How to prevent cross charging when a spotlight with LiPo is connected to 12V source?

[richardcpf]

I am planing to build a 50W HID spotlight based on an old spotlight host and I want a 4S lipo battery in it and make it able to run directly from a 12v source, the point is to prevent the battery pack to make contact with the 12V source when it is connected to a car.

Which is the simplest way to do that?

*This might sound very dumb, but I'm no engineer and my knowledge in electronic circuity is very limited.*

 

[FILIPPO]

Maybe you can use 2 switches.. One for the light (on/off) and the other for the power source (batt/12V plug).
probably the first one is already in the spotlight you are using.. So you just need to add a new one..

 

[richardcpf]

Yes I have thought of a 6 pin, two position switch, but the only problem it is manual, I was looking for an automatic device like an reversed relay, that cuts contact on two terminals when a source is detected. I guess I'll stay with the manual switch

http://upload.wikimedia.org/wikipedia/commons/thumb/e/ec/DPDT-symbol.svg/330px-DPDT-symbol.svg.png

 

[Slotguy]

How about a 12V relay and a Diode? The Relay wired so the coil is energized when you plug it into the 12V source and the Diode to prevent backfeed into the coil from the Lipo.

 

[MikeAusC]

A switching socket which disconnects the battery when you plug in external power ?

 

[Everett]

The simple way to prevent the battery pack and DC input from interacting is to put a diode (Schottky for low Vf penalty) in series with each source, such that the anode of each diode connects to a source and the cathodes are connected together at the load. The problem with this setup in your case is that this arrangement simply chooses the higher voltage of the two sources, but a 4-cell lipo pack can be higher than 12v, so power will still come from the batteries until they drain to less than 12v and then it will switch over to the dc input. A relay could be used to select the DC input by connecting the DC input to the relay coil, so the relay is on when DC is connected and off otherwise. Then you can use one of the NO relay contacts to connect DC in to the load and a NC contact to disconnect the battery when the relay is energized.