Dummy battery from wood + capacitor + resistor.
How to use ZTE MF90 without battery, powering only by electricity power.
ZTE MF90 is a mobile hotspot device, http://www.ztedevices.com/product/Mobile_Hotspot/6712c639-c861-412b-8f4d-db6836995e3d.html
It supports LTE (4G) and wifi.
In some countries the price is around $200, but we are lucky enough to have this mifi for just $30 in a promotional offer by a local LTE mobile broadband Internet Service Provider (Bolt, this service is not active yet until this post is made).
Update: The Bolt TD-LTE service 2.3 GHz is now already active as per 25 December 2013.
The device is bundled, hence it's locked to the Bolt ISP. We have to unlock it to be used for other ISPs. Dc-unlocker did it very well, https://www.dc-unlocker.com/zte-mf90-detect-and-unlock-guide
So, because of LTE service from the default Bolt ISP is not yet active, meanwhile I'm using the mifi with another HSPA+ mobile broadband service.
This mifi uses battery, it lasts for around 7 hours in normal usage (wifi). To save the battery life, I must find a way to bypassing the battery and powering it from only the wall line electricity. (Unfortunately, this mifi cannot be turned on by using charger only without battery)
Then I made a dummy battery. It's a simple capacitor and resistor circuit, with a thin wooden block to mimic the battery form factor.
There are 3 pins, (+), (-) or ground, and another (+) pin. Not sure if this is correct, but I refer to some resources to say, the middle pin is used as a temperature measuring mechanism.
The resistivity between middle pin and ground must be correct in order for device to be able to turn it on.
So, I came to 33 K ohm conclusion as the resistivity between middle and ground. The circuit,
Capacitor should be 1000uF or larger. The larger the better, I use a 4700uF capacitor (polar).
To replace the body of the battery, I made a dummy battery made from wood. It's easy enough to attach a 3 connectors to the right place to mimic the original battery. It's done with the help of unused IC socket and strong glue, solder a rainbow cable to it and voila... it's now ready to replace the original battery.
Solder the (+) pin to the (+) terminal of the capacitor, and ground to the (-) terminal of capacitor. Solder the resistor between middle pin and ground. That's all.
Now we need to find a case to make them easier to be attached to the wall or desk. I use a case from older 3.5" floppy disk case. It's transparent so it looks pretty enough.
Connect the micro USB cable to the device. Connect the other side of cable to the provided wall socket. Plug it into electricity line. Wait for some seconds, the capacitor needs time to be charged. The larger the capacitor the longer time needed to charge the capacitor.
Turn on the ZTE MF90, it runs normally. No battery, no heating, no continuous charging (on the battery).
Disclaimer: This circuit is developed only for the ZTE MF90 device. Don't use it for other type of devices.
Implement the circuit only if you know exactly what to do, consult an electronics expert if you don't know about it.
