Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

12V charging circuit between two batteries limiting max amps

  1. Dec 29, 2016 #1
    Hello, I was reading a previous thread that is now closed. What I'm curious about is a simple wiring schematic. I want to charge a battery on my trailer while it's hooked to the truck from the trailer 7 pin rv connection. The rv connection has a 12v hot all the time wire that I could use to trickle charge my winch batteries while towing. The issue is the feed wire is 16ga and fused at 15amps. To avoid running heavy wires I read where you can simply install an incandescent light bulb to limit the amps. My question is- I assume just wire it in series in the positive wire? If that works it's great the bulb will show that I have juice plus charge batts and not overload wires when trailer batts are low from either use without charge and or sitting extended period of time. Here is the original post I was reading:
    Charging 12V from 12V, but limiting the current https://www.physicsforums.com/threads/charging-12v-from-12v-but-limiting-the-current.364343/

    Thank you in advance.
    Last edited by a moderator: Dec 29, 2016
  2. jcsd
  3. Dec 29, 2016 #2


    User Avatar

    Staff: Mentor

    Hi David Ward. smiley_sign_welcome.gif

    The bulb may not glow sufficiently or reliably to be a dependable indicator of charging, though doing the job just fine. The voltage across the bulb can be expected to vary somewhat, according to state of charge of the respective sides. Yes, in series with the positive cable.

    A little LED indicator circuit might be better as an indicator of charging.

    Good luck with it!
  4. Dec 29, 2016 #3
    Thank you and even if it doesn't glow that's fine too. I plan to use a simple 1156 bulb readily available anywhere. Thanks again
    Last edited by a moderator: Jan 1, 2017
  5. Dec 30, 2016 #4


    User Avatar
    Gold Member

    Sounds OK to me using a bulb rated at 12V 5A which is 60W, or just say a headlight bulb. When you crank the engine the bulb will limit current drain from the trailer battery. I would not normally expect the bulb to light up.
  6. Jan 5, 2017 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    1156 lamp is inexpensive , easy to mount, and it's easy to find a socket for it.
    It will limit current into a dead short to around two amps so I would use three of four of them in parallel with no fear of overloading #16 wire.

    If you want to get creative,
    this lamp mounted in a PAR36 tractor headlight housing would do the job

    and by connecting one side to ground it'd make a spectacular work light for nighttime
    and use a metal housing for it could get pretty warm.

    PAR36 is bulb shorthand ... PAR for parabolic shape, diameter is given in eighths of an inch, so that #4537 PAR36 lamp is 36/8 = 4.5 inches diameter, common size on farm and construction equipment.

    Have fun

    old jim
    Last edited: Jan 5, 2017
  7. Jan 10, 2017 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    If you want to avoid discharging the trailer battery whilst cranking, you can always fit a Voltage Sensitive Relay, which disconnects the trailer battery when the main battery volts fall below 'charging' value. You presumably don't want the trailer battery to be draining the start battery without the engine running so you either need the trailer charging supply to be connected down stream of the ignition switch or use the VSR* I suggested. Your start battery needs to be sacrosanct, whatever the state of the trailer battery. A suitable bulb in series should save your wires but it could mean your trailer charging is 'underperforming'.
    This problem comes up frequently with Boat Electrics and there are a number of (highly overpriced) charge controls systems which will keep both batteries tip top but I guess your system is not so life-and-death as marine systems can be.
    I also had a guy delivering logs and his tipping motor battery was always letting him down because of the low mileage between calls. A more intelligent charger would have sorted him out (plus a decent deep cycle battery of course).
    * A VSR is not complicated to wire - just three terminals; one to ground and the other two to the two batteries. It consumes just a very few mA when idle.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: 12V charging circuit between two batteries limiting max amps
  1. Amp limit of a Battery (Replies: 2)