2
\$\begingroup\$

I'm currently tasked with designing a battery-powered device (i.e. it will be connected to two four-cell LiPo batteries, each with a voltage of 14.4V to 16.8V) and am having quite a hard time finding a solution for the main switch that will disconnect the two batteries from the rest of the circuit. While there are many solutions (e.g. high-side gate drivers) that can switch the main voltage of 33.6V max., almost none of them are currently available due to semiconductor shortage or can't handle the current I need (>17A) or are too big (much bigger than DPAK).

So I asked myself this very basic question: Why is it a bad idea to disconnect GND instead of VCC in a stand-alone, battery-powered application? Attached also a picture from Paint to make it clear: There is nothing between point A and B except the components drawn in the picture.

But I also asked myself the question: What do you do with a metallic chassis standing on (real) earth? My understanding is that it would be better to connect it to point B, because a) if another part of the chassis happened to touch GND, that wouldn't short the NMOS but only create EMC horror and b) even if you used a power supply with its negative terminal connected to mains earth, it wouldn't short the main switch, but only force both its SOURCE and DRAIN to the same potential via big old capacitor earth, instead of creating a low-impedance path.

So am I right or are there any other grounding / whatsoever problems that would occur when disconnecting GND instead of VCC?

Disconnecting GND instead of VCC

\$\endgroup\$
10
  • 1
    \$\begingroup\$ That depends. Is the battery not accessible elsewhere at any point? If yes, this could be a good way to save cost. Edge cases where you can charge it externally while the rest of the system is on, then you may have issues. How does the rest of your system look? \$\endgroup\$
    – winny
    Mar 18, 2022 at 11:37
  • 1
    \$\begingroup\$ Who says that is a bad idea and why it would be a bad idea? If you have an isolated system such as a flashlight with battery and LED, why would it matter which side of the battery you disconnect? \$\endgroup\$
    – Justme
    Mar 18, 2022 at 11:56
  • 2
    \$\begingroup\$ Most flashlights put the switch on the negative battery side, because it's convenient as the battery is inserted positive end first. Note if you make the battery packs yourself, you could even put the switch on the battery pack itself. It would be a nice safety feature since it would disconnect the accessible battery terminals, preventing accidental shorts. If the battery has a BMS, and that chip has an ENABLE pin, you could even use that with the BMS MOSFET acting as switch. \$\endgroup\$
    – bobflux
    Mar 18, 2022 at 12:01
  • 1
    \$\begingroup\$ Yes but you are not disconnecting GND or chassis from anything, but only battery negative terminal from GND or chassis. Same thing if you disconnect battery positive from VCC. Without further details about how it is actually built, in theory it does not matter, until you want to think about which part may short circuit and where. \$\endgroup\$
    – Justme
    Mar 18, 2022 at 12:14
  • 1
    \$\begingroup\$ The only problem is the accidental sneaky paths you didn't think of. Also, if you ever connect the oscilloscope GND to battery side GND, then try to connect it to anything on the DC-DC, you will be bypassing the FET through GND. Did you rule out high-side PMOS switching? \$\endgroup\$
    – user57037
    Mar 19, 2022 at 3:06

1 Answer 1

2
\$\begingroup\$

Why is switching off GND instead of VCC a bad idea in battery powered application

It certainly isn't a bad idea in the case of an automotive battery.

Consider which terminal of an automotive battery is to be disconnected first (the grounded one or the ungrounded one) bearing in mind the safety of the service technician.

The grounded terminal should be disconnected first to ensure that a battery dead-short does not occur, should the spanner disconnecting the other terminal contact a nearby grounded metal part.

A battery dead-short will result in short circuit currents of the order of hundreds of amperes and heavy sparks. This could result in the service technician suffering burns or other injuries due to involuntary movement.

That's the reason why a battery disconnect switch is always installed on the ground end of the battery.

Extending this reasoning to the battery-powered device in question, and presuming there is a chassis ground, a disconnect switch on the ground side would certainly be a good idea.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.