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Old 07-27-2008, 10:25 AM   #11
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Location: Northern BC Canada
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Re: Powering off inverter while in motion

All batteries are brand new, however they went dead on me twice because of an as of yet unknown open circuit somewhere. My Bus isnt going on the road yet since I'm not ready yet and to many things are laying around loose.
The Voltage was measured at the battery post with 2 different digital meters to make sure they where reading correctly.
I guess I have to get somebody to kick up the RPM's while I measure and see what happens.
Proud owner of a: 1996 Thomas Safe-T-Liner,Cummins 6CTA 8.3Lt diesel, Allison AT.
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Old 07-29-2008, 03:32 AM   #12
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Re: Powering off inverter while in motion

What is the normal output of an alternator in Volts? I know on my pick ups its normaly around 14+ somewhere. However my Thomas at high idle only puts out 12.95V
I think the normal regulator setting is about 14.5 volts, plus or minus. A maintenance float charge is 13.5 volts down to maybe a minimum of 13.25. A regulator set at 12.95 will run your bus loads up to the alternator output limit , but is too low to refill discharged batteries properly.

If the batteries have been drained down , or the load is excessive, the voltage reading out of a good alternator will be dragged down until the load is removed and/or the batteries recharge a certain amount. This is what happens when the headlights dim and the [electric] wipers slow when at idle waiting at a stop light. Check for a full charge with a hydrometer, or charge the batteries full up with a shoreline charger, and then see if the load on the alternator releases the voltage to go higher.

Since you measured the 12.95 volts across the battery terminals, did you also measure at the alternator between the output post to the alternator case? If the reading at the alternator is higher, it could be 13.65 volts less isolator drop, loose or corroded cables, or failing disconnect switch, plug, etc. If the measurement across the alternator is just as low, the alternator may be overloaded, have a loose drive belt, or have internal diodes open in one or more of the AC phases.

If you see a voltage difference, check for bad wiring by measuring from the alternator output (meter +) to the battery positive (meter -) with the engine running, and see if the reading is nearly zero, or less than 0.8 volt with an isolator in line. Repeat the test with the meter (+) on the battery ground, and the (-) on the alternator frame. This will show up a bad ground if it's not nearly zero. (A sensitive voltmeter might pick up a small fraction of a volt just because of the wire resistance.)

If you see 2 or 3 volts anywhere measuring plus to plus or ground to ground, move the meter until you find where the drop is. For example, measure from the alternator frame to the bus frame, then bus frame to battery ground. Make sure you get a good connection with the meter leads through paint, rust, and battery terminal waterproofing. If the voltage drop is hard to find, don't forget to check from each contact stud to its wire terminal at the 'same' spot for a loose or rusty connection. Anyplace that reads zero difference with the alternator charging is a good connection, anyplace that shows voltage means there is resistance and voltage drop in that connection.

However my Thomas at high idle only puts out 12.95V. Is my Alternator going bad or is this common at high idle, will output increase if I'm on the road?
The alternator output increases with RPM, not vehicle speed. If you can't get the voltage to come up parked in neutral with the engine at cruise RPM, it won't be any higher in gear going down the road.

The AGM's might be a bit of a concern because of their charging algorithm. They really don't like to see more than about 13.5 volts for charging.
You might put the voltage drop from an isolator or blocking diode to good use. Connect the starting batteries direct, and charge the AGMs through one pole of an isolator. About 14.2 volts to the wet cells will be 13.5 volt to the AGMs. 14.5 will be a little bit high, but not as bad as direct wiring. Make sure there's a disconnect, or the AGM loads will draw down the starting batteries through the one pole when you are parked.

If you already have a two-pole isolator, and the alternator regulates both outputs to 14.5 for the wet cells, you can put one pole of a second isolator between the AGMs and the main isolator output to get the extra voltage drop. A hefty diode (rated for full alternator output amperage) on a proper heat sink will also work.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
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Old 07-30-2008, 09:33 AM   #13
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Location: downriver, detroit mi
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Re: Powering off inverter while in motion

the skool of hard knocks says: never forget that battery cables and all other wires have 2 ends and that if one one is not clean and tight their is going to be a voltage drop or loss, that being said check for max voltage at the alternator by full fielding the alternator and reading the voltage from the power stud to the case, then start checking either grounds or hot terminals for voltage loss or drop and go thru the system until you find and correct all of the current robbing voltage droping pieces/connections.
look in particular for defective cable ends(crimps) and or frayed wire ends at the connectors, if the wire looks green or otherwise corroded it is probably a wise decision to replace the wire with a new properly sized one, tie wraped onto the factory harness.
after all it's your bus and temporary fixes always seem to come back at inconvient times to haunt me.
good luck
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Old 07-31-2008, 12:56 AM   #14
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Re: Powering off inverter while in motion

the best battery cables i've found are 00 welding cable. Flexible, carries tons of current, but they are a bit pricey
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Old 07-31-2008, 08:20 AM   #15
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Re: Powering off inverter while in motion

the best place to buy automotive electrical stuff in the midwest is , they stock just about any terminal or plug that is made as well as automotive wire that is smaller dia conductors so it is more flexable for automotive use. if you just want welding cable try your local welding supply co. for the best deal.
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