Can I share my battery ground wire with a DC DC converter?

[Offroader]

I have a long wire run for my 12 volt DC DC converter. I was wondering if I can use my battery main ground wire for the ground on my 12 volt DC/DC converter?

I would use separate positive wire obviously. This would eliminate me having to run an extra wire, as the portion of the wire is for my charge port and doesn't get used anyway.

Would this be OK?

 

[flangefrog]

It depends on the DC-DC converter. Try measuring the voltage between the positive output and negative input. If it's 12V then it should be fine.

 

[teklektik]

I was wondering if I can use my battery main ground wire for the ground on my 12 volt DC/DC converter?

Yes.
There are three common forms for DC/DC converters:

1. 4 terminal, isolated: The input and output connections have no internal connection to one another. The is no measurable voltage between the input and output connections. This is more expensive to build and not common in automotive-style '12'V' (typically 13.8V) converters
2. 3 terminal, not isolated: The input and output connections share a common ground. This is less expensive to build and is a common form for 12V converters.
3. 4 terminal, not isolated: This is actually just a 3-terminal converter like (2) but with and extra tap provided to the same Gnd. Unfortunately, many folks look at the four wires and assume they are getting the fancier isolated type.

In the case of (2) or (3), the Gnd is already shared - the difference is simply an extra common Gnd wire.

Case (1) can be downgraded to type (3) by tying the input and output negative connections together. This is harmless to the device and simply defeats the isolation that was the expensive part of the design.

So - you can make voltage measurements if you wish, but if you have a 4 terminal device, it must be type (1) or (3) and in either case just wiring the in/out (-) connections together will work without digging into things any further.

Beyond the converter itself, beware of sharing Gnds for electronics and high current devices like motors, headlamps, etc. This generally works okay but noise and Gnd voltage offsets can make some electronics flaky or yield slightly inaccurate measurements with electric sensors (e.g. temperature) where small analog voltages are in play through Gnd. There are different schools of thought on this, but I prefer to run an extra small gauge Gnd for electronics. Anyhow - harmless to try without it and add it if the results are unsatisfactory.

 

[Offroader]

Thanks for the responses, exactly what I was looking for.

 

[Offroader]

Yes it does seem my power supplies, actually two of them, are both non-isolated.

Both my 12 volt and 8 volt power supplies (8 volt powered by 12 volt) all share the same ground. there is continuity between the negative input on my 12 volt and the output on my 8 volt.

Both power supplies share the same ground as my 82 volt battery.

I assume then I can use my 82 volt battery ground wire to connect both of my power supplies 12 volt and 8 volt.

My 8 volt power supply will power LED lights (2-4 amps) and also an 2s (about 8 amps) electronic speed controller for an RC fan placed inside the motor for cooling. Can I assume that noise shouldn't be an issue here?

 

[Eteck]

I assume then I can use my 82 volt battery ground wire to connect both of my power supplies 12 volt and 8 volt.

Maybe.

Can I assume that noise shouldn't be an issue here?

No. You'll have to try it and see if there are any problems. If so then try star grounding everything at one point near the battery.
It's not just noise but the actual currents will tend to float the ground and could conceivably cause a problem somewere.

Another hazard is things accidentally get together in a network containing two non-isolated power supplies, so make sure you fuse everything properly.

You are really not giving enough information for a definitive answer.
If you have actual problems you could eventually post a schematic and people will be able to figure it out.

 

[TheBMallory]

I often run into the same question. Am I right to assume that, using a multimeter, if I see little to no resistance between input ground and output ground it means it's shared?

Envoyé de mon XT1092 en utilisant Tapatalk

 

[Eteck]

if I see little to no resistance between input ground and output ground it means it's shared?

Not sure what you mean exactly.
With a DC converter you will usually know whether the design is isolated or not. If it's installed in a system
then that measurement tells you the output is tied to the common ground somehow, but little else.
If it's just the bare supply then yes that would indicate a non-isolated converter, unless there is like a
user jumper strap somewhere on or in the unit connecting the input and output grounds.

 

[teklektik]

If you have actual problems you could eventually post a schematic and people will be able to figure it out.

No -- that is, in fact, the exact problem.

• 
  The question is not whether two points are electrically connected, it's how they are connected.

What is necessary is an exact wiring diagram, not a schematic.

Again, if it was me, I'd run something like a #18 Gnd for the fan and headlight and a #22 Gnd for CA or anything similar connected to the controller. This is an easy 'best practices' approach that pretty much eliminates the need to analyze all the possible electrical ramifications of device interactions when sharing a single Gnd. For a different configuration, things might change. For instance, if you have a front mounted controller for FWD, then just hook the fan and headlamp up to that controller main Gnd.

If you are concerned about busy-looking exposed multi-wire runs, just invest in some braided plastic sleeving and make up your own custom cable run. It's like a Chinese finger puzzle, so buy it a bit small so it hugs the wiring tightly.

Am I right to assume that, using a multimeter, if I see little to no resistance between input ground and output ground it means it's shared?

Ya. That works fine on the bench with nothing connected.

 

[Offroader]

Finally hooked up everything, two power supplies, 12 volt and 8 volt,

small electronic speed controller for an RC hobby EDF (electronic ducted fan).

Everything works fine except one small problem.

To cool my hubmotor I have an EDF fan which is used on remote control airplanes. They spin at around 50K RPM and blow a lot of air. My hub motor is setup to suck in fresh air and blow the hot air out.

You can see how it works in the video.

Problem is when the fan is running and cooling like shown in the video, if I give any ebike throttle the EDF fan just stops. It resumes as soon as I let off the throttle. Even just a slight movement, couple of amps, of the throttle will shut the EDF fan off.

Any idea why is it doing that? Voltage is definitely not affected because I hooked up two LED lights on the front of my bike and they work fine and don't even dim when giving full ebike throttle.

Would it be the signal wire going to the ESC that is being affected? The servo tester that controls the EDF (What I am turning in the video) is powered by the 5 volt regulator in the ESC.

This used to work fine before on my old build, with all same components.

I posted a picture of the ESC I am using that powers the EDF fan and is powered from the 8 volt power supply.

[youtube]72EbskPCNcY[/youtube]

The two power supplies mounted under the seat.

 

[Offroader]

Just to update I fixed the EDF cutting out problem.

I swapped out the ESC that was giving me issues with a different higher quality one and it works fine even with throttle.

Not sure why giving throttle, and even regen would cause the ESC to cut off. The ESC would keep resetting itself when I gave my bike throttle.

 

[TheBMallory]

If the incriminating esc is indeed a simonk, it is supposed to be fed a 400hz pwm signal (it's designed to be used on drones). I'm not sure if your potentiometer thing was giving that.

Perhaps it worked fine when the motor was off "by chance", and perhaps there was too much electrical and electromagnetical noise with the motor on (especially if you're running long cables) .
Anyway, glad it worked out

Envoyé de mon XT1092 en utilisant Tapatalk

 

[Offroader]

Actually it was the signal wire after all. I had to get rid of my long extension to the front of the bike.

 

[thankyoudude]

Case (1) can be downgraded to type (3) by tying the input and output negative connections together. This is harmless to the device and simply defeats the isolation that was the expensive part of the design.

You have no idea how much time you just saved me for posting this, thank you so much!
I had a Renogy DC-DC (ISOLATED) convertor separating my front and rear 12v batteries in my car. The Renogy turned itself on properly with the front battery's remote-turn-on-wire, but all the other rear battery components wouldn't turn on using the remote-turn-on wire! I measured the voltages of the remote-turn-on wire between the front and the rear grounds, one was 12v, the other was 0.5v. This is because the rear battery was isolated from the front, and a circuit couldn't be completed between the turn-on-wire and the negative of the rear battery (which the rear components were tapped into).

Finally, I connected 1 wire from both battery grounds, and yay! it's fixed! No longer are my two batteries isolated at the ground. Now the remote-turn-on-wire works!

 

[E-HP]

You have no idea how much time you just saved me for posting this, thank you so much!

Sadly he’s not with us anymore, but his contributions continue to provide great benefits. I learned a lot from his posts even before joining ES, and refer to the unofficial Cycle Analyst user guide all the time; so I agree, thank you teklektik.

https://neworldshop.com/wp-content/uploads/2016/09/CaV3UserGuide.pdf