and great in the wille waggs. I guess my point is 80% of the campers do not need that much power so a stock system in good shape works fine for them and is simple. My 60 amp Toyota alternator charges my 120 amp coach battery fine and my drives are any where from 3 to 10 hours but there is not a shot it would handle 4X 120 amp batteries even if I drove for a week! For those that camp away from power run 2000+ watt inverters, and other high demand items more batteries and bigger alternators no doubt are the way to go. I like the KISS principle and that is how I treat camping. If I'm at a camp ground I'll go looking for a socket and let my 45 amp charger deal with the batteries. I do have a 400 watt inverter to run the laptop have LED lighting, run the water pump and some times the heater and must say I have never had a battery issue even after a few days. Arcing should not be a problem with a key actuated relay as they are closed before they are loaded how ever most of the stock relays are only rated to a 75 amp load. My system consists of a combiner it is a smart relay that reads both batteries and only combines when the truck battery is charged then works on charging the coach battery it is time set to deal with fluctuations. When the engine is off and the voltage drops to 12.7 it disconnects that's the only concession I made to simplicity it is also bidirectional so when plugged in it charges the truck battery too. No right or wrong just different applications.

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Here's something to think about concerning the mechanical isolators. Imagine your in the boonies. You have dropped your coach battery down to about 11 volts or lower which is basically dead. With the mechanical isolator as soon as you turn on the ignition the coach battery gets connected to the starting battery before and during start-up. All of a sudden your coach battery is taking allot of current from your starting battery. Your engine has a problem and won't start right away, the coach battery continues to take current. And your starting battery is partially discharged because one of the kids left the cab light on all night.

I bring this up because I think there are a couple of different ignition circuits. Some that are on all the time and some that are only on in the run position of the key switch and off in the start position. It would be an interesting test to discharge the coach battery then do a current reading with ignition on before the alternator kicks in. Another test is to re-create the above scenario and just energize the mechanical isolator and let the batteries equalize and see what voltage they end up at. The mechanical isolator only isolates when its off.

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Well I hear what you are saying but here are some variables. The current flowing thru a #8 wire would be a good deal less then the rated contacts of the relay and the 30 amp circuit breaker would not stand the high draw even if it could be delivered thru the wire. Now if you had them joined with a #2 or greater wire with out a breaker it maybe be an issue. The charge rate also would be a lot less say then the same type of relay turning a starter that might exceed 300 amps and they close full load. Many of them are still in service after hundreds of starts and thousands of miles. My biggest complaint with the relay type is the poor build quality, they rust! Why they do not use copper studs and SS nuts is beyond me of course the breakers suffer form the same dumb ideal. Maybe if you bought them from a boat yard at 3 times the price they would be better quality. I guess you could use the same scenario of a dead/weak truck battery and turning the key on to steal some power from the coach battery to start your truck! I guess what I’m saying the transfer of power between the two batteries is not all at once it takes awhile a battery has a fair amount of resistance particularly in a weak state.

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Well I hear what you are saying but here are some variables. The current flowing thru a #8 wire would be a good deal less then the rated contacts of the relay and the 30 amp circuit breaker would not stand the high draw even if it could be delivered thru the wire. Now if you had them joined with a #2 or greater wire with out a breaker it maybe be an issue. The charge rate also would be a lot less say then the same type of relay turning a starter that might exceed 300 amps and they close full load. Many of them are still in service after hundreds of starts and thousands of miles. My biggest complaint with the relay type is the poor build quality, they rust! Why they do not use copper studs and SS nuts is beyond me of course the breakers suffer form the same dumb ideal. Maybe if you bought them from a boat yard at 3 times the price they would be better quality. I guess you could use the same scenario of a dead/weak truck battery and turning the key on to steal some power from the coach battery to start your truck! I guess what I'm saying the transfer of power between the two batteries is not all at once it takes awhile a battery has a fair amount of resistance particularly in a weak state.

That makes sense. Personally I prefer that both coach and start batteries are isolated 100% from each other as far as back-flow current. If I need to get some power to charge my start battery I can always jumper at the isolator for awhile for a charge.

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Ok so i fried the replacement alternator this weekend while en route to a campsite... known facts; fridge was on (dometic original 1986 vintage). i installed another autozone duradie alt and drove home with no isolator and made it to the camp and home ok; some facts with voltimeter after 15 minutes of idle with fan on full and headlights on:

  • battery 1 + when touched to A post reads .82 volts constant; alternator manual calls for any reading under .7...
  • on cab battery the same reading to A post from positive is .53 volts; well within spec. batteries seem to both be charging albeit at different rates; truck battery at 13.57 ish and cab battery at 14.01 ish.
  • with no isolator and car hooked up as normal stock wiring the + battery post to alternator batt post reads .3 volts
  • all battery negative to alt body or isolator body reads were below .1 volt usually .03 on all. no issues with grounds i guess

now here is the new alternator $$$ question...
  • when running the fridge with warmed up car at idle with headlights on and fan on, the cab battery drops to 12.19 volts and struggles to rise... is this normal? this load condition i had when i lost the "durawontlast" new alternator; was in a traffic jam when it died. so the question is ; should a new reman alternator be able to take that load of having the fridge on or do i have a bad fridge or do i have a shadey isolator?

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