How can I limit the amperage from alternator to RV "house battery"?

jackbombay
jackbombay Solar Expert Posts: 46 ✭
in Off Grid Solar & Battery Systems #1
After a few wrong combinations of charge controllers and panels, and batteries, I now have a system that I think should be trouble free for some time, that system consists of a 130 watt panel, a 30 amp pro star charge controller with remote temp sensor and the "battery sense" hooked up along with a 200 AH "Universal battery" AGM battery mounted up under the van. The panel and battery work well for our power needs, the largish charge controller was used so I would have enough CC capacity to add another panel in the future if we need more power in winter months.

I do have a a 4 gauge wire run to a large solenoid so I can charge the 200 AH battery from the alternator, the van has a 150 amp alternator, my hunch is that when driving along if I flip the switch I'd have somewhere around 100 amps of power heading towards the battery. I purchased the battery from NAWS and I just called them and they said the charge current shouldn't exceed %15 of the capacity of the battery, 30 amps in my case.

Is there any way I can limit the amount of current that flows to the battery from the alternator?
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Comments

  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    #2
    Re: How can I limit the amperage from alternator to RV "house battery"?

    I use an agm battery in my race car. Never had an issue with the alternator over charging it.

    If you are really concerned you could place another charge controller between your engine and house battery. That would replace your solenoid. I think my first step might be to install an amp meter between the solenoid and the house battery. You can get one at just about any auto parts store.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
    Re: How can I limit the amperage from alternator to RV "house battery"?

    The only way you'll have 100 amps going from alternator to battery, is if the battery is really, really run down, and even them, the resistance from the connecting cable etc would develop a slight voltage across it, adding to the battery voltage, and further limiting current flow from the alternator. Otherwise as soon as current starts flowing into the battery, it's voltage will rise to counter the voltage from the alternator, and automatically limiting current flow. Alternators in my part of the world regulate at 14.5 volts, so as soon as the alternator sees that voltage, current flow all but stops.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: How can I limit the amperage from alternator to RV "house battery"?

    My suggestion would be to start the van with the AGM battery already connected, or at least only throw the switch when the van is idling to limit a step change in voltage/available surge current from the alternator.

    Check the charging voltage at the AGM battery... If it is in the 14.2 volt range (common here in the warmer climates), the battery should be perfectly happy. If it is over 14.2 volts in hot weather/hot battery, it could be an issue. But from what little I have seen, most automotive alternators are not able to output 14.5 volts at 100+ amps for more than a few minutes before the alternators get hot and reduce output voltage/current.

    In fact, I would be more worried about the alternator "surviving" vs the AGM battery being damaged by the alternator.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: How can I limit the amperage from alternator to RV "house battery"?
    BB. wrote: »
    My suggestion would be to start the van with the AGM battery already connected, or at least only throw the switch when the van is idling to limit a step change in voltage/available surge current from the alternator.

    Check the charging voltage at the AGM battery... If it is in the 14.2 volt range (common here in the warmer climates), the battery should be perfectly happy. If it is over 14.2 volts in hot weather/hot battery, it could be an issue. But from what little I have seen, most automotive alternators are not able to output 14.5 volts at 100+ amps for more than a few minutes before the alternators get hot and reduce output voltage/current.

    In fact, I would be more worried about the alternator "surviving" vs the AGM battery being damaged by the alternator.

    -Bill

    A typical alternator will be protected against overcurrent by a fuse. (No active current limiting as with some older generators). Too much current will destroy the expensive diode bridge inside the alternator. A 50A alternator is considered large for a passenger car. I have not seen an alternator that shut itself down as you describe, but it may be a feature common to really large alternators.
    SMA SB 3000, old BP panels.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #6
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Powerstream makes a 12vdc to 12vdc battery charger that should do the trick: http://www.powerstream.com/pst-pb1108.htm
    This one is a little more sophisticated, but it's twice the money: http://www.powerstream.com/DCC.htm It doesn't seem worth the extra money for your application.

    Note to moderators: If there is a problem linking to the manufacturer of a product that NAWS doesn't carry, please feel free to edit this post accordingly.
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #7
    Re: How can I limit the amperage from alternator to RV "house battery"?
    waynefromnscanada wrote: »
    The only way you'll have 100 amps going from alternator to battery, is if the battery is really, really run down, and even them, the resistance from the connecting cable etc would develop a slight voltage across it, adding to the battery voltage, and further limiting current flow from the alternator.

    I hope to never let the battery get below 12.2V so the battery should never get really run down, but even at 12.2 there is still roughly 100 AH of capacity in the battery for the alternator to throw current at.
    BB. wrote: »
    My suggestion would be to start the van with the AGM battery already connected, or at least only throw the switch when the van is idling to limit a step change in voltage/available surge current from the alternator.

    Check the charging voltage at the AGM battery... If it is in the 14.2 volt range (common here in the warmer climates), the battery should be perfectly happy. If it is over 14.2 volts in hot weather/hot battery, it could be an issue. But from what little I have seen, most automotive alternators are not able to output 14.5 volts at 100+ amps for more than a few minutes before the alternators get hot and reduce output voltage/current.

    Throwing the switch at idle was something I was thinking about too, I would never be fully charging the battery with the alt so the max voltage isn't much of a concern, I would be using the alternator to give the battery a boost in the event of long periods of snow/rain/coulds.
    vcallaway wrote: »
    If you are really concerned you could place another charge controller between your engine and house battery. That would replace your solenoid. I think my first step might be to install an amp meter between the solenoid and the house battery. You can get one at just about any auto parts store.

    This is really helpful, thanks for the idea, I do have an extra charge controller around, but I think starting with an amp meter makes the most sense as the current may stay under 30 amps if I keep the battery above 12.2V.
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #8
    Re: How can I limit the amperage from alternator to RV "house battery"?
    Endurance wrote: »
    Powerstream makes a 12vdc to 12vdc battery charger that should do the trick: http://www.powerstream.com/pst-pb1108.htm
    This one is a little more sophisticated, but it's twice the money: http://www.powerstream.com/DCC.htm It doesn't seem worth the extra money for your application.

    Note to moderators: If there is a problem linking to the manufacturer of a product that NAWS doesn't carry, please feel free to edit this post accordingly.


    That is a nifty device there and certainly a possible solution as well.

    I'll let you guys know what I find with an amp meter, I think I might be able to borrow a handheld one to run some preliminary tests.


    Thanks for all the info :-)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: How can I limit the amperage from alternator to RV "house battery"?

    It is not that the alternator has a current limiting circuit... It is just that they get hot which increases internal resistance (copper windings) and (probably) reduces the charging set point for the car battery (and by extension, the AGM house battery).

    http://www.electronics-cooling.com/2002/05/thermal-design-challenges-in-automotive-alternator-power-electronics/

    http://jgdarden.com/batteryfaq/carfaq5.htm#voltage

    You will also have issues where the car battery and the house battery are different temperatures... Cold batteries require higher charging voltages, warm batteries less.

    A very cold car with a house battery in a heated compartment could be over charged as the alternator has boosted output for the "cold" engine battery.

    None of it is perfect--But if you monitor your AGM voltage and charging current--You can probably get by well enough. It this becomes a bigger issue, then you will have to think about a DC to DC or other type charging system for your AGM.

    For very new vehicles... I gather that there is a new requirement for cars to turn off when stopped in traffic / at a light. So, they are now having to beef up the car's electrical system. AGM batteries, pushing charging voltage to 15 volts, more reliable starters (used more often to start engine in traffic), etc...

    Here is some more information from an Amateur Radio (HAM) point of view (a high power radio set can draw upwards of 2,000-3,000 watts from vehicle power system--so the HAMs are paying attention too).

    http://www.k0bg.com/alternator.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #10
    Re: How can I limit the amperage from alternator to RV "house battery"?
    BB. wrote: »
    You will also have issues where the car battery and the house battery are different temperatures... Cold batteries require higher charging voltages, warm batteries less.

    My house battery is outside, under the vehicle so the car and house batteries should stay fairly close in temp to each other.
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #11
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Ok, so I think I have found an economical way to measure the charge amps when the alternator is charging the batteries.

    I can install this shunt at my solenoid, one end connected to the solenoid and the other connected to the cable that runs to my house battery, and I can use this gauge to read the signal from the shunt.

    Is there anything I am overlooking with this?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Normally, shunts install in the negative side of the battery wiring... If you install this in the positive side--then each of the two sense wires should have a fuse to limit current if one of the sense wires gets shorted to ground (like 1 amp fuse, or smaller, if your wiring is very fine awg).

    With negative mounted shunt, not much will happen if one of the current sense wires gets shorted to ground.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #13
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Thanks for the tip, I could install in on the negative lead, but it will be far easier to install in on the positive lead. I will fuse both sense wires.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #14
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Make sure they are fuse close to the shunt (near the source of "high current")... Fusing near the meter will not be of much help.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jackbombay
    jackbombay Solar Expert Posts: 46 ✭
    #15
    Re: How can I limit the amperage from alternator to RV "house battery"?
    BB. wrote: »
    Make sure they are fuse close to the shunt ...

    Definitely, I'll wrap the whole shunt in 1/8" thick vinyl/zip ties too.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #16
    Re: How can I limit the amperage from alternator to RV "house battery"?

    Shunts can get very hot (they are big power resistors):

    0.075 volt shunt drop * 100 amps = 7.5 watts of dissipated energy at full power

    You will run the risk of melting zip ties.

    Many shunts have some sort of base with mounting holes.

    For this one, I guess you have to mount the cables in clamps and suspend the shunt between them.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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