This is a remix of SMARS with USB rechargeable battery. The battery charging integrated module can adjust the output voltage to fit the voltage of the Arduino board and motors. The minimum voltage required (from the DC input) for a regular Arduino UNO board is 7v.

In my case, I have two 6v motors, so I adjust the output voltage to 6.3v and connect the supply voltage to pin Vin instead. This will bypass the serial diode which lower the required voltage for the Arduino board to 6.3v (7-0.7) but still allows the regulator to work properly and provides 5v output. See the circuit diagram.

You can recharge the battery from the usb port on the side without taking off the battery, but the slide switch needs to be turned on when charging. You may connect the switch to the output of the charging board instead, which may consume a little bit energy when the switch is OFF.

Space is a little bit tight, so take care for the cable and wiring. Using hot glue can be useful.

I use the WeMos D1 board instead which allows me to control the robot via Wifi from my smart phone.

Part list:

• 1x Arduino UNO or WeMos D1 ESP8266 board
• 2x N20 motors (6V 150 or 200 RPM)
• Adjustable step up lithium battery charging integrated module https://www.aliexpress.com/item/32998389598.html?spm=a2g0s.9042311.0.0.34e44c4dnylIg4
• 1x 503450 lithium-ion battery 3.7V 1000mah (size 50x34x5 mm)
• 1x Dual H Bridge Motor driver https://www.aliexpress.com/item/32944408444.html?spm=a2g0s.9042311.0.0.bb094c4dPNtmON
• 1x mini slide switch https://www.aliexpress.com/item/32859663187.html?spm=2114.search0604.3.14.349c7461npBSca&s=p&ws_ab_test=searchweb0_0%2Csearchweb201602_7_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_537_536%2Csearchweb201603_53%2CppcSwitch_0&algo_expid=be07d9b5-bf68-401c-9225-568a880b562b-2&algo_pvid=be07d9b5-bf68-401c-9225-568a880b562b