The Wemos battery shield seems an easy way to start battery feeding your Wemos D1 mini, but after using one for a while, it became obvious it is not for any serious battery use, mainly for two reasons: It is not efficient in using the battery power and the Wemos D1 itself is not efficient to be used with batteries.
The battery shield like all shields is to be plugged onto the Wemos D1 mini. It has two jacks: one for a battery and one for an USB plug that can be used to charge the battery. The charging circuit is built around a TP5410 Lipo charger. One therefore would expect a standard LiPo cell connector on the board but the LiPo cell connector on the board is an XH2.54 connector, whereas most LiPo cells come with a JST-PH connector. Another disappointment was that it came with male headers only. Apparently it is thought this shield should come on top of every other possible shield
Apparently he 5410 cranks up the battery voltage to 5 Volt and feeds this to the 5 Volt pin of the Wemos D1. Though this makes the Shield handy to use in combination with other shields that may rely on the voltage coming from the 5 Volt pin, it is likely a less efficient way of using the battery power as opposed to bringing it to 3.3 Volt directly via an LDO.
There is not much technical information on the battery shield, but I did find a circuit that seems to almost come right out of a chinese datasheet of the TP5410, in which several configurations are shown.
Studying the circuit it seems that the 5 Volt coming from the USB, is connected to the 5 Volt pin of the shield, only through an SS32 Skottky diode. In itself that is no problem, unless one decides to use another input to that connector, e.g. a solar cell. The Wemos D1 mini has an RT9013 LDO regulator that has a max input of 5.5 Volt with an absolute Max rating of 6 Volt.
Considering that the SS32 Skottky diode has a forward voltage of 200 mV@200mA, a voltage of > 6.2 on the USB connector of the battery shield (say a 6 Volt solarpanel on a bright day) could already kill the RT9013 LDO on the Wemos D1 board, Eventhough Wemos states the shield can be supplied with 10 Volt, but maybe that is without it being connected to the Wemos D1 mini
Anyway, I hooked up a 720mAh Lipocell to the battery shield and uploades a sketch that measures the battery voltage (through a resistor on A0) and did an upload to Thingspeak every minute, being in deep sleep in between, just to see how long it would last.
My first observation was that the shield apparently does not fully charge the LiPo cell. It came to a max of 4.05 Volt before it switched to ‘Standby’. I made sure by using a Multimeter and indeed, only 4.05-4.1 volt on the cell.
It worked pretty well after that, uploading the voltage to Thingspeak for 5 days and 5 hrs, when suddenly it stopped, with the battery voltage at 3.56 Volt, far above the 3 Volt minimum charge of a LiPo. Strange. The RT9013 has a drop of 250mV at 500mA, so even at 500mA the voltage on the ESP8266 still would have been 3.31 Volt. I have fed the 8266 with as low as 2.9 Volt and it was still working, so this was a strange finding. Resetting the Wemos did not bring it back to life… which was not so surprising because when i measured the LiPo (this was almost a day after the sketch stopped working… I was busy shopping for Newyears Eve), it was at 1.3 Volt. A quick connection to a USB port brought it back to life. A very strange finding indeed that warranted me to repeat the test (See below). But for now it doesnt seem like the battery shield is the best solution for a serious battery dependend project.
As said before, the Wemos D1 Mini, also is not the most suitable for a battery operated project. True, the ESP8266 can be put to deepsleep and will only use around 77uA, but the Wemos board also has the CH340 FTDI to TTL chip on board. As that is directly connected to the 3.3 Volt line, it will always be active, drawing around 50uA which is sort of in the same ballparc as the ESP8266 itself. Without that CH340 it could operate 1.6 times longer.
The repeated test showed similar results: Charge to 4.05 V (should be 4.2 Volt) and apparently shuts off at 3.5 volt (should be 2.7 Volt)
A final test, in which the Wemos was put to sleep for 10 minutes, the battery lasted 18 days before it was shut off at 3.59 Volt
Apparently Harald has a more positive experience with the shield.