Pi battery power & usb hub questions

RussellH · #1 14-07-2019, 07:26 AM

So I’m setting up Astroberry on a Raspberry Pi 3B+ And looking at power it and the rest of my accessories with a 12V battery. I need to power the mount, the pi, a usb hub (with guide camera, focuser, mount and SLR attached), focuser motor and possibly a dew heater down the track.

I was looking for an appropriate powered USB3 Hub and since Startech seem to be recommended, was looking at this one https://www.startech.com/au/Cards-Ad...hub~ST53004U1C

Q1. This got me thinking - it has a 2.4A charge port. Could I use this port to power the Pi and save having an extra power adapter straight from the battery to the Pi (which I haven’t got yet)? I’m not sure if plugging the hub into the Pi, and then the Pi back into the Hub will create any kind of feedback loop issue. The charge port is supposed to be powered independent of it’s data ports, so that would suggest it is isolated so should be OK, shouldn’t it?

Q2. That hub also comes with an AC to 12V 2A DC adaptor to run it, but I can just make up an appropriate DC plug and connect straight to my battery to power it, can’t I? Maybe I need to throw a voltage regulator in-line is all, in case the hub is expecting a regulated 12V input.

Q3. I’m also a little concerned how these hubs handle the weather, and if moisture could be an issue, but I guess it’s no worse than the Pi itself, so some measures are needs to protect your electronic gear from the elements. What have others done regarding weatherproofing their electronics?

Thanks.
Russell.

RussellH · #2 14-07-2019, 04:08 PM

Hmm, maybe this question should have gone in the DIY section. I’ve ordered that hub anyway, so I’ll just have to figure it out when it arrives.

dikman (Richard) · #3 16-07-2019, 09:26 AM

Q1. As the charging port is only supplying DC for charging something it shouldn't have anything to do with the data lines in the other ports so I don't see why your idea shouldn't work. Just make sure the Pi doesn't draw too much current.

Q2. 12v rechargeable batteries generally supply just over 13v when fully charged, which will drop slightly to 12v as they are used. Your hub should be designed to have a bit of leeway with the supply voltage (for safety) so I don't see why it shouldn't run ok from your battery.

Q3. It's not designed for outdoor use so yes, a suitable housing would be a necessity.

RussellH · #4 16-07-2019, 11:11 AM

Thanks for that.

Yes the main hiccup might be the hub charge port is rated 2.1A but the Pi power supply is rated 2.5A. But given the hub is self powered and if I only use self powered inputs such as the mount and SLR directly into the Pi USB ports, most of the Pi current draw should be just the main board and wifi signal, so shouldn’t exceed 2.1A I’d think.

sil (Steve) · #5 18-07-2019, 10:25 AM

best to run powered hub from power, pi isnt designed to be a power hub plus they are not plug and play so you may struggle to get the hub available on the pi, take power out of the equation and power separately.

RussellH · #6 18-07-2019, 11:00 AM

I’m not talking about running the hub from the Pi, I’m talking about Running the Pi from the Hub. But I am also getting a 3A USB charger as well, so ai could run the api from that and the Hub from another power plug if needed.

sil (Steve) · #7 23-07-2019, 08:47 AM

still same deal, its hit and miss with how usb hubs, even power only hubs, distribute their amps to each port. it's a situation most likely to end up with problems with the Pi due to low power, may work for a while but not after time or the moment you need to charge your phone or whatever. even the power hubs that claim 2A per socket I've seen in practice that meant "while all sockets are unused". dedicated psu per device no matter how you want to quibble over my answers.