Thread: trickle charging one battery from another

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What you are doing is just what we did in the marine industry because most boats have more than one battery. The problem with just hooking up a wire to the second battery is that the alternator regulator will see the first battery that gets fully charged and think it's job is done. Here is a very good write up from West Marine that will explain some options better than I could:

Charge Management

Battery Isolators used to be the best method of distributing charging current to multiple battery banks while assuring that they remain electrically isolated during discharge. These devices are electrical "one–way check valves" that allow current to flow to, but not from, the battery. Their disadvantage is that diodes cause a voltage drop that wastes charging energy, creates heat and causes batteries to be undercharged. Alternators with external voltage sensing can correct for the undercharging problem, but voltage drop and the heat generated remain a problem.

Automatic Charging Relays (ACR) are becoming a popular method for achieving the same goal as isolators, but they work on a different principle. Instead of using diodes to block current from flowing in both directions, ACRs use mechanical relays combined with a circuit that senses when a charging source is being applied to either battery. When a charge is being applied, the ACR closes; and when the circuit senses that the charge is no longer present, the ACR opens (after a short time delay to assure that the ACR doesn't open due to temporary voltage sags caused by load startups, like a refrigerator turning on).

Automatic Charging Relays have lots of other uses in addition to isolating the Start and House batteries, and Blue Sea Systems now produces two relays for two kinds of jobs:

* CL7600 Current Limiting: Great for automatically charging a remote battery for a windlass or bow thruster, for combining or isolating two banks on a smaller outboard, I/O or inboard. Connect it to your high–powered stereo to prevent the subwoofer system from draining the House bank. Handles 60 continuous amps. User–adjustable open and close voltage settings, for 12V systems. Includes Current Limiting feature.

Don

Or, you can add a simple battery selector switch like this one and simply remember to alternate between 1 and 2 as you travel to charge both up. Here is a simple wiring diagram how to hook that one up. Omit the parts about "house distribution panel, etc" and use only the part about the two batteries.