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tev123's avatar
tev123
Explorer
Oct 14, 2015

100 amp alternator through battery isolator issues

Hello.

Bought a 1990 class B a few weeks ago and as you could imagine first order of business was to dismantle all the systems and learn how everything is hooked up and how it works (first rv for me) :)

I have an issue and a few questions regarding charging the coach's side battery using the vehicle's alternator:

The camper has 2 batteries, one standard in the engine bay and a deep cycle in the coach to run the appliances, etc. The coach battery gets charged from a battery charger that sits just under the transfer switch/circuit breaker panel. Either the generator or an external 110v line supplies power to it.

The prev. owner had a 100 amp alternator recently installed

There is a battery isolator in the engine bay that is connected a bit suspiciously. Both vehicle battery lead and alternator lead are attached to the same battery post of the isolator. The alternator post has nothing attached. And the aux battery post has a wire that goes through 2 inline 50A circuit breakers to the + post of the circuit breaker panel in the coach's side, where the coach's battery + cable is also connected.


Issues:

I reconnected the alternator lead to the alternator post of the battery isolator. I also noticed that one of the inline circuit breakers was blown, so I bridged it since there was another along the line.

When I turned on the vehicle and checked the readings on the isolator posts, I got 14.4v from the alternator and the aux battery post, but only 12 on the engine battery post. After about 10 seconds the aux battery cable started getting very hot (ie. smoke), so I shut off everything and disconnected the coach battery from the isolator... The wires off the isolator seem to be 8 or 10 gauge. Everything down to the coach's battery + post was warm/hot to the touch.


Questions:

Is it recommended to have the alternator also charge the coach's battery when the engine is running via the isolator? While I travel, it would top off or recharge the coach battery, but I don't know if it is not recommended to charge the deep cycle battery straight out of the alternator (eg. without a sensing battery charger...)

Could the isolator have gone bad? I would have expected the inline circuit breaker to have tripped if > 50A was going through it. Maybe that went bad too? but either way, I would not expect the battery to charge at 50A, right? The isolator is the solid state type and does not look very old; maybe a few years based on corrosion amount compared to other components...

What am I missing?


Thanks!

14 Replies

  • Is this a diode-based battery isolator or solenoid-based battery isolator?

    And by putting alternator and both battery wires on the same lug, it is the same as no isolator-- they are all always connected. This is often done when the isolator has failed.
  • It sounds like you have eliminated the circuit protection by bypassing the circuit breaker and have the isolator miswired/failed. This, along with a coach battery that is depleted, would be the cause of the heat.

    Yes, isolators do go bad. There is also good probability that you will never get 50A from the alternator to the coach battery through a diode isolator when everything is wired correctly.
  • RoyB's avatar
    RoyB
    Explorer II
    This is what happens to all trailers after a few years. Many undocumented MODS to deal with...

    I would put it back to normal but not remove any of the mod items. I would love to have a second 100AMP ALTERNATOR SETUP in my truck for my additional battery banks if that is what I reading from your comments...

    Also you mentioned battery isolator - with todays smart mode charging schemes you cannot use the old DIODE TYPE three port Battery Isolators... The large diodes inside have to much DC VOLTAGE for you to be able to provide the three required charging modes for the additional batteries being 14.4VDC-13.6VDC-13.2VDC.

    Todays so-called battery isolators are now accomplished with automatic smart relays that direct connect to the source when needed... Read-up on the BLUE SEA SI-ACR Automatic Charging Relay on-line to see how they provide you charging voltages for additional batteries.
    BLUE SEA AUTO CHARGING SMART RELAY SI-ACR

    MORE INFO

    With the cable getting hot sure sounds like a bad wiring concept or batteries wired in the REVERSE ORDER... I think you found the reason it was disconnected.

    I would get it all back to normal and then have the experts tell you what you need to do with what you got...

    Just my thoughts - not an expert on any of this...
    Roy Ken
  • The diode based isolators are battery killers. Those large finned heat sinks are your hard working alternator amps going up in heat, instead of into the battery. The diode also reduces voltage, basically making it impossible for the battery to ever reach full charge via the alternator, and greatly reducing the amp hours returned into the battery whenever you drive.

    Some of the isolators try and trick the alternator into allowing a higher voltage to negate the diode's voltage loss.

    You would be better off with a heavy duty continuous duty solenoid rated at 100 amps continuous or higher.

    There are voltage sensing solenoids like the surepower 1314 and 1315 or the Blueseas ACR.

    I chose the fully manual option with a 1/2/BOTH/OFF switch

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