Save battery with powered portable hdd

[enonod]

MBP 13" Retina early 2015
Using a Portable HDD for compactness means no external power supply and so power is drained from the battery, thus more recharges, less time etc.
To overcome this I use a desktop HDD but still use a Samsung T1 SSD also.
I am concerned that if I use a Y USB 3 cable (using one leg to supply power only) there may be slight differences in voltage at the Mac port or that the Mac port will still supply half the power.
Can anybody please suggest the safest solution.

 

[Cory Cooper]

Hello and welcome.

If the desktop hard drive has an AC power cable, you won't drain any power from the MBP. The Samsung T1 SSD does require power from the MBP.

Does that help?

C

 

[enonod]

Thank you Cory; I am not sure whether you are saying that the T1 'must' for some reason take power from the same USB as the data or simply that it does (which I know because I use it). If the former, then it suggests that it could not be used on a powered USB 3 hub; is that the case?

enonod said: I am concerned that if I use a Y USB 3 cable (using one leg to supply power only) there may be slight differences in voltage at the Mac port or that the Mac port will still supply half the power.

The general theme is that I am trying to externally power a 'portable' HDD because it is small but never comes with a separate power supply.
I suppose a simpler way might be to ask whether a USB port supplies half the DC current when something is connected via a powered hub, or is there some mechanism in the port that shuts off the current when external power is present.

 

[Cory Cooper]

Correct, if external power is present, then the USB port will not need to supply power. The USB 3 connection is higher powered than an older USB 2 connection, and does supply/use both power and data from the single USB 3 port.

You mentioned a USB 3 Y-calbe - did that come with the drive?

I wouldn't suggest trying to externally power a USB 3 portable drive if it wan't designed for that. They are bus-powered, and will have the necessary ports/cables if they can be externally powered. The Samsung T1 SSD isn't designed for external power.

If you are concerned about battery life of the MBP, then using bus-powered devices isn't a good option when running on battery power alone. If you are using the Samsung connected to the MBP in a desktop situation, where the MBP is connected to its AC power adapter, then it will bus-power the T1 just fine with any additional connections other than the USB 3 cable that came with it. I use my older MBP connected to AC power in closed-lid mode, connected to an external monitor and a bus-powered two drive portable RAID enclosure via FireWire without any external AC power to the RAID without any problems.

C

 

[enonod]

Thank you Cory, no the Y cable did not come with the drive it is something I read about. My question regarding the power sharing came from my electronic background rather than newbie ignorance (not being sure of the design of USB ports and possible use of gates or diodes to cut off local supply in the presence of external power or whether the whole works as batteries would and share the load), however, we move on.
My concern was that I have read that the MBP should be charged then used until charge required again rather than powered constantly (say, when a bus powered HDD is present). Is that correct?
Drawing up to 900mA when not plugged in certainly reduces the time and thus requires more charges. I don't have an old MBP unfortunately, but have recently discovered the closed lid situation when using a larger screen (at first I did not know what was happening when the screen came back on when the lid was closed. I don't know how much power that saves.
Overall I have found myself in the habit of constantly closing the lid to save power. Do you know how many charges the battery can take in its life, the MBP is in use most days most of the day? I worry because this MBP has the battery spread all over it which would make it expensive to replace (being beyond the user to replace).
[EDIT] I note that the large screen goes off when power is removed from MBP, does that mean it is OK to leave power on??

I am grateful for your input.