Limiting current flow

tkc100
tkc100 Solar Expert Posts: 67 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
I am faced with a bit of a problem that I hope there is a simple solution to.
I wasn’t sure in which forum I should post this question because even though I do not consider myself a beginner a clear solution is a present eluding me.
I need to charge a 12 volt battery from a limited source.
The source is only capable of supplying 20 amps and it is fused for protection.
How can I limit the current flow to the battery being charged to avoid overloading the charging source and blowing the fuse?
I know that only a given amount of current can flow over any particular wire size but after that things get confusing.
If I place a properly size wire at the battery being charged would that effectively limit the current flow and keep the 20 amp fuse from blowing?
If that is the case what size link (gauge and length) would do the trick?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,640 admin
    #2
    Re: Limiting current flow

    How much do you want to spend?

    It sounds like the MorningStar 15 Amp MPPT Solar Charge controller would be about the ideal fit... But not cheap.

    Charging from a battery bank/other power sources has been tested and found to work well.

    Is the 20 amp supply filtered to some degree (not a lot of AC ripple)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • XRinger
    XRinger Solar Expert Posts: 529 ✭✭✭
    #3
    Re: Limiting current flow

    I used an Astron power supply 12v 20a power supply to charge a 12v car battery for many years.
    I did have to make one small mod to the Astron's board.

    Anyways, I used the setup to power my 100w Ham radio transceiver.
    So, even when the grid failed, I could still use the radio.

    I don't think that I ever ran the battery down so low that it took more
    than 20A of charge current, since I never blew the 20A fuse.

    If I was you, I would go to the AutoZone store and buy one of their
    little 12V 15 or 20 amp circuit breakers. (They look like fuses).

    PS:
    Forget the wire idea. That would cost you a small college fund in copper..
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  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    #4
    Re: Limiting current flow

    Good morning B.B.
    The charging source is a truck with the standard 12 volt battery and alternator.
    There is a auxiliary circuit factory wired to the trailer hitch that provides 12 volts and is fuse to supply no more than 20 amps.
    It would save me a lot of additional work and it would be nice if I could use this circuit to charge an auxiliary battery.
    A 15 to 20 charging rate would be adequate but I'm concerned that initially a low battery would over tax the 20 amp fuse.
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  • XRinger
    XRinger Solar Expert Posts: 529 ✭✭✭
    #5
    Re: Limiting current flow

    One common practice (with Hams anyways) is to charge up a deeply
    discharged battery with a 12V lamp in series with the supply.

    The load of the lamp limits the current flow into the battery.
    Works like a trickle charger. Once the lamp grows dim, you flip a switch to connect it directly.
    Larger wattage lamps will charge faster.

    This method will also work to prevent arcing-sparking when connecting
    an inverter to a battery bank. (To pre-charge the capacitors).
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #6
    Re: Limiting current flow

    http://smartercharger.com/battery-chargers/#CTEK D250S DUAL

    You could use one of these but they are not cheap.
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  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    #7
    Re: Limiting current flow

    I know that this is exactly backwards to most of the issues we face with low voltage supplies. Normally the quest is to get the most current with the least amount of voltage drop to the battery being charged.
    In this particular situation I am trying to charge a battery without over taxing the existing circuitry.
    I am aware that placing a resistive load in series will limit the current flow.
    Nice tip on avoiding the arching when hooking up an inverter.
    However in the particular application it would be a two step operation that would require manual intervention.
    The existing wire within this current is 12 gauge and is about 15 feet in length.
    Is it possible to calculate the resistance of the existing wire and then if necessary place an even smaller gauge wire directly at the battery being charged to effectively limit the current flow?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Limiting current flow

    Install a circuit breaker.
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #9
    Re: Limiting current flow
    tkc100 wrote: »
    The source is only capable of supplying 20 amps and it is fused for protection.
    How can I limit the current flow to the battery being charged to avoid overloading the charging source and blowing the fuse?

    He wants to limit the current that a potentially dead battery would draw so as not to exceed 20A. A circuit breaker will not allow him to charge the battery. His fuse will protect the vehicle wiring. He needs to regulate the charging current to less than 20A. The CTEK will do this because it is a buck boost switcher.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Limiting current flow
    Joe_B wrote: »
    He wants to limit the current that a potentially dead battery would draw so as not to exceed 20A. A circuit breaker will not allow him to charge the battery. His fuse will protect the vehicle wiring. He needs to regulate the charging current to less than 20A. The CTEK will do this because it is a buck boost switcher.

    Spending money on something to cover the possible problem of a battery being too low and drawing too much current on a 20 Amp circuit is likely a waste of money. Yes a fuse will protect the line. So will a circuit breaker. Either will handle over-current for a short period of time. The difference is you can reset a circuit breaker.

    If the battery is kept charged there is little chance it will need to draw more than 20 Amps. That is the simple, inexpensive solution.
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #11
    Re: Limiting current flow

    Providing he never discharges the battery then I agree with you. However, if he does have a battery that is discharged to the point where it will draw more than 20 amps then he is not going to be able to charge the battery at all. The CTEK (or similar device)although an expensive solution will limit the current no matter the state of the battery. This will provide completely automatic operation.

    And TKC, never under size a wire to limit current through its resistance, it is VERY dangerous and a fire hazard. Size the wire gauge accordingly and limit current some other way.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Limiting current flow
    Joe_B wrote: »
    And TKC, never under size a wire to limit current through its resistance, it is VERY dangerous and a fire hazard. Size the wire gauge accordingly and limit current some other way.

    Truer words were never spoken!

    I just reviewed the thread and don't see any mention of the size of this auxiliary battery is? :confused:
    20 Amps could charge a couple of T105's without trouble.

    If the battery is really down, take it out and hook it to the vehicle battery with a set of jumper cables.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,640 admin
    #13
    Re: Limiting current flow

    Usually, the cable from the vehicle battery to the house battery in a trailer just does not draw that much current because of voltage drop...

    Just to make some numbers up and see what happens... Say 14.2 volts charging from the car alternator. Say 11.5 volt battery (you should not take it lower, or else you will kill the battery anyway):
    • 14.2 volts - 11.5 volts = 2.7 volts drop
    So, how long can a 10 awg wire be with 2.7 volt drop and 20 amp maximum current limit... Using a generic voltage drop calculator and some trial and error, I get:

    • 55 feet (one way) with 20 amp limit on 10 awg wire will give a 2.6 volt drop.
    So--it is very conceivable that a 20-30 foot run of 10 awg wire and a well discharged house battery could draw more than 20 amps and damage the wiring.


    To be honest, I never have really liked the vehicle charging system to recharge the house batter... It can take a while (many hours of driving) and really does not do a good job of finishing the charging on the house deep cycle battery (vehicle alternators are usually around 13.8 to 14.2 volts--and a deep cycle flooded cell battery should be upwards of 14.5 volts or so).


    I would suggest that running an inverter on the vehicle and an AC charger back on the house battery would give a more satisfactory rate and finish charge.


    But, that is not cheap and requires more equipment and things to turn on and off and not that much charging current unless you use large inverters and AC chargers (20 amps * 12.5 volts = 250 watts of charging).


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Rondu
    Rondu Registered Users Posts: 1
    #14
    Re: Limiting current flow
    BB. wrote: »
    • 55 feet (one way) with 20 amp limit on 10 awg wire will give a 2.6 volt drop.

    I think this could be your solution, a 27 foot run of 10 awg wire with a 20 amp breaker.
    With the house battery anywhere from 14.2 to 11.5 V (based on BB's calculations) you will get under 20 A charging current. If the battery drops below 11.5 V the breaker will have to be reset.
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  • XRinger
    XRinger Solar Expert Posts: 529 ✭✭✭
    #15
    Re: Limiting current flow

    Install a cheap 12v 20a circuit breaker,
    and put a 12v lamp (old headlight) in parallel with it.

    If things go well, the lamp never comes on.
    If it does turn on, you might notice it
    and reset the breaker after the light dims.

    If you fail to notice it, your battery still gets a pretty good charge,
    it just takes a little longer.. 8)
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #16
    Re: Limiting current flow
    XRinger wrote: »
    Install a cheap 12v 20a circuit breaker,
    and put a 12v lamp (old headlight) in parallel with it.

    If things go well, the lamp never comes on.
    If it does turn on, you might notice it
    and reset the breaker after the light dims.

    If you fail to notice it, your battery still gets a pretty good charge,
    it just takes a little longer.. 8)

    deleted deleted deleted
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