Robot Batteries – Power vs Weight

I’ve always been tall so have at times been on the receiving end of a driver saying “you can tell you’re in the car” referring to my size/weight having impact on the car’s speed/power.  So this is what this decision is broadly based on, what batteries should we use in Mr Bit to make sure we get the best power weight combination?

We’ve got some requirements for our robot:

  • Pi is 5V and needs about 2000mA (though running headless we may need less)
  • Low power micro metal gearmotors, stall current ~360mA, free run 40mA (assume ~200mA under normal use)
  • 4x motors => ~800mA under normal use (stall 1440mA)
  • Motors can be run with 3V-9V (typical 6V)

Our initial thoughts we to use separate batteries. A 2A output power-bank for the raspberry pi and a set of 6 AA batteries for the motors to ensure we have high voltage and enough current as the batteries deplete (and in the hope it gives the motors an extra boost).

+2A power-bank and 6AA cells in holder

+2A power-bank and 6AA cells in holder

This works well for our power needs but we are concerned about the weight of the batteries as well as their size. The bigger the battery the bigger the robot (or less room for other components). More importantly to us is the how the weight might affect Mr Bit’s speed and power on the ground. The stall torque for our low powered micro metalgear motors looks quite low (compared to medium/high powered ones) so we are going to need to make our robot as light as possible to get best performance out of these motors… And the power-bank + 6 AA cells weighs in at 0.5Kg!

We looked at the following battery options so that we can have 5V – 9V outputs and ~3000mA draw:

  1. power everything just from power-bank: not enough V for the motors
  2. power everything from the AA batteries: at ~1500mA draw not enough current
  3. have 2 sets of AA batteries in parallel and 5v regulator (12 cells): similar weight
  4. use lithium based battery and 5v regulator: good weight and could meet both V and A requirement

We’d want to have a 7.4 lithium battery and something which is safe and easy to use. We don’t have need for really high output current with our motors so we don’t really need a high drain RC battery like a li-po, plus this would just add a lot of cost and stress from the complexity of using it. Li-po need to be carefully managed, they need to be monitored for low power and charged carefully in fireproof bags. Maybe if we want a very high power robot in the future we will learn more about li-po.

Our preferred battery is a li-ion cell and the 18650 offers a neat package similar to a big AA battery. They are 3.7V so we will use two in series and they offer ~3000mA draw (perfect for our needs). The batteries have a built in circuit protection which won’t allow them to be charged over 4.2V or discharge under 2.7V which means we can actually have +8V on a full charge.

We are going wire this into our motor driver directly and for the pi we are going to use a step down/up 5V regulator. We think the one we’ve chosen is amazing, it’s the Pololu S18V20F5 and outputs a consistent 5V whether from a source 2.9V – 32V. It means it will shut off if 5v can’t be delivered which will protect the pi from low voltage issues, it will make use of the batteries as much as possible without their protection needing to kick in at low voltage, it is future proof for us if we build bigger robots and it adds only a mere 7.5g weight to our overall robot.

2 batteries and regulator module

Small, powerful and light, 2x 18650 batteries and S18V20F5 5V regulator

2 of these 18650 batteries in a battery holder + 5V regulator weighs only 100g – that’s a whopping 80% reduction compared to our original option!

We know this is probably a little less scientific approach to selecting batteries for our robot but we feel we’ve made the right decisions for Mr Bit and hope to get the best performance for him from this configuration.

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