We used DualSky 1300 mAh 30C 2s1p 7.4 V battery (Figure 2-11). What do all these values mean? 1300 mAh (milliamp hours) or 1.3 Ah is the battery capacity. Higher value means longer running time. 30C is the discharge capacity for one hour. In this case 1300 mAh * 30 = 39000 mAh = 39 Ah. 2s1p means that battery has two 3.7 V cells in parallel. So together they give you 7.4 V, the output current. The recommended input voltage for Arduino Uno is 7–12 V.

Figure 2-11. Rechargeable lithium-polymer battery pack

Batteries often use one set of cables for charging and other for discharge (powering your device). In our battery, the blue connector is for discharge and small white one is for charging (Figure 2-11). Be careful to use the correct one.

Let’s connect the battery and the key switch. The ground wire from the battery connects directly to Arduino, but the positive wire has a power switch in the middle (Figure 2-12). You need to use a connector for the battery, so that you can disconnect it for charging.

Figure 2-12. Circuit diagram of battery and power switch

The battery cable has a connector on one end and free wires on the other. It connects the battery to the power switch and to Arduino GND. Use a connector suitable for your battery. Our battery came with a male DC3 connector, so we use a DC3 female for the battery cable.

Leave the black free wire 25 cm long. Cut one red free wire to 8 cm and another one to 15 cm. Strip the wires and solder the black wire and shorter red wire to the switch.

Put heat shrink tubing over the soldered joints (Figure 2-13). Leave some tubing for the other end of the red wire, too (Figure 2-14). You can’t add tube after you’ve soldered both ends.

Solder red wires to the power switch and put some heat shrink tubing on them (Figure 2-15). Connect the free red wire to Arduino VIN and the black wire to Arduino GND (Figure 2-16).

Now you have connected the battery and the switch (Figure 2-17).

The power source is often omitted from circuit diagrams. We didn’t use a symbol for USB power earlier, so we won’t be using one for the battery all of the time either.

Well done! You have now built a complete line-avoiding robot. Let it play in the arena for a while and enjoy a cup of coffee (or appropriate celebratory beverage).

Figure 2-13. Putting heat shrink tubing on the cable for connecting battery to Arduino and power switch

Figure 2-14. Adding a piece of heat shrink tubing before soldering to the power switch

Figure 2-15. Positive wires soldered to the power switch

Figure 2-16. Switch wires connected to the ScrewShield

Figure 2-17. Battery and power switch connected