From: John De Armond Subject: Re: Installing second battery Date: Tue, 21 Sep 1999 01:13:24 EDT Newsgroups: rec.autos.tech DrSilver wrote: > > I bought an '86 Ford F-250, and I'm fixing it up for fishing expeditions. I > put a camper shell on the back and I seem to be accumulating lights and > other electrical doodads at an alarming rate. > > I'd like to install a second battery to run all this stuff. Can anyone > point me towards instructions on how to do this? Visit your RV dealer (or some car parts places) and get a heavy duty battery paralleling relay. This relay looks quite like a starter relay but has a lower power winding that can be energized continuously (a starter relay can't.) Install the new battery and parallel it to the existing battery through the relay. The relay should be energized only when the alternator is actually generating. The proper way to do this is to pick up the signal (if it exists) from the alternator or regulator that has voltage on it when the alternator is generating. On the Toyota-based camper I just installed a system on, the regulator had a terminal that had 12 volts on it when the alternator was charging. Many alternators have a separate half-wave diode pack that supplies a signal when the alternator is charging. A bit of poking around with a voltmeter should find the right place. If you can't find a suitable signal from the alternator, you can install a separate oil pressure switch that will energize the relay whenever oil pressure is present. Just tee it in with the existing oil pressure switch/sender. Another possibility if your engine has fuel injection is to pick off voltage to the fuel pump. The ECU typically will run the fuel pump for a few seconds when the ignition key is turned on to build fuel pressure and then de-energized until the engine is cranked. The object in this exercise is to make sure the batteries are paralleled only when the engine is charging. If they are paralleled when the engine is stopped (if the relay signal is taken from the ignition switch), then both batteries are discharged. I recommend installing a SPDT momentary contact switch that when pushed, will manually energize the relay while breaking the circuit from the regulator. Label this switch "emergency start". This is your built-in jumper cables :-) and will allow the accessory battery to be used to start the engine if you leave your lights on and run down the truck's battery. The diode battery splitters that used to be popular do not work well (at all) with most integral regulator alternators (most everything modern.) The reason is that the 0.7 volt drop across the diode prevents the battery from properly charging. The old systems worked because the external regulator took its voltage measurement from the ignition switch instead of the alternator terminal and automatically raised the alternator terminal voltage whatever amount was necessary to bring the ignition switch voltage back to 13.8-14 volts. John From: John De Armond Subject: Re: Elec connection, truck to trailer Date: Wed, 10 Nov 1999 00:28:59 EST Newsgroups: rec.outdoors.rv-travel budhop wrote: > > Mike: > > Thanks for your input. > > I have a spare isolator from a boat upgrade. I'll look into the suitability > of using that as an improvement and to cover the possibility of having a > senior moment and leaving the two systems connected during extended trailer > use. A diode type isolator will remove any possibility of charging the trailer battery because of the 0.7 volt drop across the diode. The only way to isolate the trailer system and still charge the battery is to use a relay isolator. Most relays I've seen are actuated by the vehicle's ignition switch. The batteries are paralleled when the switch is on and isolated when the switch is off. This works but is not optimal because if the ign switch is accidentally left on, both batteries are discharged. The optimal connection is to actuate the relay with a signal that the engine is running. Some alternator voltage regulators output such a signal. Another option is an oil pressure switch. This is what I used on my MH. You'll need the type switch that CLOSES on pressure increase. I don't remember what model engine this comes from - I just looked in the catalog at the parts house until I found one. A third, slightly more expensive option is to use a racing RPM switch. MSD and Mallory make nice switches that will work. John From: John De Armond Subject: Re: Elec connection, truck to trailer Date: Thu, 11 Nov 1999 00:50:34 EST Newsgroups: rec.outdoors.rv-travel Don Dickson wrote: > > Neon John wrote: > > > > > > A diode type isolator will remove any possibility of charging the > > trailer battery because of the 0.7 volt drop across the diode. > > Then a lot of people are being fooled because the most common isolator > is the diode type. The battery will still charged even with the .7v drop > because the alternator output is in the 14v range. It just makes it > harder to get a 100% charge on you trailer battery. Sorry, that's not the way it works. The alternator terminal voltage is mostly irrelevant (see below) and is a dependent variable. The independent variable is the voltage at the regulator reference terminal. The alternator voltage simply is whatever voltage it takes to reach the regulator's setpoint plus the voltage drop through the B+ wiring. On an old car with small wiring such as my 68 Fury, the alternator terminal voltage is over 17 volts when charging at near full output. The issue is where the voltage regulator measures the system voltage. All external regulated alternators and some internal regulated alternators take the system voltage measurement at the ignition switch (actually the +12v terminal of the regulator but since there is essentially no current in that lead, it is the same as at the ignition switch.) Many modern alternators, particularly the 1 wire self-excited alternators, take the voltage measurement at the alternator output terminal. This is a cost saving design "feature". Evidence of this feature is an unusually heavy B+ wire to the battery, used in an effort to minimize the voltage drop across this wire. Internally referenced regulators do a much poorer job of battery management and battery life suffers as the result. My wife's toyota camry is an example. The reference voltage is 14.4 volts. With a typical complement of accessories operating, the battery voltage is the proper 13.8-14 volts. But when the lights are off and the AC is not running, the battery terminal voltage is 14.4. This car eats a battery every 2 years or so as a consequence. Diode isolators will work on alternators with an external voltage reference. The reason is the 0.7 volt drop across the diode is seen as just another voltage drop in the system wiring. The regulator simply increases the field excitation to the alternator so that the reference voltage is again 13.8-14 volts. Diode isolators will NOT work on internally referenced self-excited alternators. This includes most foreign cars that use the Bosch pattern alternator and most late model US designs. The ONLY isolator that will work with these alternators is the relay isolator. Lacking a factory manual or other listing of suitable alternators, the easiest method of determining what kind of alternator one is dealing with is to put a heavy duty diode (such as a diode isolator) in series with the alternator B+ terminal. If the B+ terminal voltage RISES, then the diode isolator will work. If it stays about the same, then the regulator is internally referenced and the diode isolator will NOT work. It is the path of least resistance and hassle to just use the relay isolator. No testing is required and one knows that it will always work. it is not unusual for ham radio operators to install a second battery in their cars to run their rigs from and because of that, I have been involved in dozens of dual battery installations. I've found that the diode isolators are not worth the effort. The relay isolator works on ALL vehicles. Simply sticking a diode in the +12 volt lead going to the trailer, as I interpreted the recommendation of the original respondent to be, is a situation different from this discussion. Such a diode will introduce the additional voltage drop only in the trailer wiring and will cause the battery not to charge. This drop is different from ohmic voltage drop across the wiring because the 0.7 volt drop is essentially constant regardless of current. If the wiring causes a 0.7 volt drop at, say, 30 amps, the drop would be only 0.35 at 15 amps and zero volts at zero amps. The charge current to the trailer battery will be reduced until the drop and the battery voltage equals the 13.8-14 volt regulator reference. The battery will be charged, only more slowly. Not so with the fixed diode drop. >In fact there > probably is even more resistance(voltage drop) in the wire going to the > trailer if you don't make sure to get at least one size larger than the > standard gauge that installers use. The guy who put mine in looked at me > kind of funny when I told him to go to the heavier wire. It made a whole > $2.50 greater cost. True. However, as I note above, the ohmic drop in the trailer wiring is distinctly different than the junction drop across the diode. Ohmic drop slows the battery charging; junction drop stops it. > The > > only way to isolate the trailer system and still charge the battery > > is to use a relay isolator. > > This is a more efficient way to do it. Not the only way. I have a relay > isolator myself but because it has a moving mechanical part it will at > some time eventually fail. I usually disconnect the power from the relay > if I don't intend to use the trailer for several months. That way the > relay isn't constantly turning on and off for no reason other than the > ignition is being turned on and off. But the starter solenoid, a much less robust relay, is being cycled essentially the same number of times (one time per ignition-on, plus or minus) and the failure rate of these solenoids is practically nil. I carry a relay in my spare parts kit but I've never used it over the last 15+ years of owning vehicles with 2 batteries (radio cars and now a MH). I suspect you put the relay at more of a risk of damage by pulling the wire off than just letting it operate would. Couple other thoughts. If a diode isolator fails open, a common failure mode, the charging system to the vehicle battery is also disabled and the alternator is put at risk. This is because the vehicle charging current must also go through the isolator. If a relay fails open, the most common failure mode, the only result is the house battery is not charged. Relays fail-safe while diodes will commonly fail-dangerous. Another reason I prefer the relay. I mention this only because I've seen three instances of this in the last year. A starter solenoid, though it looks like an isolation relay, is NOT interchangeable. The isolation relay is designed to be energized continuously. The coil typically draws an amp (about 14 watts). A Ford-type solenoid is designed for intermittent energization. The coil typically draws 4+ amps (about 56+ watts). The Ford-type will quickly overheat and fail. A starter solenoid can be used in a pinch if the coil is powered through an ignition resistor. This will drop the voltage to a safer level. John From: John De Armond Subject: Re: Elec connection, truck to trailer Date: Fri, 12 Nov 1999 04:04:20 EST Newsgroups: rec.outdoors.rv-travel bob_woodsman@my-deja.com wrote: > > Good information John... > > Although you may have had good luck with your solenoids, you may be > interested in seeing a comparison to a new type of solid state relay > isolator. It only uses much less current to turn on. And, since it > monitors the voltage, you can even mount it in the camper... > > They have some resistance comparisons here: > http://www.hellroaring.com/nomoving.htm Thanks for pointing me to that URL. After reading a few pages of this web site, I decided to take an engineering look at this product and the company's claims. I think the results are quite interesting and illustrate why when a small company claims to have invented a better wheel, they usually have not. It also shows misleading advertising using selective and distorted date at its worst. Pull up a chair - this will be a long one. This company very carefully tries to present selective test data favoring their product with not reporting other relevant data not so favorable and while not disclosing how the device actually works. Nonetheless, with a careful review of all the data presented and some educated guesses, we can figure out a lot of things. They claim a very low "on" resistance, too low to be using power MOS-FETs for switching. We may then assume that the switching device is either a bipolar transistor or an insulated gate bipolar transistor(IGBT). The claim that the relay requires only 12 ma to operate indicates that IGBTs are used, as that would be insufficient drive for even a darlington pair bipolar. IGBTs are new and "cool" so I'd expect an outfit like that to be attracted to 'em. A heavily driven IGBT will act like a resistor at low currents but will become more of a constant current limiter as the current approaches the saturation level for the applied gate drive. This causes its effective resistance to rise which leads to higher power dissipation, which will eventually lead to thermal runaway and the ultimate destruction of the device if allowed to run to completion, quite unlike a relay. And when overloaded, the semiconductor will fail catastrophically. Again, quite unlike a relay. Inconsistent specs On their page http://www.hellroaring.com/bic75300.htm, they claim a voltage drop of "< 0.05 volts at 5 amps". That implies an internal resistance of 10 milliohms or less. We'll assume they're reporting the lowest figure they can support with data and use 10 milliohms. Not bad. However, elsewhere on that same page, they claim that at 120 amps the 75150 will dissipate "about 36 watts". This computes to a resistance of 2.5 milliohms! They show 5 milliohms of resistance on page http://www.hellroaring.com/nomoving.htm in the graph. Hmmm. Which is it? Misleading specs. The chart and discussion on page http://www.hellroaring.com/nomoving.htm is what really got my attention. They claim that a mechanical relay's coil draws 1 to 3.5 amps (we've already addressed this) and that the contact resistance might vary from 1 milliohm (best case, new relay) to 500 milliohm (worst case, old relay). This would correspond to from 0.12 volts to 60 volts drop at 120 amps (the number of amps they quote repeatedly). What is very interesting is that they charted their data taken from a test current of only 0.5 amps. A half amp is effectively like dry switching for a solenoid relay. That is, there is not enough current to burn off surface films on the contacts and so the apparent resistance will be erratic. This disappears when the current rises to several amps. BUT! A half an amp would make their solid state device look artificially good. Hmmm. I didn't like the look of that chart nor the discussion so I decided to test a relay secured from my spare parts bin. My Fluke 8800a lab DVM had just returned from the cal lab so I decided to press it into service. this instrument can measure millivolts to 3 decimal places. For my first test, I set the relay up on my bench with a power supply capable of supplying a regulated 5 amps DC. I configured a Fluke 88 DVM to measure the current and the 8800A to measure the voltage drop across the relay contacts using a 4 wire Kelvin connection which cancels out the test lead resistance. A separate power supply supplies coil power to the relay. The first measurement was to check the coil current. At 12.5 volts, the coil drew 0.7 amps or 8.75 watts. Significantly lower than their claim. For the next measurement, I actuated the relay 15 times in a row and recorded the current and voltage drop. I plugged the test values into an Excel spreadsheet to compute the effective resistance and power dissipation. Here is the table of figures: Test 1: Amps MV dc Ohms Watts Watts @ 120 amps 5.00 2.968 0.0005936 0.01484 0.356 4.99 115.000 0.0230461 0.57385 13.800 4.94 2.861 0.0005791 0.01413 0.343 4.94 58.540 0.0118502 0.28919 7.025 4.93 1.867 0.0003787 0.00920 0.224 4.93 5.542 0.0011241 0.02732 0.665 4.94 5.586 0.0011308 0.02759 0.670 4.92 4.712 0.0009577 0.02318 0.565 4.92 51.502 0.0104679 0.25339 6.180 4.89 29.400 0.0060123 0.14377 3.528 4.88 26.502 0.0054307 0.12933 3.180 4.88 7.201 0.0014756 0.03514 0.864 4.70 3.310 0.0007043 0.01556 0.397 4.87 5.995 0.0012310 0.02920 0.719 4.87 5.125 0.0010524 0.02496 0.615 I computed the power at 120 amps so that the numbers could easily be compared to their claim that their device dissipates "about 36 watts" at 120 amps. These results are quite interesting. The first thing we notice is that even at its worst, the relay drops almost no voltage and dissipates almost no power. The variable resistance shows that a) this relay is still on the edge of operating dry due to the low current and b) this relay's contacts were somewhat dirty because it had been stored for quite some time. Note that by the end of the test, the voltage drop was becoming quite consistent. Test 2: Amps MV dc Ohms Watts Watts @ 120 amps 74 0.04082 0.0005516 3.02068 4.898 74 0.04460 0.0006027 3.30040 5.352 74 0.04200 0.0005676 3.10800 5.040 74 0.04820 0.0006514 3.56680 5.784 74 0.09000 0.0012162 6.66000 10.800 74 0.07402 0.0010003 5.47748 8.882 74 0.06024 0.0008141 4.45776 7.229 74 0.04867 0.0006577 3.60158 5.840 74 0.03833 0.0005180 2.83642 4.600 For test 2 I decided to test the relay at a higher and more typical current level. I used my DC welder to supply the current and a Fluke 80i-410 DC clamp-on amp clamp to measure the current. Here the figures become more consistent. The higher drop is due to the slight heating of the relay contacts which causes their resistance to become slightly higher. The power dissipated and the voltage dropped is still completely insignificant. This is in stark contrast to the "bad" numbers presented on their web page. We've proved claims 2 and 6 on page http://www.hellroaring.com/nomoving.htm to be false. Claim 1 is valid only if the relay is improperly configured and really has nothing to do with the relay itself. Claim 3 is irrelevant as far as I can tell for RV use. Claim 4 is of no consequence either way. Claim 5, "During extreme cold weather, solenoids or relays can fail to operate, especially if not environmentally sealed" is hilarious and an indication of the stretch this company will go to in order to promote their product. What makes it so funny is that the starter uses a relay of essentially identical construction. If the cold keeps the relay from working, it will also keep the vehicle from starting! And if the relay has frozen, the power dissipated in the coil will thaw it quite quickly. Some things they didn't mention: * As they indirectly acknowledge in claim 5, the device has a positive temperature coefficient of conduction. What this means is as the temperature rises, either from ambient temperature or internal heating, the resistance of the pass transistor increases. This causes more heat which causes more heat and so on until the transistor either fails from thermal runaway or else protective circuitry operates. Either way the relay quits working, hopefully only until it cools. * Semiconductors are easily damaged from high voltage. Something as simple as having a spark plug wire fall against the wiring and sparking during service operations might do the trick. * Since the device dissipates significant power, it must be mounted where it can receive cooling air. With a relay it really doesn't matter much where it is mounted. * Semiconductors are easily damaged by reverse polarity voltage. Such as hooking a battery up backward or accidentally reversing the jumper cables. Voltage reversal will almost always smoke the diode pack in the alternator and many times the ECU if the engine has one. Nothing like smoking your expensive relay in the same incident. Speaking of price, * Price. Last time I was at the Shipp's RV sales, I noticed that an isolation relay costs about $13. According to their web page, the smaller device costs $114.95! And the larger one, $139.95!! Wow! I can buy a lifetime supply of mechanical relays for that kind of money. In Summary Their data is internally inconsistent, test conditions are chosen to make other products look their worst while hiding the products' warts and they make irrelevant comparisons while hyping their highly overpriced product. Think I'll pass on this one. John From: John De Armond Subject: Re: Elec connection, truck to trailer Date: Tue, 16 Nov 1999 01:54:05 EST Newsgroups: rec.outdoors.rv-travel bob_woodsman@my-deja.com wrote: > Although I don't believe it will change your opinion, I must point out > an error in your calculations. In your tests, you miscalculated the > power levels for an equivalent resistance at 120 amps. > > > > > Amps MV dc Ohms Watts Watts @ 120 amps > > 5.00 2.968 0.0005936 0.01484 0.356 > > 4.99 115.000 0.0230461 0.57385 13.800 Ooops, sorry. Knew I should have done those calcs by hand and not relied on the spreadsheet. > > These should be 120*120*resistance which equals 8.55 Watts and 331 > Watts respectively for the first two above. Your figures appear to be > result of multiplying the new current by the old voltage drop. Of > course, this calculation is not an accurate method on a relay for such > a different current. > > Anyway, it appears to me that your "engineering look" is not entirely > without bias. Many of your own claims appear exaggerated to strengthen > your own position on the subject. Claim? I'm not selling anything. I'm pointing out that irrelevant claims (see below) are being made to justify a product that costs over 10 times the product it is trying to replace without showing any significant benefit and with several major deficits, probably the most significant of which is the possibility of thermal runaway. I'll be interested in seeing if they respond to your mailing. I got the distinct feeling of technical obfuscation as I read their site. They could present the same product with a lot less hype using conventional technical terms. > > What seems ironic, is that your test data, even at 5 amps, tends to > support the erratic resistance data given rather than "prove" false > statements. Pass if you like, but I'm glad that I didn't. I still > think it's a pretty neat product. Variable contact resistance over the range of interest for a good relay IS irrelevant to this application. We're charging a battery, ferchristsake! I found comparing their good relay to a bad mechanical relay to be particularly disingenious. Now if we compare a bad mechanical relay to a burned out solid state relay.... Like I said in the oil discussion, whatever makes you feel good is what is good for you. For me, I'll buy an extra mechanical relay and put the $100+ in my pocket for some other toy. John From: John De Armond Subject: Re: Need help with battery isolator Date: Wed, 08 Mar 2000 16:09:25 EST Newsgroups: rec.outdoors.rv-travel sbourg wrote: > > In article <38C6A015.4BA9CCC1@bellsouth.net>, Neon John > <johngd@bellsouth.net> wrote: > > > >You'll be a LOT happier if you install a relay between the main and aux > >battery instead of an isolator. The relay, energized by (preferably) > >an oil pressure switch or your ignition switch, simply parallels the > >batteries when the engine is running and separates them when the engine > >is off. You don't have to figure out where the alternator's sense > >terminal is or worry about the voltage drop across the isolator or > >worry about diodes burning out or any of the other problems that go > >along with solid state isolators. The relay you want to use is called > >a "battery isolator relay" and looks like a round GM-type starter relay > >except it has a coil rated for continuous duty. DO NOT use a starter > >relay - the coil will quickly burn out, as it is not rated for > >continuous duty. My local full retail+ RV store charges $13 for the > >relay. The only downside, that the relay is mechanical and may > >occasionally fail, can be addressed by buying a second relay and > >tossing it in your spare parts box. Still cheaper than an isolator and > >MUCH less work. > > > > The relay may be easier to install, but it can allow heavy > current to flow between batteries when one is in a state of > greater discharge than the other. This is stress on both the > batteries and cabling - and could blow a breaker. Maybe it could (I don't think so but lacking 100% proof) but in practice it doesn't. Some current will flow if one battery is completely dead but the current isn't enough to hurt anything. One can get an idea of the magnitude of this current by observing the current flow when one jumps off a completely dead vehicle. A few minor sparks but certainly nothing that could possibly damage any component in the system. > The isolator is > an elegant solution which absolutely prevents this and allows > better distribution of charge current where it is needed. The > diodes are likely to blow only if something is miswired or > shorted to ground. Else they will outlive the mechanical relay > contacts by a wide margin with no degradation in performance - > which is inevitable in a relay. > > The controversy over which method is better is ongoing, but all > the relay has going for it is cheaper initial cost, and in some > cases - simpler hookup. A relay is always a simpler hookup. The diode isolator is a very inelegant kluge that seems to have a life of its own. It wastes power (a little less than a watt for each amp flowing), it is relatively fragile (just as fragile as the diodes in the alternator), is subject to destruction from even a momentary reverse polarity (such as when one reverses jumper cables) and is difficult to make work correctly. It might be acceptable if the charging system is engineered (and not just kluged together) from scratch to accommodate the 0.7-1 volt drop across the isolator but it is a pain in the ass in the aftermarket. This is 20 years of experience speaking. Old external regulator alternator systems work just fine. Internal regulators may or may not and often times, when "not", the problem is a just barely undercharged battery that seems to go dead every week or so. The problem is that some internal regulators STILL sense the voltage at the alternator terminal even though it has a sense terminal. This is probably due to an internal connection that guarantees the alternator will charge at some rate even if the sense lead becomes disconnected. The alternator used on 80's and 90's vintage Volvos were horrible in this regard. If the alternator insists of sensing at its terminal, the charge voltage at the battery will be 0.8-1 volts low. The battery(s) will be charged but only partially. An indication of how much of a problem this is is to take a look at the insert that comes with the most popular brand of aftermarket isolator. It lists dozens of configurations to try to work around this problem. Another problem that rears its head when the alternator is required to produce near full output over a long period of time, such as when driving an inverter, is that the isolator gets very hot to the point that it may burn out. 100 amps at 1 volt drop is 100 watts and that's a LOT of heat for the skimpy heat sink that comes on most isolators. This is 100 watts of power that is NOT available to the inverter. Diodes almost always fail shorted. When one leg shorts, the battery on that leg gets all the charging current while the other oen gets practically none. This will fool the average joe into believing the battery is bad. I fought these damn things for years, first on emergency vehicles, then on ham radio installations and finally on RVs before it finally dawned on me that the relay solves all these problems. The relay is trivial to install. simply run a suitable gauge wire from the existing battery to the relay and thence to the new battery. Connect the coil, preferably through an oil pressure switch (so the relay is only on when the engine is running) or to ignition switched 12 volts. There is effectively no loss through the relay and the coil typically draws less than 10 watts. A MAJOR second benefit is that if a manual energizing switch is provided, one can parallel the two batteries so that the house battery can boost the vehicle if one runs down the vehicle battery. Or, as happens to me occasionally, one can use the vehicle battery to boost the generator after the house battery becomes too low to crank it. Can't do any of this with a diode isolator. I suppose mechanical failure is possible but I've certainly never experienced it. If I ever do, the extra relay laying in my spare parts box can be installed in minutes at a cost of about $13. Compare that to the cost of a new diode isolator after you smoke it by accidentally hooking jumpers up backwards or overload it. To me, the relay option is a no-brainer. John From: John De Armond Subject: Re: Need help with battery isolator Date: Thu, 09 Mar 2000 05:01:22 EST Newsgroups: rec.outdoors.rv-travel sbourg wrote: Since you're in face-saving mode now, I'll only comment on your errors and let the merits of the devices stand on their own. > > Any externally sensing alternator has absolutely no problem with > the voltage drop across the diodes - most unregulated alternators > have hundreds of volts voltage compliance, and the alternator > output simply goes a bit higher than before to compensate. The > voltage drop works in your favor by allowing a larger voltage > drop to the higher current draw side without reducing needed > charging to the other. Compliance voltage is not the issue. What voltage the regulator's sense lead sees IS the issue. If the sense terminal sees the generator voltage BEFORE the diode isolator, either because of the way it is wired or because there is an internal resistive or diode link inside the regulator between the output and sense terminals, then the charge voltage applied to the batteries WILL BE reduced by the 0.7-1.0 volt drop across the isolator. I know that this is the case for some (all?) Bosch internal regulated alternators. I suspect it in many others. I particularly suspect it in nippon-denso and Hitachi alternators because both of these alternators will regulate the output voltage even with the sense lead disconnected. > True, there is a power loss in the isolator - 100 Watts or so > when the alternator is producing 1500 Watts - much less as the > battery charge goes up. All isolators I have experienced are > ruggedly over-designed, so thermal runaway is no problem. I have > never experienced nor known of anyone to have a problem. The term "thermal runaway" has no meaning in the context of a diode. This is a term that refers to a transistor's increased gain with temperature causing it to run away and destroy itself unless externally ballasted. Good old fashioned thermal failure in the diodes IS the issue. Perhaps my definition of "ruggedly over-designed" is a bit more conservative than yours but I find the aftermarket units to be way under-heat sunk. Battery charging isn't the issue. Running an inverter or other heavy load which draws heavy continuous current is. I'd hate to try to count how many of these things I've replaced on vehicles where the load is high and continuous. > > Certainly, if you jump a battery reverse polarity in any modern > vehicle, the least of your problems will be the isolator. Maybe, maybe not. But not having the isolator on the vehicle is one less thing to damage. > > Relays, on the other hand, have mechanical points - subject to > pitting and developing a high-resistivity contact. This can allow > a series drop of many volts, lengthening recharge times and > preventing a fully charged state. The first you may realize this > problem is when your batteries have been so chronically > undercharged that they prematurely fail. Rubbish. You're regurgitating that crap put out by Hellroaring Technologies here: http://www.hellroaring.com/nomoving.htm. This is the typical crap I'd expect from a company trying to sell a multi-hundred dollar electronic gizmo to replace a $13 relay. It is a strawman argument that can't stand even mild scrutiny. The last time this issue came up, I tested the battery isolation relays in my possession using a Biddle milli-ohm bridge. This bridge passes up to 10 amps through the unknown and reads the voltage drop (aka Kelvin connection) to measure fractional ohm devices. Probably the worst case relay would be the one on my 82 Itasca which is, as best I can tell, the original relay. It certainly looks grody enough. Yeah, the resistance bounced around a bit but was never over 1 milli-ohm (that would be 0.1 volt drop at 100 amps). Even when a contact closure resulted in a contact resistance near 1 milli-ohm, invariably, the next closure was much less. The nature of the construction of the isolation relay, with a loose disk of copper making the connection between fixed studs when energized, practically guarantees that the relay won't make the connection at the same point on the disk twice. Thus, if there is a bad spot on the disk that causes a relatively high connection, the next time, a different area makes the contact and the connection is normal. The new relays pushed the limits of my bridge, with contact resistance in the dozens of micro-ohms. At this range, the resistance of the studs and how well the Kelvin contacts are made must be considered. I should add that when I actually measured the voltage drop across the relay in my MH, the drop was on the order of a few millivolts for a 50 amp load. What happens is the higher current burns through any oxide or arc residue that may be on the contacts that would distort any measurement made at lower current. Next, let's take a look at your claim that a relay may drop "many volts". Let's say that "many" equals 2. That would have 14 volts on one battery for 12 volts on the other - reasonable. If I drop 2 volts at 100 amps, that's 200 watts of loss - a level of dissipation that would quickly render the relay a crispy critter. Since we don't see reports of flaming relays, we can safely assume that your claim is both hyperbole and incredible. > A relay will cost less, but it is only initially cheaper. Thus it > is the method of choice by cost-cutting RV manufacturers In RV > use. It will need replacing eventually and will cost you more in > battery failures, neither of which is the manufacturer's concern. > > Bad opinions of isolators can always be traced to improper > application, installation, or simple misunderstanding of how they > work. At this point, I'll allow the reader to determine the relative credibility of either side of this debate. It's the last I have to say on the subject. John From: John De Armond Subject: Re: Need help with battery isolator Date: Sat, 11 Mar 2000 04:18:14 EST Newsgroups: rec.outdoors.rv-travel Wes Stewart wrote: > John, in one of your previous posts, you mentioned measuring relays > using Kelvin connections, so I know that you know that you have a > "forcing" lead and a "sense" lead in that situation. > > The purpose of the sense lead it to monitor the voltage at the junction > of the forcing current and the device under test (DUT). In theory, the > sense lead carries no current and therefore suffers no voltage drop, so > it correctly measures the voltage at the junction. This is true in > power supply circuits as well, where the sense lead is connected to the > load or to another point at which regulation is to be maintained. > > That said, why would you want to connect the sense lead to anywhere but > at the battery terminal? It is the battery voltage that needs to be > maintained at the correct level to ensure battery longevity. Using your > method of connecting somewhere to a load "common" defeats the > zero-voltage-drop-in-the-sense-lead principle. From a manufacturer's perspective, I speculate that it is viewed to be more important to keep the voltage constant at the loads rather than at the battery. I imagine that they'd get far more complaints and warranty claims if the brightness of the lights went up and down with engine speed along with blower speed. Not to mention shortened bulb life. The battery is pretty tolerant of charge voltage so it can just sorta come along for the ride. Again, this is speculation on my part but it is also the way I'd design things. BTW, my old Fury's voltage regulator is referenced to some point near the battery (not sure where, never run it down). The lights do run up and down with engine speed and with electrical load. Somewhat annoying. > In other words, the alternator should be charging the battery; the > battery should be supplying the loads. That can't happen, of course, because current can't flow in both directions at the same time over the same wire. Under stead state conditions, the power flows from the alternator to the loads and the battery is floating off on the side. > With respect to the basic question of which is better, a diode "isolator" or a > relay, there are advantages and disadvantage both ways. I can't argue with that. It's a shame that people like sbourg must go to such extremes to defend his position. Makes the issue much more polarized that it needs to be. I'm certainly not 100% against diode isolators. Heck, my Datsun Z car still has one in it to run my radios. Good application - external regulator, well defined sense point, room to place the thing in a good stream of cooling air and a relatively low current draw (60 amp alternator). If the diodes fail, they're gone to be replaced with a relay. I'd certainly never tell someone to rip out a functioning diode isolator, especially if it is an OEM installation. OTOH, I'd never buy a replacement diode isolator when I can replace it with a more suitable relay. Hopefully, THE END of this thread :-) John |
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