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Old 04-22-2008, 04:55 PM   #1
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How do you choose the battery isolator size?

My altinator puts out 145 amps I was told, so do I get an Isolator that is 140 amps or 160 amps( the only two sizes close) or do you pick the Isolator based on something else. Does the brand matter? I can't seem to find reliable info. Any help is appreciated, and maybe ideas as to where other people have mounted theirs...thanks
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Old 04-28-2008, 12:38 AM   #2
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Re: How do you choose the battery isolator size?

When you have a choice, ALWAYS pick a size larger than your alternator. If the 160 Amp is the biggest you can get, buy that one. I would suggest a 200 amp one, if there is one available at a good price. To be conservative, I personally would try to get one about double my alternator size, but cost and mounting location space may be issue. You might also want to allow for an alternator upgrade if that is in your plans.

The average isolator is basically two heavy-duty silicon diodes mounted back-to-back on heat-sink fins. These diodes act as electric one-way valves. If the voltage at the output is higher than the input, the flow shuts off, unless the voltage (pressure) rises to such an great extreme that the diodes are destroyed. This reverse-flow prevention keeps a good (i.e. starting) battery from discharging into a run-down (i.e. lighting) battery in an attempt to re-charge it. That's why we use them.

When the voltage at the input rises to about 0.7 volts higher than an output, the current begins to flow. Each diode 'uses' this 0.7 volts of pressure to keep the current flowing. If both batteries are equally discharged, both diodes will conduct and the alternator output will be split between the two systems.

Suppose one battery is deeply discharged. You wake up one morning, and the coach battery is down to 11.0 volts, but the starting battery is still fully charged to 12.7 volts. When you start the engine, if the discharged battery drags the alternator down below 13.4 volts (12.7 + 0.7) at the input, the diode for the starting battery will not conduct, and all of the 145 amps goes through the other diode to the coach battery until the voltages equalize. Unlike systems with a solenoid or switch that joins the batteries when the key is on, the good battery will not discharge to help the alternator fill the bad one.

The devil is in the 0.7 volt drop used inside the diodes. With a 145-amp alternator running at full tilt, your isolator is, in effect, a 101.5-watt heater (145 amps x 0.7 volts). The 140-amp isolator is only rated to dissipate 98 watts. The diodes can overheat and fail. The bigger sizes really indicate that the beefier units can handle more heat.

If a free 140-amp unit were to fall into your hands, you could probably get away with using it. With good batteries, light loads, a cool location, good airflow, and balanced charging, a slightly undersized isolator may last months, maybe even forever. If you have one battery go dead, and run full output for an extended time into one of the undersized diodes, it may overheat and fail. You're now in the market for a new isolator. Since it was free, you aren't out anything.

If you're paying good money, starting with an isolator that can handle more heat than you think it will face is a good idea, especially if it is going into a hot area like the engine compartment. It's not worth saving a little on an undersized unit.
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Old 04-28-2008, 03:13 AM   #3
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Re: How do you choose the battery isolator size?

WOW Redbear...you know your isolators. I've been waiting for anyone to answer my post. I saw someone mount it on the back of their battery box, is that a good place? Is there any brand better or worse or better suited for bus purposes? Thanks again Redbear.
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Old 04-29-2008, 07:57 PM   #4
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Re: How do you choose the battery isolator size?

Well, I guess everyone is good for something. I've made my living in electronics (radio) for almost 30 years. I wish I knew how to weld, or to paint decently. I don't have experience with specific isolator brands. It's not like there's much of a difference, except how well the diodes attach to the heat sink, and whether the manufacturer's rating is conservative, with lots of reserve, or at the enthusiastic edge.

There is no universal best place - it depends on your layout. The things to think about are:

1. Keep high-current wiring short. Just like the diodes will turn some of the current into heat, so will wires. The fatter and shorter high current wires are, the more electricity reaches its destination (and bigger sparks if you mess up! ).

2. Water and condensation are your enemy. Most, if not all isolators are splash-proof, but moisture can get inside the jackets of the wires if they aren't sealed well. I can't count the number of times I measured a good 12 volts in a mobile radio's copper wiring, but then saw drops to near zero when connected to a load. Feel for a lump, cut it open, and find green powder inside where the metal used to be.

3. Avoid high heat areas. The less environmental heat it's exposed to, the better it can get rid of the heat it generates in use. Lower heat means greater current capacity. The only failed isolator I ever dealt with was mounted by the RV factory on the firewall above the radiator of a Chevy 454-powered motorhome. Being near the exhaust manifold or turbo would be worse.

My only concern with a battery-box location would be if out-gassing caused acidic build-up on the wiring. I don't know how well-vented your battery box might be.

Take a look at what fits you best, and give it your best shot.
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Old 04-29-2008, 11:02 PM   #5
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Re: How do you choose the battery isolator size?

Excellent posts, Redbear Now we have an electronics expert to add to out knowledge base.
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Old 04-29-2008, 11:16 PM   #6
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Re: How do you choose the battery isolator size?

Quote:
Originally Posted by the_experience03
Excellent posts, Redbear Now we have an electronics expert to add to out knowledge base.
Agreed! Most definitely cool, and glad to have on this excellent board!
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Old 04-30-2008, 12:14 AM   #7
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Re: How do you choose the battery isolator size?

Again, many thanks Redbear!! I've been looking on the web and can't seem to find one bigger than 200amps. I may have to settle for that one for now, I don't plan on changing out my altinator any time soon. I guess I ment to say that I was thinking of mounting it on the back(outside) of the battery box. My box has limited ventilation, the hole in the back of the box is all. It's the stock box that came built in the bus and I added only one large battery for now to run my fridge controls, water pump,ceiling fan and maybe some lights soon. I'll be looking to you for more electrical genius when new problems arise(and they usually do!). Good looking out Redbear, thanks.
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Old 04-30-2008, 01:48 AM   #8
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Re: How do you choose the battery isolator size?

Found this on eBay....

http://cgi.ebay.com/ebaymotors/200-AMP- ... enameZWDVW

Is this what you might be looking for?
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Old 04-30-2008, 08:36 PM   #9
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Re: How do you choose the battery isolator size?

Good info on the isolator. I need to clarify something as I am confused. When we are discussing a battery isolator, it is a system that charges the starting batteries as well as the house batteries. The isolator keeps the two systems from interfering with each other. In a couple of post, I wonder if the poster is talking about a battery disconnect?

I have a nagging electrical issue in that something is draining my batteries. I have tried to identify the culprit but I have not found it yet. I have been thinking about installing a battery disconnect as a for sure way to easily keep my batteries from draining. I have shopped for one and I do not understand proper sizing and where it is intended to be installed.

Originally , I thought the disconnect should disconnect the battery as close to the battery as possible. The sizing of the disconnect vs. the size of my battery leads are not a match so I am thinking this disconnect is intended to be installed perhaps in the instrument panel. Could someone set me straight?
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Old 04-30-2008, 09:18 PM   #10
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Re: How do you choose the battery isolator size?

ideally a disconnect would be as close to the batteries as possible, but it has to be a switch capable of handling the high current draw required by the starter.

disconnecting the main wire feeding into your switch panel may cure the problem if the item that's draining your batteries gets it's power from there.
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