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Old 12-22-2010, 12:54 PM   #1
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Battery Boost Switch

Hi all

I was wondering if others run different types of batteries in their rigs for the starting and coach power systems?

I found that my Coach is wired so that the Battery Boost Switch is always activated while the ignition switch is in the RUN position. A simple check is to press the boost switch before starting and listen for the click, then press if after the engine is running, no click. It has been activated by the ignition switch. To verify I checked the solenoid power line it was indeed activated.

A question that comes to mind.
Why have a boost switch at all if indeed it is always activated when starting the motor?

The alternator does as it should and outputs loads of power. The problem is that is outputs to much. It was factory set at 14.5Vdc at the starting battery. This excessive voltage was measured with highly accurate digital meters and my TriMetric 2025.

Both sets of batteries get this excessive voltage all the time the engine is running. Not something good for the AGM batteries supplying power for the coach.

Just wondering if others have found this to be true with their machines?
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Old 12-22-2010, 03:50 PM   #2
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the boost switch connects the batteries when the key is in the start position. hard to hear when cranking the engine, but the boost relay drops out when cranking. the boost relay should be pulled in when the key is in the run position to allow the alternator to charge the chassis and house batteries. as for the AGMs, sorry, dont know.
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Old 12-22-2010, 04:43 PM   #3
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That is normal voltage for a alternator to put out.

Have you had any problems with any of the batteries?
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Old 12-23-2010, 07:33 AM   #4
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Yes - the first set of cheap RV/Marine batteries that were connected in parallel died after two years.

I added a Morningstar Tristar MPPT-60 solar charge controller that has very flexible charging capabilities and just wonder why a constant 14.6 could be the right thing for the new batteries. Unlike the alternator system the Morningstar outputs multiple different voltages depending on the batteries need.
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Old 12-23-2010, 09:04 AM   #5
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Many vehicles have AGM batteries. I have one as a start battery in my Jeep Wrangler and pickup. In cold weather the charging voltage is about 14.8 volts. Have not had any problems with them. If the isolation relay didn't do that, the only charging you would have is from the converter.
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Old 12-23-2010, 09:27 AM   #6
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I just replaced the two Interstate house batteries that came in my motor home after 7 years and 46,000 miles. One had a dead cell and the other had one cell with a very low specific gravity reading.
My alternator output averages 14.4 volts.
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Old 12-23-2010, 09:27 AM   #7
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Thanks for the responses
Still trying to wrap my head around the alternator thing - guess I have a ways to go before I fully understand the issues

Another question
"Why do battery manufacturers and solar charge controllers promote three stage charging if the stock alternator system does the job?"

I don't like seeing 14.6 Volts at the battery for 5 or 6 hours at a time while driving, yes sure the current drops to a very low value since the battery voltage rises up. My solar charge controller recommends 1 to 2 hours max duration at that higher voltage otherwise the batteries gas / boil excessively.

Yes agreed the voltage should increase in colder weather the reason for battery temperature sensors. The stock alternator system had no temperature sensors, my solar system does.

I guess my system is a bit different from others. I tossed out the cheap "Converter" that was basically a 12Vdc power supply, replaced it with a proper three stage battery charger / converter and installed a proper PV powered Morningstar MPPT-60 Charge controller, they both do a great job at keeping the coach batteries up.

My thought is to simply add a cut off switch at the Boost Switch to disable the alternator from charging the coach batteries. Then maybe adding a Sterling Power A2B charging system.
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Old 12-23-2010, 10:57 AM   #8
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Normal alternator charging should be in the range of 13.7 - 14.6V. It is normal to see slightly higher periodically but never constantly. If you have anything higher, replace the alternator
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Old 12-23-2010, 01:09 PM   #9
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Optima Charging Recommendations

It looks like your charging voltage is within the limits recommended by Optima

Optima Recommended Charging Voltage and Current
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