So if someone have a let say 400V pack, a C-A and a DC-DC to supply it with the 12V out from the dc-dc, and connect it with resistor divider to that Aux input with the 400V battery , that mean that the 12V ground and the 400V ground will be linked ?
so the dc-dc will lost his isolation advantage.
which way would you suggest to use this Aux V input and keeping the HV and 12V isolated ?
maybe using a little DC-DC just for supplying the C-A board?
Joe Perez wrote:What is the problem (or potential problem) that you see with having the low-voltage and high-voltage grounds linked together?
I think it's not acceptable usually for safety.
First, to be clear, your link is referring to automobiles with a live chassis ground. In the case of an ebike, there is no good reason to use a grounded frame, so the 12v lighting etc is (should be) already isolated from the frame and the dangers of joining the motor and battery grounds are substantially reduced. However, sidestepping a discussion of the 'why' in favor of your wish to keep the vehicle Vbatt isolated from the 12v utility power:
We assume that the Vaux and throttle are being driven with the on-board CA +5v supply (as intended) and accept that various CA inputs will share a ground with the high voltage vehicle Vbatt. This is inescapable by the CA design and without a -Vbatt tie to chassis ground, the dangers appear minimal or at least manageable.
Justin indicates that the internal CA v3 electronics draw about 10ma. Assuming a resistive throttle of about 5K and a resistive Vaux divider of about the same resistance, then we are looking at 5v/5K + 5v/5K = 2ma for a total of 12ma. For a 12v CA power source, the on-board regulator will need to dissipate (12v-5v)*12ma = 84mw, well below the device limit of 1500mw.
The easiest solution is simply to tap the vehicle battery to drive the CA with around 12v. The low 12ma draw is not going to materially unbalance the affected cells and this would leave the existing 12v supply unused and unaffected.
Otherwise, your suggestion to provide a second voltage supply dedicated to the CA seems a good approach. To avoid the large power dissipation to drop Vbatt (400v in your example) to the allowable CA supply voltage, and to take advantage of the very low current requirement (around 12ma above), a good solution might be to use an isolated DC/DC converter component module to provide 12v from the existing available 12v supply. These are available for $10 or less on eBay ('Isolated DC Converter 12V'
). The 12v module output will then power the CA and share the vehicle Vbatt ground while the primary utility 12v supply will remain isolated.EDIT - Authoritative answer from Justin can be found here.