I installed a second battery in my boat to run a trolling moter. I want to run a wire back to the fuse panel so the battery will stay charged when the gas engine is running but I don’t want it to pull this battery down when the gas engine is not running. I was needing to know what size diode I needed to put inline.

Thanks, Shanestrong text

  • How much current, in Amps, do you need the diode to pass?
    – jwh20
    Commented Aug 8, 2022 at 17:17
  • @jwh20 That is what the question asks.
    – Chenmunka
    Commented Aug 8, 2022 at 17:21
  • Yes, I know that. But as asked we have no idea what the load is.
    – jwh20
    Commented Aug 8, 2022 at 17:23
  • 1
    To be clear - you want an automated solution that isolates the second/aux battery from the first, so that the first is always fully charged and the second battery is only used for the trolling motor. When the liquid-fuel engine is running and turning its alternator, you want both batteries to charge. If so - you want a "dual battery cutoff" as used in 4WD vehicles.
    – Criggie
    Commented Aug 18, 2022 at 1:40

1 Answer 1


Diodes are a very poor solution to that problem.

A lead-acid battery is usually float charged at a voltage of about 13.7 volts, although rapid charging could use voltages of as high as 14.5 volts.

If your boat uses the float charge voltage of 13.7 volts for the main battery, the second battery connected via a diode would see this voltage minus voltage drop of the diode. The voltage drop varies with current but is usually around 0.7 volts. Thus, the voltage at the second battery would be about 13 volts.

The second battery therefore charges much slower because of the low voltage at that battery. This persistent low charging could cause an early death at the second battery due to sulfation caused by under-charging. Also the diode would waste energy and heat up very rapidly, which could be a problem.

A better solution would be a switch such as a relay or MOSFET controlled by a control circuit sensing voltage. The switch would disallow drawing down the second battery, but whenever it sees a charging voltage at the first battery, would connect again the two batteries.

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