Originally Posted by alexwhittemore
It's probably better to use the chassis ground than wire directly to the battery. BMW uses lots of wires coming off the positive terminal for various purposes and power save modes. The biggest is for the starter/alternator, then there are 2-3 smaller fused and switched lines which will turn on or off for various power modes (on, sleep, standby, ignite, etc etc).
On the other hand, there's only ONE coming off the negative terminal which then connects to the chassis. That run has a current sensor in it to monitor battery state of charge. If you have a ground connection that bypasses that, you could very easily end up with a situation where the car gets confused about whether the battery is full or empty, and doesn't charge it appropriately.
Of course, for HUGE setups, you might want to run another ground cable for the sake of current handling, but then that ground lug handles 1kA during starting anyway, and it's not like you're putting in a more powerful alternator. So again, probably best if you don't.
Additionally, it's generally better to avoid adding ground lines anywhere you can to avoid creating ground loops. Create too many paths for ground current and you can couple unforeseen noise in. Same way you can get weird effects if you have your phone plugged into aux and a crappy charger at the same time in some cars.
This is great advice, maybe why I am getting an SES code 4A66 on my electrical management system. I am moving my ground.
Now, I am looking to improve my power connection which is straight to the nut holding the battery clamp on the positive terminal. This is pretty crude but effective. However I would like to use the MAK8 plugs which are part of the big red terminal adapter pictured below. But I have only two gray sockets left available on the right side and they have funny colored dots on top of them. If I split my power cable into two MAK8 connectors and connect to these available ports, could there be any problems? Does that big red BMW terminal adapter have fuses in it?