Boty Bot: Batteries

The DFRobot platform that I used for Boty Bot has the batteries placed inside the bottom compartment, near the motors. It would be a real hassle to exchange batteries, as you’d have to take the whole thing apart. And the kit does come with a DC power jack plus a hole for it in the back, next to another hole for a SPDT switch.

So it was obvious to me from the start that I’d want a rechargeable battery. And I was quite proud of myself for working out the following wiring diagram for the battery.

When the switch is in the up position, the battery is connected to the motor shield, and the motors receive power. When the switch is in the down position, the battery is connected to the DC power jack and can be charged. I guess the scheme is pretty obvious, but I was still pleased with myself.

After a bit of searching, I chose a 6V 2000 mAh NiMH battery pack that is like 5 AAs shrink-wrapped together, plus a related charger. I considered a more expensive universal charger, but decided to just get this cheaper one, as I didn’t think I’d be using lots of different kinds of batteries, and I’ve been spending too much money on this hobby.

The 6V battery worked great. But then a day after putting the robot together, the robot started acting crazy: going backwards and not responding to the remote, giving crazy Ping distance measurements, etc. I finally concluded that it was the 9V battery driving the arduino, ping sensor and servo: the battery was running low, but rather than stopping working completely, the robot was just stopping working properly.

So, I got a second rechargeable battery to drive the arduino, ping and servo (i.e., everything but the motors): a 7.2V 2000 mAh NiMH battery pack just like the 6V one driving the motors, but with 6 AAs shrink-wrapped together rather than just 5.

I soldered a 2.1mm male plug onto the 7.2V pack, then realized I’d forgotten to thread the plastic sleeve onto the wires first, and so had to cut them, desolder, and solder again. That won’t be the last time I screw up in that way, I’m sure.

I then made a couple of adapters for the charger, since one battery is connected to a 2.1mm female jack and the other to a 2.1mm male plug.

The robot has been working well since I got the 7.2V battery pack.

I also got a mini volt meter so I can monitor the battery charge. I’m not sure whether I should monitor the power to the arduino (the Vin pin) or the regulated power (the 5V pin). Currently, I’m monitoring the Vin pin.

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