Have seen similar questions but not found a definitive answer, so here goes. Split charge and leisure battery fitted in my Volkswagen T5, and I want to run a USB charger, cigarette lighter charger and LED light set all from the fuse box connected under the drivers seat (with the battery). My question is whether it would be safe to ground all the accessories to the earth bolt under the drivers seat, where the leisure battery is grounded.
Electrical – Grounding accessories in an automotive (~12V) system
12vautomotivebatteries
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This is more of a comment than an answer because your requirements are made fuzzy by your illogical (don't take it to heart) circuit:
Are you looking for this type of basic configuration of your batteries: -
If so, then add the relay contact either in series with the 12V battery or in series with Vout. I'm assuming that your arduino will always take power from one of the supplies. I guess you'll need some kind of charge circuit for the 12V to keep it topped up too. This can be done with a schottky diode in series with a resistor from 13.8V directly to 12V BUT it depends on your battery type and I'm no expert on battery chargers.
a. grounded negative chassis is a convention, it can be a grounded positive instead, but every connection has to be reversed. ground is a concept, a point of reference, but not a physics reality.
b. the current passes through the battery positive terminal and the fuse, and so it doesn't matter what the return path is; a negative cable or chassis. the fuse will still blow on fault. the fuse has to be part of the circuit for it to blow.
c. there is no alternative route, the current must pass through the fuse. it may also pass through other things, but this is not important; the fuse will still operate if the current and time allow it to.
d. the alternator has a cable connected to the battery positive, and either a cable connected to battery negative or the chassis is used as the path. it works because the alternator generates a voltage higher than the battery, and the voltage carries power into the battery. during charge, the battery is a load with respect to the alternator.
Best Answer
In most automobiles the metal chassis is the common ground (earth). All accessories should be connected to it for return current, even if there are multiple power sources. A typical car already has an alternator and battery, two power sources both connected to ground.
simulate this circuit – Schematic created using CircuitLab
It may be easier to add new ground connections to the frame rather than wire them back to an existing ground. It is possible that the ground wire under the seat is not designed to handle the extra return current you would be putting through it, which would cause problems for all devices running on it. That said, drilling into the frame is not always safe either. (Where are your fuel lines and brake lines?)
Safety is about ensuring that the positive terminal of your leisure battery is never connected to the main battery. The leisure battery should have a main fuse and per accessories fuses on the positive side.
If your vehicle goes for service, remove the leisure battery. Put a permanent label on the main battery that the car has a second battery, as a favor to anyone who has to work on it an emergency.
I would not attempt this modification on a hybrid vehicle, electric vehicle, or newer vehicles. Their electrical systems may differ.
(A few old collector vehicles have the positive terminal connected to the chassis, in which case this advice does not apply. Although if you happen to have such a vehicle, you're probably aware of this fact.)