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Old 11-14-2013, 05:39 PM   #1
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Battery Isolator Question

After hours of repairing corroded wires and testing with a voltmeter I've come to the conclusion that my battery isolator is bad. Being that the MH is on a Chevy G30 van chassis I have a single starting battery and a single 70 amp alternator and then a single deep cycle coach battery. I have 15.8 volts at the isolator coming from the alternator but at the two other terminals on the isolator only have battery voltage at them. There is just roughly 11.8 volts at the starting battery.

So being that I need to replace it is there any kind or brand that anybody recommends, or are they all pretty much the same?
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Old 11-15-2013, 02:04 AM   #2
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You can get one battery or the other charging by placing a jumper on the isolator from the alternator post to a battery post. I ran for some time on a 95' P30 chassis coach I used to have with a jumper to the chassis battery and just let the converter charge the house batteries via shore or genny power.

As for the isolator I don't know or think any particular model or brand is better than another.
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Old 11-15-2013, 11:18 AM   #3
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Perfect Switch without a doubt has the best isolator technology on the market currently. Use it in our Sprinter as recommended by Odyssey batteries and it works exactly as advertised FWIW
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Old 11-15-2013, 08:36 PM   #4
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The MH does run but is not road worthy right now due to transmission problems an it needing brake lines so replacing it is not a huge priority. I do like to run it often though so I would like it to charge my starting battery as it's running. It never crossed my mind that all I need to do is disconnect the starting battery portion of the harness from the isolator and connect it to the same post that the alternator is connected to so I will only be charging the starting battery.

I guess thirty years on the original isolator isn't to bad so I can't really complain about having to replace it!
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Old 11-16-2013, 12:01 PM   #5
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I have installed a very large, but unknown number of these over the years. You can pay more for one with a name, or not, it makes little difference. Just don't buy too close to the alternator rating. Yours is 70, look for a 90. The newer ones are much better than what your coach came with installed.

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Old 11-18-2013, 01:50 PM   #6
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There are many options for you. The main are: diode based, relay based and FET based. You can also go with a simple relay that is manually operated

Then you decide if you want one way (engine charges either battery) or two way the RV converter can charge the engine.

If I was going to recommend one, I'd say to look for a relay based two way or just a plain, continuous duty manual relay. I chose the latter, as it's cheap and effective, but some like the automatic features of the former. If you have lots of money and want the best - an FET based design is in the running.
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Old 11-19-2013, 08:12 AM   #7
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A relay based two way you might look at is the Sure Power 1315-200
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Old 11-20-2013, 08:52 AM   #8
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I think I am going to go with the Sure-Power 952 isolator. I found out the isolator on the MH is rated up to 70 amps meaning the alternator output is less than that. Catalog listing options for replacement alternators are 61, 63, 78, and 80. So by going with the bigger isolator rated for 95 amps I can upgrade my alternator too at some point.

Plus I can get one for $31 online!
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Old 11-20-2013, 09:05 AM   #9
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Who sells for $31.00 please & thanks
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Old 11-20-2013, 09:29 AM   #10
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www.ase-supply.com
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Old 11-20-2013, 11:38 AM   #11
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Battery isolators that run large diodes are not my favorite option. Firstly you need voltage to push the power or amps into the battery. A diode works but there is voltage drop across the diode. Why would you put a dropped alternator voltage into a battery? The whole idea is to charge the battery while on. By dropping the voltage through the isolator, it takes longer to charge the battery plain and simple. Buy a simple starting relay and there will be no drop in charging voltage to the battery.
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Old 11-20-2013, 07:25 PM   #12
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Quote:
Originally Posted by Paul R. Haller View Post
By dropping the voltage through the isolator, it takes longer to charge the battery plain and simple.
You can negate the voltage drop by locating the alternator sense wire after the isolator diode in the circuit, on one of the output terminals is convenient, some even have alternator terminals. The alternator will then increase voltage to cover the voltage loss of the diode.

That said, I'm also not a real fan of diode isolators. More because they're generally mounted in a very exposed location because of the cooling necessity and end up failing because of the exposure.
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Old 11-21-2013, 10:31 AM   #13
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Quote:
Originally Posted by Argosy View Post
You can negate the voltage drop by locating the alternator sense wire after the isolator diode in the circuit, on one of the output terminals is convenient, some even have alternator terminals. The alternator will then increase voltage to cover the voltage loss of the diode.

That said, I'm also not a real fan of diode isolators. More because they're generally mounted in a very exposed location because of the cooling necessity and end up failing because of the exposure.
While I agree with Argosy on both counts, as most often the failures caused by exposure are just that the terminals require cleaning and then they can be back in service. I have never seen an internal failure of an isolator that was caused by the environment. When this terminal cleaning service is required, there are also other battery and starter terminals that need some very basic maintenance. So, I don't find it an issue. I do find them to be more dependable than any isolator with mechanical contacts and less expensive than the J-Fet of similar capability.

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