Robot Battery & Power Pack

What it is

The power pack is the part beginners think least about and get wrong most often. It is simply the battery that feeds current to the motors and the electronics—but choose it badly and the robot twitches, resets, or refuses to move, no matter how good the code is. Getting power right is often the difference between a robot that runs and one that keeps rebooting.

The key idea is that a robot has two very different appetites for current: the motors gulp it in surges, and the logic sips it steadily. One battery can feed both, but they must be arranged so the greedy motors don’t starve the sensitive brain.

How it works

Follow the current from the pack:

1. The battery supplies raw voltage—6 V from four AAs, 7.4 V from two 18650s.
2. The motor rail runs straight from the pack into the L298N driver’s Vin. This is where the heavy, surging motor current comes from—kept off the Arduino entirely.
3. The logic rail is a regulated 5 V (from the driver’s onboard regulator or over USB) that powers the Arduino and sensors with a clean, steady voltage.
4. A common ground ties everything together. Every rail must share the same ground line, or the logic signals have no shared reference and readings become meaningless.

Split this way, a motor drawing a sudden amp can sag its own rail without dragging the Arduino’s 5 V down with it.

Why a 9 V block fails

The single most common first-robot mistake is powering the motors from a rectangular 9 V PP3 battery. It looks convenient, but it holds very little energy and has high internal resistance. The instant the motors draw current, its voltage collapses—the Arduino dips below its minimum and resets, so the robot lurches and reboots in a loop. A 9 V block is fine for a breadboard sipping milliamps; it is the wrong choice for anything that drives wheels.

Choosing and wiring

• Match voltage to the motors. 3–6 V motors are happy on a 6 V or 7.4 V pack; use PWM to tame the speed rather than under-volting the pack.
• Capacity is runtime. Higher mAh means longer between charges; 18650 cells last far longer than AAs but need a protected holder.
• One switch, correct polarity. Add a master switch on the pack and double-check polarity before first power-up—reversed power can kill a board instantly.
• Don’t back-feed. Avoid powering the Arduino from USB and the battery at the same time unless the board is designed for it.
• Add a bulk capacitor across the motor supply to soak up the start-up surge and steady the rail.

Internal resistance: the number that decides everything

Every battery behaves like an ideal voltage source with a resistor in series. That resistor is the whole story of why one pack works and another resets your board.

V_under_load = V_open_circuit - (I x R_internal)

A robot pulling a 2 A surge through a pack with 0.5 Ω of internal resistance loses a full volt before anything downstream sees it. Here is what that means across the common options:

This table explains the 9 V block completely. It is not that it has the wrong voltage — it has the highest voltage in the list. It is that under a 2 A surge it delivers 3 V, and the robot resets. Voltage on the shelf tells you nothing; sag under load tells you everything.

Measure yours in two minutes

1. Measure the pack's voltage with nothing connected.       -> V_open
2. Stall the motors and measure again while stalled.        -> V_load
3. Measure the stall current with the meter in series.      -> I
4. R_internal = (V_open - V_load) / I

Write the number on the pack with a marker. It rises as the cells age, and a pack whose resistance has doubled is a pack that has started causing “random” resets.

The current budget

Add it up on paper before you buy anything. Use stall, not typical.

A typical obstacle-avoiding robot — Uno, two TT motors, an ultrasonic on a servo — budgets at roughly 360 mA typical and 2.7 A peak. Size the pack for the peak, and size the runtime from the typical:

runtime (hours) = capacity (mAh) / average draw (mA)

2000 mAh of AA NiMH at 360 mA is about 5.5 hours of driving, which is far more than the battery in most robots ever gets used for. Capacity is rarely the constraint — surge capability is.

Wiring it properly

Battery + ──── master switch ──┬──── motor driver VM  (the heavy path)
                               │
                               └──── 5 V regulator ──── Arduino, sensors
Battery − ─────────────────────┴──── COMMON GROUND ──── everything

Four rules, in order of how often they are broken:

Common ground, always. Every supply, every board, every sensor ties to one ground. A missing common ground gives logic signals no reference, and the failure is silent — nothing works, nothing is warm, nothing looks wrong.

Star the ground, do not chain it. Run each ground back to one point rather than daisy- chaining through the motor driver. Motor return current through a shared ground wire creates a voltage difference along it, and your sensor readings shift whenever the motors move.

Master switch on the positive lead, before anything. Being able to kill a runaway robot instantly is worth the two minutes.

Reverse polarity protection. A single Schottky diode in series costs ~0.3 V, or a P-channel MOSFET costs almost nothing. Either is cheaper than the board a reversed pack destroys — and reversing a pack takes one moment of inattention.

Capacitors: what they do and do not fix

Add a 470–1000 µF electrolytic across the motor supply, close to the driver. It is worth doing. It is also routinely expected to do something it cannot.

What it does fix: the sharp switching transients from PWM and motor commutation, which are microsecond-scale and couple into the logic rail as noise.

What it cannot fix: a sustained surge. The arithmetic is unforgiving:

dV = I x t / C
1000 uF supplying 2 A for 10 ms -> dV = 2 x 0.01 / 0.001 = 20 V

A capacitor asked to carry a 2 A motor surge for 10 ms would need to drop 20 V, which is more than the pack has. Only a stiffer battery carries a surge. If your board resets when the motors start, a bigger capacitor is not the answer — a lower-internal-resistance pack is.

Do also add a 100 nF ceramic next to each IC’s supply pin. Different job entirely: it handles the high-frequency demand of the chip’s own switching, which the electrolytic is too slow to serve.

Choosing a chemistry

NiMH is the right first choice. Six cells give 7.2 V with low internal resistance, they tolerate mistakes, and a dead pack is an inconvenience rather than an incident.

Li-ion 18650s are the sensible upgrade: far more energy for the weight, very low internal resistance, and cheap. Use a protected holder or a BMS board — lithium cells taken below about 3.0 V per cell are permanently damaged, and a robot that runs until it stops will do exactly that.

LiPo is what performance builds use, and it is the one that requires respect. Never charge unattended, never puncture, never leave one discharged, and use a balance charger.

Troubleshooting

Monitoring the pack

The last thing worth adding: a robot that knows its own battery voltage can stop before it browns out, and it costs two resistors.

// 100k / 33k divider: 8.4 V max in -> ~2.09 V at the pin. Safe for a 3.3 V ADC.
const float DIVIDER = (100.0 + 33.0) / 33.0;

float packVolts() {
  return analogRead(A0) * (5.0 / 1023.0) * DIVIDER;
}

One caution that makes this worth doing carefully: on an Arduino, analogRead is ratiometric to the 5 V rail, which is itself sagging when the motors run. Measure a falling battery through a falling reference and the reading rises. Switch to the internal 1.1 V reference with analogReference(INTERNAL), size the divider for that, and the reading becomes honest — the analog input tutorial covers exactly this trap.