Electrical – grounding a moving robot


I'm am designing an external battery pack for a robot, Pepper, a robot by Softbank (I'm am not making the battery itself but the circuit connecting two batteries). I am going to connect a 25V (LiFe-PO4) battery to Pepper's internal (Li-ion) battery and strap it to its back. Due to the delicacy of (Li-ion) batteries I included a voltage regulator. But I also want a ground in this circuit, in case of a short circuit or anything.

The problem is: Pepper is a moving robot, so a real ground isn't possible. I was wondering whether it is possible to use its (small) metal chassis as an earth. Similarly like they do in cars.
(Just wiring the – to the chassis)
Pepper is a robot which is about 1,4m high, and only has a metal chassis in it's base, which is about 40cm broad and 20 cm high

If this is not advisable, is there another way of making the circuit more safe? (I am already going to include a fuse.)

External battery specs:

-Capacity 10Ah

-Output voltage25,6V

-Maximum pulse discharge 55A

-Maximum continuous discharge 15A

Internal battery specs:

-Capacity  30Ah

-Max charge current 8A

-Max charge voltage 29,4V

Any other tips about my project are more then welcome

Best Answer

With a moving object you can not normally have a true ground. There is one exception where the wheels are conductive and the device is moving over a metal floor or rails, but I'm going to ignore that here.

With an isolated system providing safety to users involves providing sufficient isolation to ensure that both the "positive" and "negative" side of the power source are never presented to the user. If the thing were in a metal enclosure, connecting that enclosure to either the positive or the negative side of the battery would be acceptable provided nothing exposed of the opposite polarity pokes through that enclosure.

Either way, and with a partly non conductive chassis, you need to ensure that any metal parts the user may come into contact with are either double insulated or are connected to the same terminal as the metal parts of the chassis.

The voltages in question here are non-lethal, however, you also have to be concerned about a hard short caused by some inadvertent metallic connection between exposed "terminals." Such a connection will result in a mighty spark, even with a fused power supply.

If this device is intended for out-door use, you should also consider what measures you need to take to prevent water from reaching exposed and connected metal parts.

One other thing you need to consider is static build-up on the robot. With an isolated system a significant charge can build up, kilovolts, between the chassis and the floor it stands on. That charge, will give anyone that touches it a shock, and that discharge may interfere with the operation and reliability of the robot. As such, use of a high impedance discharge belt that hangs under the robot and drags on the ground may be prudent. However, that aspect is and entirely different issue and not related to the battery isolation. Since it is intermittent at best, it should never be relied upon as a safety ground.