DIY Kit - Installing Rechargeable Battery
Please note this guide is for v1 Gecho (made in 2018 and earlier), the v2 already has a built in charging circuit.
Recharging is the way to go!
If your Gecho board also requires some soldering, please follow this guide first.
The charging module
If your DIY kit comes with rechargeable battery powering scheme, you should find one of these modules included. It is a standard circuit utilizing TP4056, with DW-01P and 8205A dual mosfet for over-current protection.
You need one more wire
Althought the charging module has its own USB micro connector, you may want to wire it in such a way that Gecho is charged via the on-board mini USB connector instead. In order to do this, you need to take the 5V voltage from somewhere. The good point is the L2 element, a ferrite bead (which filters unwanted high frequencies on the USB power line). It does not really matter which end of L2 you solder the wire to. Ideally it should be the end farther from the edge of the board, but if the other end is easier to access, then it is fine too.
How to wire it all together
Wiring is pretty straightforward. The module's ground pins (-) and (out -) are internally connected, no need to solder the wire to both of them.
For SRAM backup voltage (which is essential for user settings and generated content to be preserved) you have more options. The MCU's VBAT pin is wired to 3rd pad on the board and it requires to have 1.65 to 3.6V constantly connected, even while unit is powered off. The current consumption is very low, STM32F405 datasheet says it's typically below 1µA. You can use CR2302 battery, two AAA batteries (old and used will do) or add 3 diodes in series to lower the voltage to safe levels. Normal silicon diodes with typical voltage drop of 0.6-0.7V should be used. The voltage of rechargeable battery will cycle from 4.2V (fully charged) to 3.5-3.6V (critically low). With three diodes, this voltage gets down by cca 2V, staying in safe margins for VBAT pin.