Battery Holder "Clip-On" for mBot 1.1

From a disassembled laptop battery I had several used 18650 batteries sitting around. Two of these (paralleled) fit very well into this battery holder, which can be clipped under the mCore board of the mBot 1.1. 

Since these reused 18650s  have no protection circuit, I added as a minimal protection an automotive fuse to prevent problems in case of an accidental shortcut. Of course you have to solder on the correct JST connector (2mm distance of the contacts).

The battery pack that you plug into the 3.7V Power Connector is charged via the USB connection. During charging a red LED lights, which turns green when the cell is fully charged. 

Although the charger circuit of the mCore board is designed for a Li-Polymer cell, it should do fine with the 18650 Li-Ion cell(s). I did some measurements. The charging voltage never exceeded 4.16V. Discharging with an external load of 0.8A showed 3.7Ah capacitiy when discharged until 3.2V. Quite good for such old used cells. The mBot becomes quite slow at this voltage, therefore, in real operation you will not want to discharge it further.

→ However, do the tinkering with 18650 cells at your own risk. ← 
These unprotected cells are potentially hazardous for the inexperienced user.
To stay more on the safe side, you can use a ready-made 3.7V Li-Polymer rechargeable with attached JST connector instead, especially if you have no solder equipment.

The holder can be printed quite easily, with only 2 small supports for the teeth of the clip mechanism, see photo. As for all parts that use elastic forces, PETG is preferable to PLA. My print lasted 1h15 with a layer height of 0.3 mm.