USB battery box 2x18650 enb



I got this box from fasttech where it is sold under the "ENB" name.
Specifications:

• Color: White
• Connectivity: Connectors: USB/Micro USB
• Current Output:1.5 A
• Input Type: DC 5V
• Maximum Supported Battery Length: 67 mm
• Output Type: DC 5V
• Manufacturer: Brand ENB
• Dimensions: Depth: 27 mm, Height: 98 mm, Width: 61 mm
• Product Weight: 66 g

How does it look



The box arrived in a retail packing with specifications on the back.





All connections are placed in the same end, micro usb for charging input and normal usb for output.
This means it can share charge and cable with a mobil phone, as long as the charger is rated at 1A or above. With two cables it is possible to daisy chain them, charger to usb box and usb box to phone, after a couple of hours both will be fully charged (Very nice).



On top of the unit is 3 leds and a button.
The leds shows when the usb output is active with a "running" light.
When charging they shows charge status, holding the button down will also force display of charge state.




To replace the batteries, slide the outer cover off the box. The batteries can be up to about 67.8 mm, but shorter is better, i.e. unprotected.



With a small screwdriver it is possible to open the box.









A close look at the electronic, one interesting details is that there is no connection between the two battery +.
Because of this the batteries is not required to have the same charge state.



Measurements

• Disconnect load when current drops to 32 mA
• Automatic turn on when load is connected.
• When output is off the battery drain is below 30uA (This is more than 20 years with a full load of 3100mAh batteries).
• When charger is connected, output is turned on.
• When charger is connected it will use input current to drive load and charge/discharge depending on load current.
• When load is connected, leds does always indicate load (running sequence).
• When charging without load, all the leds will turn off when charge is finished.
• The USB outputs is coded as Apple 1A and USB charger only, i.e. if charging is slow or not working in one connector, try the other.
• Battery state on leds: below 3.63 volt 1 led, between 3.63 and 3.92 volt 2 led, above 3.92 volt 3 led.

A load sweep shows that it can deliver up to 1.8A, before turning off, with 1.5A rating this looks fine.



A test with a single battery and 1.25A shows that it shuts down after 20 minutes, this does not look very good. Let’s try with two batteries instead:



With two batteries and 1.4A load, the box works perfectly. It runs for two hours.
The output turns off when the batteries is down to about 3.1 volt, this is a bit early.



A 1A load with two batteries will stretch the runtime to more than 3 hours. Notice that the termination voltage is slightly lower.



Reducing the load to 0.5 A looks strange, there is a jump at 200 minutes and again at 390 minutes.
This is because the box only uses one battery at a time when running at 0.5A, the jump at 200 minutes is the switch between the batteries and at 390 minutes it starts using both batteries in parallel.
Voltage and Voltage2 is the voltage for the two batteries, the current measurement is common.
Note: The efficiency curve is faulty between 200 and 390 minutes, because it is using the voltage from the discharged battery for calculations.



At 0.5A load the output turns off when the batteries are just below 3 volt, this is a good value for most LiIon batteries.








At 1.4A load there is about 91mV noise, this is acceptable.



Reducing the load to 0.5A will reduce the noise to 36mV, this is also acceptable.



Charging a battery looks close to a CC/CV profile, but a very soft change from CC to CV (This increases charge time).





On the 2600mAh battery the current reduction start early, increasing the charge time to the same as a 3100mAh battery.



Charging two batteries looks funny, this is because the charger is switching between the two batteries at a slow rate. This does not affect the CC/CV charge profile.



Conclusion

The box is larger than I expected for a two battery box. The function of the box is very good with low standby current, automatic turn on, support for both Apple and usb standard. Support for simultaneous charging, i.e. both charging its own batteries and delivering output, it very useful. It works best with two batteries.



Notes

Read more about how I test USB power supplies and chargers