What Gauge Ground Wire

[Milwauken]

I need to replace the ground cable (about 15") on my MiEV's 12 v battery. Is 14 gauge sufficient?

 

[JoeS]

Milwauken, I moved your post over to here as it was off-topic in the other thread.

#14AWG is too small. Edit: WAY too small!

I just looked at the main battery ground cable and it looks to be either #8AWG or #6AWG - can't quite tell because of the thickness of the insulation, and I didn't take off the battery cover to measure the wire thickness. Thicker (#6AWG) is better in this application.

 

[WReed82]

Hi Milwauken,

At 12 volts, with an 80 amp / 960 watt load, 15" wire length, 8 gauge wire, voltage drop is 1.3% or .16 volt.
Changing to 10 gauge wire, V-drop is 2.07% or .25V.

Does anybody know what the actual maximum amps load is on this wire?

Thanks and good health, Weogo

 

[JoeS]

... At 12 volts, with an 80 amp / 960 watt load, 15" wire length, 8 gauge wire, voltage drop is 1.3% or .16 volt.
Changing to 10 gauge wire, V-drop is 2.07% or .25V.

Does anybody know what the actual maximum amps load is on this wire?...

Because of the distributed-ground configuration of our i-MiEV, in normal operation with the dc-dc supplying all the 12v power to the vehicle, I would expect very little current through this battery ground wire once the battery is charged.

A realistic high-current scenario is a badly-depleted 12v battery which the dc-dc attempts to charge, especially right after the battery was 'jumped' in order to get the car started. If all the battery cells are ok (not shorted), this very high initial current would probably quickly decrease to below 30A.

Edit: Another scenario is if the 12v was being used to drive an inverter (attached directly to the 12v battery) to power some house appliances during a power outage: the car could be turned on to READY to keep the 12v battery charged and thus provide the current to the inverter, that current going right through the ground cable.

An even worse scenario is a shorted cell in the 12v battery which then caused someone to 'jump start' the car but which subsequently resulted in a high continuous current being stuffed into that battery by the dc-dc. Best not to speculate at what awful things could then take place.

Even though I've read on this forum that 80A as the maximum output of the dc-dc, I haven't been able to confirm that (specification anyone?). I would expect the dc-dc to protect itself by limiting its maximum output current.

So, today I took the cover off the 12v battery and attempted to measure the ground wire diameter using a digital caliper. Averaging out the readings gave me 0.244inch diameter or slightly thicker than a #4AWG or in metric around 25mm^2

Woego, I came up with some different voltage drop numbers using uncoated stranded copper wire resistivity. My numbers show a lower voltage drop than yours (e.g., 15" #8AWG at 80A gave 0.0778v). I don't think a #8AWG would sustain that for too long if that 80A was continuous.

In conclusion, I would stick with the OEM ~#4AWG larger diameter wire in order to maintain the battery negative terminal and chassis at the same potential. In this application, good mechanical integrity is also important and the #4AWG provides that.

 

[Don]

I agree. The 12 volt system in these cars is already responsible for *most* electrical problems - I certainly wouldn't do anything to give it even more help. It's easy to find a factory made, standard negative battery cable at most auto parts stores. That's what I'd use as a replacement