# Grounding in a DC circuit on a vehicle

I am working on a vehicle with batteries, but i am not quite sure if my grounding solution/design is correct. It is supplied with two 12VDC batteries in series, connected to a DC-DC (24VDC-24VDC) powersupply. Basically i always ground the 0V to the chassis, and use a 2-pole MCB after the batteries and 1-pole to devices. Is this a bad design?

• Are the two 12V batteries used for anything else i.e. do they need to be grounded? Does any circuit need to be grounded? If so, why? Playing devils advocate!! – Andy aka Jun 29 '13 at 21:42
• @Andyaka, i see i was fairly too quick with the schematic. There is actually devices connected after the F1 MCB, which is protected with 1-pole MCB's, besides T1. – JavaCake Jun 29 '13 at 21:43
• Do these need to be grounded and does the output of the 24V dc-dc need to be grounded? – Andy aka Jun 29 '13 at 21:45
• @Andyaka, thats my actual concern. Im not certain whether its necessary or not in my case. – JavaCake Jun 29 '13 at 21:47
• Basically don't ground stuff that doesn't need to be grounded. I suspect the motors are fed from the 12v+12v and these might inevitably be grounded and cause ground currents that are many, many amps - keep non essential stuff ungrounded would be my philosophy. – Andy aka Jun 29 '13 at 21:48

I've taken your diagram and added two circles and a few lines: -

Firstly, the motors at the bottom - I don't think you should label the two wires to each motor as 24V and 0V - they are likely to be PWM outputs from the controller that form a H bridge and, if i'm correct there will be no current to ground or real 0V. I say no-current but the chassis will pick-up all sorts of interference from the heavy current fed to the motors and avoiding the chassis is best policy other than for a single connction (if necessary).

I've also drawn a circle around the controller and plc to indicate that there will likely be some control lines between the two or else how are you going to regulate motor speed? The type of connection needs establishing i.e. is it differential signalling? Do the signals share a local 0V? Can it be opto-coupled (least likely to give problems)?

When it comes to wiring, you have shown a bus type arrangement and this is OK for a circuit but you have to feed the 24V batteries (via over-current protection) directly to the controller. You can tee-off the lighter loads (dc-to-dc converter) directly from the battery. This means you have to star-point two 24V and two 0V conections at your battery terminals to avoid motor current giving problems to the converter etc..

Keep the wires from the battery to the motor controller as short as possible is a must.

You also, on your circuit have two over-current devices in the path from your battery to motor controller - I would avoid this - use F3 directly for the motor controller and I believe F4 is kind of redundant because it is protected by F2 in front of the dc-to-dc convertor. F1 I think would be redundant but if you have other circuits (not shown) then use an adequately rated device for those circuits.

• 1) As you mention there will ofcourse be a H-bridge, so the labels are completely wrong. 2) Communication to motorcontroller will be through RS232 from a PC. 3) The purpose of the F1 was to have a main-breaker and overcurrent protection to the batteries, maybe this should be changed to a normal circuitbreaker instead of over-current protection? 4) I understand that i should feed F3 directly from the battery. 5) The purpose of F4 is because the PLC requires 3A fuse, while F2 is much larger. F2 was to protect the DC-DC converter. – JavaCake Jun 30 '13 at 11:50
• @JavaCake The RS232 might give the odd glitch - if you could use an RS232 isolator this would be less troublesome when operational I believe BUT, like i say it's down to avoiding the pesky chassis currents that inevitably will be there due to induction, possibly from motors and / or motor wiring. – Andy aka Jun 30 '13 at 11:58
• the PC will be supplied from a PSU connected to the battery aswell, would this still cause a problem? – JavaCake Jun 30 '13 at 12:02
• Do you have any input on my comment concerning the circuit breakers? – JavaCake Jun 30 '13 at 12:26
• I have to mention that the motherboard of the PC and motorcontroller is attached directly to the chassis, this could cause grounding unless i use plastic washers. – JavaCake Jun 30 '13 at 13:50