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Old 04-22-2008, 03:55 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 04-27-2008, 11:38 PM   #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, 02:13 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, 06:57 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, 10:02 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, 10:16 PM   #6
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Re: How do you choose the battery isolator size?

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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 04-29-2008, 11:14 PM   #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, 12:48 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, 07:36 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, 08:18 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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Old 05-01-2008, 12:01 AM   #11
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Re: How do you choose the battery isolator size?

You guys got me thinking..... and confused....

Now... I was under the impression that a battery isolator would isolate the banks of batteries, (coach and house), but now, I'm a little confused.... Does the isolator isolate each battery separately instead of each bank of batteries?

So therefore I need a battery switch between the coach and house batteries and then isolate each battery in each bank... is that right? Or do I have it all sideways???
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Old 05-01-2008, 05:49 AM   #12
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Re: How do you choose the battery isolator size?

I would think that each battery or group of batteries that power the house would be considered one unit to isolate from another unit such as the starting battery or batteries. ( One isolator to separate the starting power from the cabin power).

On the other subject of Disconnects, how much currant does a starter for a 5.9 Cummins draw? The size of my battery leads are bigger than a quarter( coin size) in diameter. the lead connection screws down on a post, but that size is much bigger than any of the disconnects I have came across. And those disconnects are rated up to 190 amps continuous, 1000 amp intermittent. I can not imagine how one could connect the battery leads to one of these disconnects.
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Old 05-04-2008, 02:22 PM   #13
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Re: How do you choose the battery isolator size?

Quote:
Re: How do you choose the battery isolator size?
Sent at: Wed Apr 30, 2008 11:37 pm
by odyssess

what does this mean "Includes an additional terminal for exciting the alternator in start/run." I found a 200 amp isolater that has this extra terminal. Do you know what this means and is used for? Thanks Redbear.
Sorry I didn't see this PM sooner - I just peeked at the board a couple of times in the last few days - after 10 hour days at work. I look at my email in this house account only 2-3 times per week. Others may have the same question.

I looked at the GM version of the isolator in the eBay link posted by oldog 12, which has the fourth "Ignition" terminal you describe added to "excite" the alternator. It is listed as needed for "one-wire" alternators with an internal voltage regulator.

An alternator makes power by spinning one of two sets of windings. One set is the output, the other set of "field" windings act as an electro-magnet. The field windings are powered by the voltage regulator. (Some wind turbines being used by the alternate power bunch use permanent magnets, but the output voltage varies widely with speed. That's another story.)

The alternator makes 3-phase AC, which is fed through a set of internal diodes that put out pulsed DC. The vehicle battery smooths out the pulses as far as the devices being powered are concerned, like a pressure tank on a well pump.

To control the alternator output voltage, so it doesn't burn up all the 12-volt (or 24-volt) devices when the RPM's go up, there is a voltage regulator. This has a 'sense' input that measures the voltage at the battery feed. This wire usually (but not always) also serves as the regulator power input, too. Based on the sense voltage, the regulator has a "dimmer" circuit that feeds a controlled amount of battery voltage out to the field coil, which weakens or strenghtens the magnetic field interacting with the output coil and so adjusts the resulting voltage.

Some regulators are external to the alternator. Alternators for these regulators have at least 2 wires, the field and the output (plus grounding through the case). A standard three-terminal isolator is fine for these. Power for the field windings comes through the external regulator.

Some alternators have internal regulators, and these "one-wire" alternators use the single battery terminal as the regulator input and charging output. They adjust the output to what the battery SHOULD take, and don't allow for losses in the wiring. If you looked at the link posted by oldog 12, the vendor had a different version of isolator for certain years of GM models, which used 'one-wire' alternators, and have the extra terminal.

WIth the diodes in an isolator creating "one-way" valves from the alternator to the batteries, a one-wire alternator cannot draw voltage to the internal regulator to turn on the electro-magnet and get the alternator started. By having an 'ignition" or "start/run' terminal, the alternator can get voltage from the battery when the key is on, create a magnetic field, and get the alternator started. This is likely a diode working in the opposite direction from the charging ones.

The one-wire alternator is simpler, and is easier for mechanics (many of whom don't understand wiring, in my experience) to work with. The alternator system with an external regulator is more complicated, but can give more precise control as components deteriorate. Neither will adjust for a corroded battery cable, because it's unusual for the separate sense wires going right to the battery terminals.

The four-terminal isolator could be used for either type of alternator, just leave the ignition post disconnected if it's not needed. With a one-wire alternator, BE SURE that the ignition input turns on and off with the engine. If you connect the terminal direct to the starting battery, and if the house battery discharges down to 1.4 volts below it (0.7 for the house battery diode plus 0.7 for the ignition diode), the starting battery will start to flow current to the house through the ignition terminal, and defeat the isolator purpose.
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Old 05-04-2008, 02:49 PM   #14
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Re: How do you choose the battery isolator size?

Quote:
Re: How do you choose the battery isolator size?
by oldog12 on Thu May 01, 2008 12:01 am

You guys got me thinking..... and confused....

Now... I was under the impression that a battery isolator would isolate the banks of batteries, (coach and house), but now, I'm a little confused.... Does the isolator isolate each battery separately instead of each bank of batteries?

So therefore I need a battery switch between the coach and house batteries and then isolate each battery in each bank... is that right? Or do I have it all sideways???
Your original impression, and Train-Train's response are correct - the isolator separates the two battery systems. A "battery" is a group of cells (or cannons) working together as one, whether they are in the same case or not. Your car battery is 6 cells in one case, a big skoolie "house" battery might be 18 cells housed in three pairs of golf cart batteries.
(Those alkaline thingies marked D, C, or AA are really 'cells' and individually are not true 'batteries.')

If you use an isolator system, you will need one battery switch if you want to shut off the house battery system, and another one if you want to shut off the starting battery system.

The simplest DC system, which mirrors how tow vehicles charge trailers, is this:
Instead of using an isolator, simply have a switch or solenoid that joins the two systems together for charging (and possible boost starting) when the engine is running, and separates them when it's not. This is usually sufficient, but can lead to the good battery (bank) and alternator being drawn down, and the resultant vehicle low-voltage problems, if the other one is really dead when you start out.
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Old 05-05-2008, 02:37 PM   #15
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Re: How do you choose the battery isolator size?

A much better technology than the old isolators used in the past is the VSR or Voltage Sensing Relay; also known as an ACR or Automatic Charging Relay. These units sense the voltage of the battery (or batteries in the case of dual sensing units) and combine them when a set of conditions is met. Often when the "starting " battery is over 13.7 volts as an example. They break the connection when the voltage is low to protect the other battery (or batteries) in the system. They do not have the voltage drop across them that the isolators do and can be mounted anywhere.

Here's a link to just one of many...

http://bluesea.com/category/2/productline/overview/389

There's lots of good info in the tech papers on this site; they're one the of the premiere suppliers to the boat industry and there goods are often used in high end RVs.
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Old 05-06-2008, 11:28 AM   #16
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Re: How do you choose the battery isolator size?

Quote:
Originally Posted by Les Lampman
A much better technology than the old isolators used in the past is the VSR or Voltage Sensing Relay; also known as an ACR or Automatic Charging Relay. These units sense the voltage of the battery (or batteries in the case of dual sensing units) and combine them when a set of conditions is met. Often when the "starting " battery is over 13.7 volts as an example. They break the connection when the voltage is low to protect the other battery (or batteries) in the system. They do not have the voltage drop across them that the isolators do and can be mounted anywhere.

Here's a link to just one of many...

http://bluesea.com/category/2/productline/overview/389

There's lots of good info in the tech papers on this site; they're one the of the premiere suppliers to the boat industry and there goods are often used in high end RVs.
Yeh sure now you tell me, after i just ordered one of those Isolators.
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Old 05-18-2008, 02:19 PM   #17
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Re: How do you choose the battery isolator size?

Quote:
Les Lampman wrote:
A much better technology than the old isolators used in the past is the VSR or Voltage Sensing Relay; also known as an ACR or Automatic Charging Relay
I've been chewing on this one for a couple of weeks. I guess the hot button for me was the term "better technology" as opposed to "more advanced technology," which I would certainly agree with. "Better" is a subjective term, and what is better for any purpose depends on the use. I think we agree a chainsaw cuts 'better' than a steak knife, but I wouldn't cut a steak with a chainsaw, nor a tree branch with a steak knife (MacGyver excepted). Like most things, there is usually not a "one size fits all" solution.

I hadn't been exposed to ACRs. They are a nifty technology. I'm sure an ACR would be great for a full-timing relative of mine, intelligent but not an electrical whiz, who goes from genset to power pole due to necessary medical equipment onboard. The ACR can be installed in bi-directional mode, so if either the starting battery and alternator or the house battery and shoreline charger reaches 13.7 volts, the battery banks are joined to share the charge, until such time they both discharge to 12.6 volts and disconnect.

A word of caution for anyone joining battery banks with an isolator, relay or switch - be sure the cell types are the same. Connecting expensive gel-cell or AGM house batteries to an alternator or smart charger calibrated for wet cells (starting, golf cart, or deep cycle) might result in cooking out the capacity of the cells. If you are going to use sophisticated cells, be sure to do your homework on the battery manufacturer's requirements for charging.

What I don't like about ACRs is that when you shut off the bus, the ACR doesn't disconnect until the required discharge has taken place. They also draw a watt and a half to hold the coil in while joined.

If the opposite battery is very low, and it bogs down the charging source, the ACR will disconnect, and after the battery being charged is back up for 30 seconds, it will try again. It will continue to pulse on and off until the low battery bank gets at least a surface charge. I don't like all the make/break events. An isolator will let all loads run off of their own battery banks while it devotes full output to the low battery bank.

Quote:
They do not have the voltage drop across them that the isolators do and can be mounted anywhere.
I love the fact the ACRs are sealed (to prevent explosions on gasolene yachts, no doubt) and don't require a protected location. Howver, the voltage drop on an isolator is only an issue in that 1) the heat must be dissipated, and 2) with a "one-wire" alternator, the lost charging voltage is not compensated for. An alternator with an external regulator which senses the chassis system voltage will be "turned up" to compensate for the lost voltage. You have voltage drop across the diodes inside the alternator, anyway.

Another disadvantage of isolators is that if you want to top off both battery systems from a charger on a shoreline or genny, you should have a second isolator. If you tie your alternator output to your charger output to share an isolator, you are braver than I am.

An advantage of an isolator is that with dissimilar cell technology, you have a separate charging output that might be able to be separately conditioned to protect the high-tech cells

The ACR installation instructions show that you can wire an LED to indicate the "joined" condition, and can add a Boost/Normal/Disable toggle switch. The "Boost" command will force a join provided either battery bank measures over 9 volts. The "Boost" function could be used with careful monitoring while going down the road to stop the make/break events while a very weak bank charges back up.

So, to share a charging source, our choices are:

1. A heavy-duty battery disconnect switch from plus on one bank to plus on the other, operated by a smart operator. No voltage drop or power used anywhere, but if left on can result in both systems becoming discharged.
2. A solenoid joining plus to plus, operated by a smart operator via a remote switch as in # 1 above.
3. A solenoid from plus to plus powered from the ignition accessory circuits, which connects whenever the bus is in the "run" position. This assumes the alternator is running whenever you leave the key on.
4. An ignition solenoid as in # 3, adding a Force on/Disable/Automatic toggle switch. Turn off the solenoid feed from the key if you're troubleshooting the wipers, or the kids are playing the driver's radio, etc.
5. An ACR from plus to plus in automatic voltage sense operation.
6. An ACR from plus to plus with the mode switch added.
7. An isolator that steers alternator power to the lower voltage battery system (or both if near equal).
8. Two isloators, one for an AC-powered charger, and one for the alternator.

I personally would probably not choose an ACR, I probably would use either a battery switch or an isolator. So far, we always dry camp, and I don't like the idea of any extra battery draw. Also, the contacts will eventually wear out.

If I were using a solenoid, I would probably use a plain-Jane one from NAPA or Tractor Supply, so if (when) it went bad I wouldn't have to find an RV dealer or Camping World to be back in business. I would probably wire it to the key with an override switch as in choice # 4.
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Old 05-24-2008, 07:56 PM   #18
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Re: How do you choose the battery isolator size?

Since I already have the isolator, I want to know if mounting it sideways will be an issue. The instructions say to mount it so the cooling fins run vertically and attached to metal. I can attach it to metal, but there is nowhere close to the alt or batteries that I can mount it vertically. It's a cool spot under the bus away from the heat of the engine, but would this be bad and if so, how bad? I have a 145 amp alt and a high performance norco 200 amp isolator, so it wouldn't be too much for the isolator to take(hopefully not heating it up as much.) Thanks
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Old 05-25-2008, 09:49 AM   #19
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Re: How do you choose the battery isolator size?

I haven't seen a picture, and its just my opinion, but Ithink you should be fine.

Remember. 145 amps is the PEAK output of the alternator, only available at higher RPM with either a very discharged battery or a full DC load (driving in a blizzard with all lights, wipers, and heater going while dinner cooks in the microwave running on an inverter. ). You are not going to put that much current out at idle, even if it's needed. I'd be surprised if the AVERAGE current on a long daytime drive was over 100 amps.

The isolator, if the manufacturer was conservative, should be able to withstand more than 200 amps continuous to either battery system, with proper airflow. If the manufacturer was cheap, he may be expecting the duty cycle to be low and put higher numbers on a lower capacity unit, counting on the fact that it won't be stressed except occasionally. Either way, you already have 27% reserve capacity built in, before you consider the reduced duty cycle.

As far as fin position, the unit will get a little better cooling if the air passes between the fins (along their length) instead of hitting the first one at right angles and then passing the tips of the others. The vertical orientation is good for installations that stand still (like in a closet), where the only airflow is convection from any inverter heat. If your "under the bus" mounting is on the frame, the front-to back movement of air as you travel is greater than any inverter-created convection would ever be. Mounting it vertical there would be counter-productive. Mounting the isolator to metal will cause some of the heat to conduct away from the fins, making the mounting a part of the heat sink.
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Old 05-25-2008, 11:46 AM   #20
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Re: How do you choose the battery isolator size?

Seriously, it's scary how good you are with electrical stuff. I'm really glad you are a part(BIG PART in my opinion) of this community. I get better, faster answers from you(as well as others) than going down to the R.V. supply store!! Thanks again Redbear!!!! I'm going to install that Isolator today.
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