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01-09-2018, 14:42   #1
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12v Diode Question

Hi all, not sure if this it the right place to ask..

I have a trailer with a battery and small 12v water pump mounted in it.
I have tied in to the lighting circuit to supply power to the battery when driving (lights on). This should maintain the battery and give me the option to run the pump off the car if the battery is flat. (engine running, lights on)

Obvious problem is that the lights on the trailer will be on all the time, using the power from the onboard battery unless I disconnect the terminal.

Would adding an inline diode to the positive supply from the trailer to the battery solve this?

If so what size diode do I need?

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02-09-2018, 21:38   #2
Capt'n Midnight
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Is the trailer being powered from the car ?

What wattage is the pump ?
Startup current may be more.

1N400X series can take an Amp
after that its usually 3 or 5 Amps, but they are cheap so no harm in getting one with a higher rating than you need
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03-09-2018, 18:38   #3
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Trailer is powered from the car.

Not sure of the wattage, but it is 15 amp max.
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20-05-2019, 23:02   #4
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The wattage of the pump is relevant, but the big issue is the charging current. If the battery in the trailer is low, then there will be a lot of charge current.

Edit: If suitable a small trailer mounted solar panel connected to the trailer battery will give a constant trickle charge, and a simple change over switch to allow the pump to work from either car or trailer battery.

If you are going to pull more power from the lighting circuit you need to look at fuse ratings.

As for charging the battery via a diode I think the voltage drop of a diode will be a big issue there, I am assuming the trailer battery is lead acid ? If so to charge via a diode you would need a solid 14.4 volts.

The solar seems the best idea if the trailer is parked outside.

Last edited by SlowBlowin; 20-05-2019 at 23:15.
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20-05-2019, 23:12   #5
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Upgrade to a 13 pin trailer socket

( gives permament 12v and 12v on/off with ignition switch )

then put a split charge relay on it so it only charges when the engine is running :

and something to limit the charging current

Last edited by gctest50; 20-05-2019 at 23:19.
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