Dual Battery System Questions

Within my project, I have a +36V and +12V battery, I have the following questions:

1. Is it the best decision to keep the grounds separated?

2. Is the chassis/frame still the most viable central ground point if it's made of carbon fibre (there's no engine)?

3. If not, which battery is most suitable to ground to? The +12V or +36V?

4. The +36V system has a maximum current of 300A, what wire size would be suitable for this? I can't find relevant AWG wire size.

1/ You can use a common ground, 0V point, negative termination, or whatever you want to call it - this is the normal practice.

Remember, each side is a loop, and provided the rest of them is kept isolated/there is a SINGLE common point between them, there will be no issues. It's most commonly used for applications where both 12V and 24V systems are used on the same vehicle/vessel, such as campers on truck chassis, and diesel power boats, where the engines run on 24 V and the interior runs on 12V.

2/ Carbon fibre is [edit] resistive, and has poor conductive properties, so you shouldn't use it as a ground/earth, but use return/ground wiring for all the circuits - a common ground point and return "may" work well, i ground loops are avoided [/edit].

3/ You can use a common ground for both voltages (technically any point so long as there is just ONE common point), but if there are electronics involved, be aware of ground loops being potential issues.

4/ It will also depend on the length of the wiring runs - INCLUDING the ground return, and the acceptable voltage drop. As a general rule, for a motor you want to minimise that by going a size or two larger - the power loss is approximately proportional to the square of the voltage drop, as a percentage. IE, 10% total voltage drop is ~ a 19% power loss. Off hand, I don't have a specific recommendation, but rather than look at the automotive options, I'd suggest contacting companies working with industrial level electrical power equipment - you're probably going to be looking at 000 or 0000 (3/0 or 4/0), and they'll also be able to supply and crimp the terminals you'll need. Other options for sourcing the cable and fittings could be solar power companies, chandlers, or companies working with electric vehicles.

A 36V system is quite unusual, may I ask what it's for?

As a guy who has dropped plenty of hot wires accidentally onto carbon fiber bodywork / tubs, I can tell you carbon fiber is certainly conductive. However, I would not use that as a designed ground path -- only using metal bulkheads embedded within a carbon tub. However, I have not tried to measure the resistance of a carbon tub -- might be an interesting data point.

Please tell us more about the carbon tubbed vehicle with a 36 volt power system.

I mis-stated what I meant, thank you David, and edited it.

It depends on the specific weave, resin, etc, but it's generally a very poor conductor.

+1 for tell us more about carbon tubbed vehicle with 36 volt system