Not very long ago, I salvaged a pair of eX-power laptop power banks, but it didn’t come with any output cables. As a result, I was only able to test the USB power output on them, which was underwhelming in terms of current supply capabilities.
As the power bank is designed to power laptops with a “16-19V” output to a 2.5mm DC barrel socket, I decided to order a few hefty pigtails from eBay as I intended to at least try to use the units.
But first thing’s first – lets see what it actually puts out and whether it can handle the load.
To begin, I had to use one pigtail to connect to my B&K Precision Model 8600 DC Electronic Load. I had a large number of crimp connectors in stock, but as the pigtail came pre-tinned, it would not crimp securely! As a result, I opted to clip off the shrouded insulation and solder it for a durable high-current connection.
The pigtail itself measures about 28cm and is made of 18AWG stranded copper. As a result, the resistance is estimated to be 11.76 milli-ohms (excluding connector resistance). At the maximum claimed 3.5A supplied, the voltage drop due to the cable is expected to be 41.16mV which is extremely negligible. It’s likely to see three to six times more voltage lost at the connections, which is still not that much (total loss of up to ~0.28V of 19V is 1.5%).
I decided to subject it to the maximum claimed supported load of 3.5A, in constant current mode. The unit didn’t shut down (to its credit), although it was not able to maintain a steady 19V output for long, falling into an “unregulated” state after 4-6 minutes. The voltage did decline to a knee just above 16V, where it abruptly fell and the unit shut down. The unit did become quite warm supplying the power, and at the rated load, lasted about 55 to 56 minutes. Delivered power is about 55-57Wh, which is only slightly less than the ~60Wh achieved via the low current USB output. This was despite the warmth of the unit during discharge, which may be due to the Li-Ion cell’s internal resistance and losses in the power conversion circuitry.
Some strange voltage swings were seen around the 18.25V mark, the cause being unknown but I suspect it was an auto-range “freakout” of the Model 8600 as the unit was flickering between three/four decimal place display. The actual voltage probably did not swing wildly at that time … or so I suspect.
As many laptops accept about 18-20V input and often do not use the complete power available from a 65W power adapter, the voltage may be able to be maintained better at the lower load. The reducing voltage as the unit depletes may be a positive sign as well, as many laptops may cease charging but remain powered externally upon sagging DC voltage (e.g. from aircraft power adapters), with further loss causing it to change over to internal batteries almost seamlessly.
Interestingly, the switching converter seems to be “always on”, just like those in modern USB power banks. This means the output is always “hot” at 19V ready for load, but might also mean a higher quiescent current than absolutely necessary and the potential to spew small amounts of EMI despite not being in use (and being unable to quell this without running it down until the Li-Ion pack protection kicks in and cuts the circuit out).
Putting it to Use
I ended up making a few makeshift cables – this one used a cut-off thin connector from a dead Bluetooth headset charger so that I could power my Asus R011CX netbook from the pack. The netbook itself suffered a battery failure, but I could not source a decent replacement battery. As a result, it regains some “nomadic” freedom as a result.
I also used two of these thick pigtails back to back to allow me to connect to an older BenQ Joybook Lite netbook, as well as being able to connect to a 2.5mm to Lenovo square adapter to power my Lenovo ThinkPad Edge E431. It works quite well, surprisingly, which is a good thing since the E431 of mine has a battery life of at most three hours.
I suppose it’s good that I’ve finally been able to assess how the unit performs when fulfilling its intended purpose. The description of output voltage is surprisingly honest and seems to be a sign of power converter limitations when sourcing the maximum current. It seems sufficient, however, to run most laptops especially netbooks and regular laptops at lower loads (i.e. not charging an empty battery, not running processor intensive tasks with the screen at full brightness). Of course, going through a few stages of power conversion is less efficient than using a dedicated battery for the laptop, but seeing as these were free (minus the cost of buying the pigtails at about AU$13 all up with a few more tails to spare), I think I did well.
The one thing I still haven’t been able to test is the 9-12V output, which I suspect is fused and protected but not converted from the internal lithium pack. Unfortunately, again, it’s a case of not having any of those dang connectors on hand!