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Current Regulator

Discussion in 'Electronic Basics' started by Dystopia, Dec 10, 2007.

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  1. Dystopia

    Dystopia Guest

    I have a motor home which has the main battery for the engine and an
    auxiliary battery to supply the lighting etc in the living area. With
    the ignition off a relay switches the auxiliary battery to supply the
    living area but when the vehicle is running the relay connects the
    auxiliary battery in parallel with the main battery so that it is
    charged. The circuit is badly designed because if the auxiliary
    battery is flat and the main battery is fully charged (which it always
    is) there is a very high current surge from the main battery to the
    auxiliary which blows the 30A inline fuse. I, therefore, need to
    insert a circuit to restrict the current to say 20A max.

    Can anyone say where there is a suitable circuit diagram on the web?
  2. You need a battery isolator, Typically 2 diodes back to back, like...

    The charge current for a lead acid battery group 24, can be quite high >100 amps, depending upon the alternator. That said,
    I think 30A is a little under sized. A Properly sized circuit breaker would be better.

    I recommend a isolator such as

  3. John Larkin

    John Larkin Guest

    A 12-volt light bulb, which is a nice nonlinear resistor. Car or
    motorcycle headlamp maybe. It also serves as a charging indicator!

  4. Ross Herbert

    Ross Herbert Guest

    You need a "smart" dual battery isolator. A simple "back to back
    diode" isolator is no good, it does exactly the same as your existing
    relay controlled unit.

    Guest make a range of smart isolators starting around 70A rating and
    you couldn't build your own any cheaper. Most marine or RV dealers
    will sell battery isolators.
  5. Ian Malcolm

    Ian Malcolm Guest

    I have a similar setup that cross connects the engine and house battery
    on a small yacht. I never have any trouble with it BUT it differs from
    the OP's in two crucial details:

    1. The relay is rated 80A and fused at 70A (greater than the alternator
    rated output). All the wiring is rated over 80A.

    2. The alternator output goes to the house battery, as it the one that
    needs the most charging.

    (This may be more trouble to set up than its worth on a motorhome - the
    engine battery can end up low quite easily if the lights are left on
    with the house battery still fully charged so the combiner has to be
    designed to handle EITHER battery flat with the other one charged. Also
    fitting wiring between the batteries and from the alternator to the
    house battery capable of handling the full alternator output and
    protecting that wiring against short circuits may be a quite difficult
    undertaking. The alternator MUST either be machine sensed or the sense
    wire moved to the house battery. If the capacity of the house battery is
    much greater than the engine battery its worth it though for better and
    quicker charging)

    Commercial units usually have quite strict restrictions on the MAXIMUM
    wire size and MINIMUM length to use which ensure there is sufficient
    resistance in circuit to keep the surge current below their switching

    Assuming the existing unit is controlled by the ignition switch,v a flat
    battery at nominally 10V (one isn't supposed to take a lead acid battery
    below 10.5V) paralleled with a fully charged one at an absolute maximum
    of 13.8V only gives 3.8V differential. It needs a total resistance of
    0.1266 ohm to limit the current to 30A.

    Even 0.1 ohm in series would probably cure the OP's problem.

    (WARNING, at 30A a 0.1 ohm resistor will dissipate 90 watts - This will
    be a problem if one of the batteries has a shorted cell. Its a lot
    simpler to get rid of the heat safely over several meters of wiring
    rather than at a single component.)

    There are only two occasions (neglecting short circuits) that the fuse
    should blow:
    If you are trying to pull STARTING current through the relay, e.g. the
    house battery is fully charged and the engine one is bad or has a loose
    If your house battery has a shorted cell.

    If the existing unit is controlled by either the alternator warning lamp
    output or senses the charging voltage, then the battery being charged
    gets up to nearly 15V and you either need a little more resistance in
    the circuit (0.166 ohm) or a relay rated higher than the alternator
    output (the alternator cant hold the voltage on a battery over its
    resting level at currents higher than the alternators rated output)

    Fitting a Guest unit (or equivalent) rated greater than the vehicles
    alternator output, *AND* rewiring to the correct specs for the new unit
    would probably be the easiest and safest solution though.
  6. Warren Weber

    Warren Weber Guest

    I had the dual diode isolator on my motor home. Due to the voltage drop in
    the diode the 2nd battery never seems to reach full charge. Changed to the
    relay set up and put a 50 amp breaker in series to 2nd battery. every thing
    work fine now. W W
  7. Dystopia

    Dystopia Guest

    Thanks for all the ideas. An isolator is the real answer and if I was
    designing it from scratch that is what I would use but the relay
    serves as an isolator albeit a very poor one. I had thought of using
    a resistance to limit the flow and the bulb is a good idea but I am
    concerned about the heat generated in the small space where the
    circuits are installed.

    For info. I have draw a wiring diagram of the circuit, as it is, and
    put it here:

    I had thought in terms of a simple circuit using a 2N3055 with a zener
    in the common base circuit and if you scroll down below the wiring
    diagram on the web site I have sketched what I mean. Anyone any ideas
    about this one please?
  8. Guest

    I was hoping someone would come up with a circuit since I could use
    such a box myself. I really don't think you can make a decent
    controller without some current limiting. I did a few back of the
    envelope designs for this application awhile ago and concluded
    anything linear makes too much heat. You would probably need a
    hysteretic current-mode controller. It is best to have a working
    design since there are plenty of gotchas in switchers.
  9. Dystopia

    Dystopia Guest

    As I have written a resistive device would radiate too much heat for
    the confined space I have to install a circuit. I did not think that
    my problem was very demanding and am surprised that no one has come up
    with an active circuit so I have installed a large choke in series
    with the fuse hoping that this will be sufficient to remove the spike,
    time will tell.
  10. ehsjr

    ehsjr Guest

    You could use the resistive limiting, and use a circuit
    to spread the charging out over time so that the temperature
    does not increase as much.

    But I'm curious. If, as you say, the real answer is
    an isolator, why not use that?

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