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First, I'm a much better woodworker than electrician hence my question. The PM has an 180A alternator. I see in searching that some people use 70A battery isolators. Well - shouldn't the isolator have a peak rating of at least the 180A of the alternator?

Lastly, I saw the battery doctor being used a bit. Is this the isolator that you would recommend for adding an AGM house battery in the 100Ah range?
 

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Watercamper,

Welcome!

I have used the 150A Battery Doctor isolator, as have others with great success.

Even though the alternator delivers up to 180A, it doesn't send that much current (amps) to the aux battery(s). On my install, I have an 80A circuit breaker between the isolator and my (2) 100Ah AGM batteries. It has never popped. While driving, a top-off charge is usually low current. When your aux batteries are almost completely discharged (not a good thing to do), they would draw the most current from the alternator, but in my case, it has never exceeded the 80A breaker.

I would recommend a similar install, with your battery(s) as close to the van battery as possible. My Battery Doctor is mounted on a board attached to the driver seat base.

Ed
 

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The battery has internal resistance and that is what is preventing the rush of high current through it. Greater discharge level means less internal resistance and more potential difference between the battery and the alternator so more current. Most batteries are rated for how fast they should be charged. For flooded lead acid and AGM batteries 1/10 the ampere hour capacity is safe. Your battery manufacturer has a site with the safe charging rate four yours. To keep the charging rate down when it might blow that beaker or fuse you might want to begin the connection at idle when the alternator is perhaps making 30 or 40 amperes. Even a few minutes of that will raise the internal resistance of your battery enough to keep the rate within safe bounds. Lithium batteries- whole other situation!
 

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If you have lithiums, you definitely need the isolator to be able to handle 180A. I have seen 175A go to my lithiums when they are down below 80%. If you have lead-acid batteries, you can probably get away with 80A or lower depending on the size of your battery bank and the type of battery.
 

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2014, 138WB, High Roof, Gas, SW MT
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Hi,
I use 60 amp breakers on the line from van battery to house batteries. I picked the 60 amp breaker because the battery manufacturer says I should not be charging at more than 35 amps (this is for to Flooded Lead Acid 220 amp-hr 6 volt golf cart batteries) as it will damage or shorten the life of the batteries. I wanted to know when the charging current was much over the recommended level. I've also measured the current with a clamp on meter and not seen more than 38 amps. AGM batteries might pull somewhat more current than my FLAs.

Important to note that this wire from the van battery to the house battery that your isolator goes in should have a breaker on BOTH ends. There are large current sources on both ends (van battery on one end and house battery on other end), so to protect the wire you need a breaker near each current source. If these breakers are the 60 or 80 amp range, then your isolator is never going to see more than that.

Be thankful that you don't have to contend with 180 amps, as this would require at least 2/0 wire between van and house battery :)
http://www.cerrowire.com/ampacity-charts

Gary
 

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Just finishing up my build. I went for the battery doctor with it 150 amp rating. Though charging the battery may not draw that much, the inverter and microwave I installed will. It seemed that the house battery alone would not provide enough amps so I figured I would only use the microwave when the van was running. I expect that my 700W microwave draws about 100 amps. The 150 amp rating gives me some extra headroom. I have run it once to check it and did not blow any fuses.
 

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kmm,

Interesting... I tried the aux battery + van battery, van running idea before I got my second aux battery. It didn't work well and did pop the 80A breaker between the van battery (and isolator) and the aux battery. I added teh second 100Ah AGM and the microwave works really well! My 700W microwave draws 1000W... seems like most do.

Ed
 

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Yes, 700 or 750 watt "cooking power" microwave ovens actually draw over 1,000 watts. Some inverters such as the Xantrex 1,000 watt sine wave are actually rated to carry well over 1,000 watts for the short time that one usually runs a microwave oven. Mine works well drawing from a 230 amp hour AGM house battery. I never run the engine when using the microwave as I am fused for 80 amps to the Blue Sea combining relay.
 

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Hey, this is exactly the thread I was looking for! I am trying to size an isolator for my 3-way system (solar, shore, engine) with a 220A alternator and a battery bank of five 100Ah 12V AGMs (got a uniquely good deal). Does the size of this battery bank increase the isolator or fuse requirements?
 

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Hey, this is exactly the thread I was looking for! I am trying to size an isolator for my 3-way system (solar, shore, engine) with a 220A alternator and a battery bank of five 100Ah 12V AGMs (got a uniquely good deal). Does the size of this battery bank increase the isolator or fuse requirements?
Wow 500 Ah of batteries! I'm curious as to your source, if it's possible to get here in Ontario. Also what size solar panel are you using and the roof configuration. I'm going through these same thoughts as I plan the solar part of my build. Perhaps a new thread or PM is in order.

Shaun
 

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Yes it will. I’d spec the whole thing for 150 amps- 2/0 wire, 150 amp controller, 150 amp breakers/fuses. I doubt the alternator has more than that to give considering the van uses some and it's output is speed dependent and that the internal resistance of the batteries will rise as the charging begins.
Crazy lot of batteries even if cheap. I have 220 amp-hr and it feels unlimited.
 

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I'm sure it would have an impact on wire sizes and fusing.

The current that flows thru the isolator to the batteries is limited by the internal resistance of the batteries, as RD so astutely stated above.

Given that the (5) batteries are connected in parallel, and that resistance decreases when resistors are placed in a parallel configuration, lower total resistance means higher current (with a fixed 12V). So the (5) batteries in parallel connected to the charging isolator will need heavier wire and heavier fusing.
 

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Wow 500 Ah of batteries! I'm curious as to your source, if it's possible to get here in Ontario. Also what size solar panel are you using and the roof configuration. I'm going through these same thoughts as I plan the solar part of my build. Perhaps a new thread or PM is in order. Shaun
I snagged my son's employee discount.>:D

Yes it will. I’d spec the whole thing for 150 amps- 2/0 wire, 150 amp controller, 150 amp breakers/fuses. I doubt the alternator has more than that to give considering the van uses some and it's output is speed dependent and that the internal resistance of the batteries will rise as the charging begins.
Crazy lot of batteries even if cheap. I have 220 amp-hr and it feels unlimited.
150A, eh? So, you're not sure these 220A alternators really max out at 80A? I wonder how dead it would have to be to exceed 80A.

I have a 500Ah battery bank because it's going to be an all-electric boondocking machine (no propane). There also isn't much sun up here on the wetcoast (less than half AZ). And then there's all these pesky mountains covered with big trees. So I'll put 400W on the roof and pray for 200. I also hope to avoid draining the batteries below 70%-ish to extend their life, which leaves me about 150-200Ah to play with.
 

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I'm sure it would have an impact on wire sizes and fusing. ... So the (5) batteries in parallel connected to the charging isolator will need heavier wire and heavier fusing.
Thanks, Ed. It really helps when a second opinion agrees with the first. ;)
Looks like I'll be needing seriously big wire and fuses for this job.

Shawn, I'll have four 100W panels in two banks either side of the MaxxFan in the middle. The panels are roughly 2'x4' and fit crosswise perfectly. I'll use Hein's sticky roof mounts, which leave room for an awning maybe later.
 

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I was guessing of course. The 220 amp alternator reaches that output at near maximum engine rpm, lets say 3500. You are more likely to hit perhaps 2000. The output is not linear with speed either i bet. Can it exceed 80? It may not but I’m betting it won’t exceed 150 and I’m standing by that estimate. Once the system is in put in a fuse at 80 and see if it blows. No harm in that. The reverse, to build for 80 and get 150 presents all sort of problems. You are going to have a load of electricity for normal campervan stuff. I have 200 watts of solar and I could easily do with 100. My 200+amp hours of battery remain in the SOC range you suggest you want and my solar has them back to 100% in about 2 1/2 hours. I seldom close the battery interconnect as I like the charging rate and profile the Tracer solar controller gives. I have used shore power perhaps a couple of times camping in prolonged rain although I can only think of one really- Dauphin Island,MS, May 2016. Even with your low Sun climate you have twice everything I have. I think you will be pleasantly surprised.
 

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Glad I read this thread.....very informative.
I'm currently trying to size the cable from the van battery
to the house batteries (220ah total, AGM) with a Wirthco 150amp
battery Doc in the mix.
The distance from van battery to house batteries is 3 feet and
we're in a gasser.
Thoughts on cable size? I get it that I need 80amp breakers near each
battery.
 

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Troublemaker

We hate to be a troublemaker, but after reading so much in this forum about "battery isolators" we feel compelled to inquire, firstly, what really is a battery isolator? And, secondly, why is everyone installing them?

We got on-line and tried to research this topic - - found lots of flowery hype about their benefits but really no technical explanation concerning their insides and how they achieve the promised results.

In our now-being-retired CaRV vehicle - - as both vehicle and house batteries were of the same AGM type - - we simply interconnected the two systems with two series interrupt switches, one ignition driven so we wouldn't inadvertently drain our vehicle battery while camped 'engine off' and a second manual switch to interrupt house battery charging if we felt our house batteries were at a satisfactory charge level. (In our new campervan system, which we hope to report-on in this forum, we maintain the add-on lithium system completely separate from the vehicle system).

I guess one could say our switches were 'isolators', but we're assuming the 'isolators' spoken of here are more sophisticated.
 

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They are. They sense a difference in voltage and connect when their designer decided to have them. I use a system similar to your CaRV, an ignition operated solenoid and a interrupt switch to avoid it closing to make the contact. I like this because I have 200 watts of solar and seldom (almost never) need the alternator to charge the batteries. The interrupt switch is off I want the alternator to charge. The advantage of this is the solar, which has a sophisticated charging profile and output appropriate for my batteries maintains them better than the van would. Battery doctor and other such connectors make the connection instead of a brain and will charge the battery as the vehicle manufacturer wanted the starter battery charged despite what the battery needs, and at a rate that may be inappropriate for the coach battery specifications. Why anyone with solar would want to regularly do that is beyond me. Once in a while? Yes. Always. No.

See: https://www.amazon.com/Stinger-SGP3...d=1487774333&sr=8-4&keywords=battery+solenoid
 

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Glad I read this thread.....very informative.
I'm currently trying to size the cable from the van battery
to the house batteries (220ah total, AGM) with a Wirthco 150amp
battery Doc in the mix.
The distance from van battery to house batteries is 3 feet and
we're in a gasser.
Thoughts on cable size? I get it that I need 80amp breakers near each
battery.
#4 is available at H-D stranded and cheap and usually in Black and Red and will carry that no problem. Others will suggest larger wire size but really for 3 ft it is 2 sizes above the requirement for 80 amps.

https://www.tessco.com/yts/industry/products/itm/automotive/get_wired.html
 
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