Wiring 3-wire DC/DC converter

[cboy]

I have a 72 volt battery pack to run my hub motors and am using a dc/dc converter to operate my 12 volt electrical accessories. I'm intending to wire the converter outputs to a 12v battery and then run wires from the 12v battery to the accessories. As I was putting things together I discovered my dc/dc converter has three wires but most of the wiring diagrams I've seen show four wires (72 volts in / 12 volts out / a ground to the 72 volt battery and a ground to the 12 volt battery) So my question is, can the converter ground wire be grounded in common with both the 12v system and the 72 volt system? Here's a quick sketch of how I think it could be wired (sorry, I don't have any fancy electrical software to make this pretty)

As shown in the sketch, the converter ground wire is attached to both ground side of the 72v battery pack and the ground side of the 12v battery pack. Is this the correct way to do this?

 

[billvon]

For non-isolated converters, that's correct.

 

[cboy]

For non-isolated converters, that's correct.

Wow, that was fast. Thanks a bunch billvon.

 

[amberwolf]

Yep, that's the way to wire it.


Just make sure that all of your battery positive wiring is well-insulated from anything that the battery negative / ground wiring / 12v negative wiring could possibly short to.

If your frame is ground for the 12v system due to common lighting mounts, then any battery positive wiring that shorts to teh frame shorts the entire pack out.

That includes any motor phase wires that happen to short to the axle or stator due to damaged insulation from whatever cause. (though this shorts the pack out thru the controller FETs).

 

[John in CR]

What's the 12v battery for? The point in using a DC/DC converter is to give you a 12V supply, so you don't need to carry a 12V battery.

 

[cboy]

What's the 12v battery for? The point in using a DC/DC converter is to give you a 12V supply, so you don't need to carry a 12V battery.

The battery is primarily a backup in the event of DC/DC converter failure. This system is in a three wheeler which is licensed in CA as a motorcycle so the headlight must be on at all times, day or night, and the rest of the lighting system must be functional at all times as well. So in the event of a converter failure, the battery will get me home safe and legal.

 

[John in CR]

What's the 12v battery for? The point in using a DC/DC converter is to give you a 12V supply, so you don't need to carry a 12V battery.

The battery is primarily a backup in the event of DC/DC converter failure. This system is in a three wheeler which is licensed in CA as a motorcycle so the headlight must be on at all times, day or night, and the rest of the lighting system must be functional at all times as well. So in the event of a converter failure, the battery will get me home safe and legal.

The battery is more likely to fail and result in the failure of both. Plus you wouldn't know the DC/DC failed, so the 12V would die shortly afterward since 12V is a pretty low status of charge.

Just don't run near the output current limit and they're quite durable. I run at night almost daily and at highway speeds as well as curvy roads in the mountains, so a light failure could be catastrophic, and there's no foreseeable event that would make be consider running both a DC/DC converter and a 12V battery.