2010 Silverado 1500

My Silverado has an empty spot under the hood for a second battery. I’m wondering if I could install a second battery and use it to power a winch that is mounted to the headache rack in the box of the truck. The winch is used to pull an aluminum boat and trailer up off-road boat launches.

As a novice, I’m wondering if the ideal battery wiring might work like this:

  • The battery would get automatically charged when the truck engine is running, ensuring the battery is always charged before winch use.
  • The truck could be running or turned off when using the winch. Only the second battery would get used by the winch. The primary battery would never get drawn down by the winch, avoiding a possible dead primary battery scenario where the truck wouldn’t start.

Is that kind of wiring setup possible? If not, what is normally done?

3 Answers 3


The method suggested by Paulster2 will certainly work.

Another way to do it at lower cost is to use a 12 volt, 120 amp continuous duty relay.

  • Make a permanent connection with heavy cable from the second battery (+) to the winch. A fuse at the second battery (+) terminal sufficient to operate the winch would be prudent. Ground the winch (-) terminal.
  • Connect the second battery (+) to the original battery (+) (or the alternator +) through the two heavy terminals of the relay so that the second battery is only connected to the truck's electrical system when the relay is ON. Use the same gauge of wire as your alternator's (+) terminal wire. Second battery (-) goes to ground using heavy cable like the one that goes to the winch.
  • Connect the coil of the relay (the small terminals) to ground and any fused circuit in the truck that is only ON when the ignition is on, but not when the starter is engaged. Thin wire is fine. For example, use the radio circuit, cigarette lighter circuit or any other accessory circuit. The relay draws only 1.8 watts, so it won't overload whatever circuit you choose.

The winch will have power at all times but when the ignition is OFF, the second battery will be isolated from the original battery so the winch can't drain it. When the engine is running, the second battery will be charged by the alternator. When the starter is engaged, the second battery will be isolated, so you don't need to worry that heavy starting current will destroy the relay or overheat the second battery's charging wire.

No need for a cutoff switch -- the second battery is isolated whenever the engine is off and it recharges any time the engine is running.

  • 2
    A possible improvement is to use a second low-power relay (or optoisolator). It turns off the power to the main relay, when the alternator's stall light is illuminated. That way when the engine isn't running, but the accessory circuit is on, the two batteries aren't connected. It also stops the main relay from briefly closing as you turn the ignition key from OFF, through ACCESSORY and RUN to START. It also has the advantage that the main battery won't be discharged into the winch battery if the accessories are on but the engine isn't running
    – CSM
    Commented Jan 1 at 13:43

I think the easiest way to accomplish this is the get a 12vdc to 12vdc charger to use to charge the battery, then keep the wiring to the winch as direct wiring without connecting into the truck's battery wiring. You can get the charger so it will only charge the separate battery when the vehicle is running. I think this would fill your need.

EDIT: You'll probably want to get a cutoff switch to interrupt the power when the winch is not in use. If you don't, it might lose its charge over time. Also, consider using a deep cycle battery which would fit in the hole. They can stand up to a lot more usage than a standard car battery.

  • Thanks! Any suggestions about where to connect the charger? Photo of engine: i.sstatic.net/MGGrZ.jpg
    – User1974
    Commented Dec 31, 2023 at 6:37
  • Just co-locate the charger with the new battery. Not sure exactly what you're asking. Commented Dec 31, 2023 at 12:22
  • Sorry, I meant, what wires should I connect the charger to? Such as the running lights.
    – User1974
    Commented Dec 31, 2023 at 14:11
  • If whatever you get has an "on" switch (meaning a wired connection to tell it when to turn on), connect that on switch to a circuit which is keyed on. If it is just power on starts the charging, then put a 40A relay inline from the battery, then use a keyed on source to energize the relay when the vehicle is on. Commented Dec 31, 2023 at 15:09

Keep it simple and stupid.

Put two maximum size that fits and maximum grade your bugget allows batteries in the two battery bays, wire them in parallel and wire the winch as per the winch manual.

Optionally: install a charge-counting battery monitor in order to avoid depleting both batteries, should your use case hints at such an outcome. If the boat winch is the only additional electricity consumer, chances are you don't really need this additional step, but it is both inexpensive and useful in a lot of other scenarios.


  • Simple wiring, few places for something to go wrong. Things in car electricity DO go wrong in a lot of unexpected ways, be warned.
  • No possibility of overloading the cross-battery relay/charger, suggested in the other answers. Depending on the winch and/or the alternator, this is pretty much a possible failure mode and a fire hazard as well.
  • No voltage surges at the cross-battery relay close/open transitions.
  • Twice the energy available for whatever electrical devices you would like to power, besides the winch (inverter, lights, etc...)
  • The discharge current and the depth of discharge is shared by both batteries, improving (pretty much non-linearily) their lifespan. A battery discharged to only 40% depth lives as much as 10x longer than a battery beaten down to 80% discharge.
  • Spare battery capacity for the unfortunate moments when the alternator fails far away from repair oportunities. 100-300km of additional range with broken alternator (or more if you know what you are doing).
  • I suppose I could just keep the truck running when using the winch, avoiding any truck starting issues.
    – User1974
    Commented Jan 1 at 16:51
  • @User1974 For a boat winch with less than 100A current draw and using it only twice per stop (you arrive, you winch the boat to the water, you sail, you winch the boat back and you drive away) you need neither a constantly-running engine nor an additional battery. You will hardly draw 2-5% of the battery capacity every time and the battery tops up on your way back. ...
    – fraxinus
    Commented Jan 1 at 17:49
  • On the other hand, with a heavy-duty 18000lbs 400A electricity hog and a long, long muddy uphill to pull yourself up by winch, you may end up depleting the battery to the point where a running engine stops because the alternator fails to keep up with the current demand and the voltage falls below whatever is needed to have sparks in the cylinders. Guess where I know from.
    – fraxinus
    Commented Jan 1 at 17:52

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