Power supplies and batteries for robot joints

By Tolerance Labs engineering team · Updated 18 October 2026

A robot’s power system must deliver its average power for the whole run and its peak power for a fraction of a second, without the bus sagging so far that joints lose speed or drivers shut down.

Choosing a bus voltage

Most joint modules run on a 24 V or 48 V bus. For the same power, doubling the voltage halves the current. That cuts I²R loss in the wiring to a quarter and allows thinner cable and smaller connectors. Higher voltage also raises the top speed of a given motor. Pick one bus voltage for the whole robot, as choosing a robot joint actuator recommends, and make logic rails from it with DC-DC converters.

Batteries set the exact range. A lithium-ion cell runs from about 4.2 V full to about 3.0 to 3.3 V empty, around 3.6 to 3.7 V nominal. A 6-cell pack is a 24 V class bus, about 18 to 25.2 V. A 13-cell pack is a 48 V class bus, about 39 to 54.6 V. Check that every joint and driver works across the whole range, not only at the nominal voltage.

Many teams stay at 48 V nominal or below. A full 13-cell pack is still under 60 V DC, a level often used in electrical safety standards as the limit for touch-safe extra-low voltage. Above it, insulation, connector and handling requirements get stricter.

Adding up joint power

A joint draws its mechanical power, torque times speed, plus copper loss, driver loss and gearbox loss. Copper loss is drawn even at standstill: a joint holding a leg up draws power while doing no mechanical work. How to size a robot joint motor shows how to estimate copper loss from torque.

Adding the peak power of every joint gives a number far larger than the robot ever draws, because joints do not all peak at once. Instead, list the robot’s main operating cases and sum only the joints that work together in each.

• Standing or holding: copper loss in every loaded joint, plus the computer and sensors. This sets the floor.
• Walking or a typical task: the average over a full gait or work cycle. This sets battery capacity and run time.
• The hardest move: a jump, a recovery step or a fast lift, when several large joints push at once. This sets peak current, battery rating and voltage sag.
• Braking: joints that decelerate return power to the bus. See regenerative braking and bus overvoltage.

Measure where you can. Most joint drivers report bus voltage and current, and a short log of a real gait beats an estimate.

Batteries: capacity, C-rating and sag

Capacity in watt-hours sets run time: required Wh ≈ average power × hours ÷ usable fraction. Planning on 80% of rated capacity is common, since the end of the discharge sags and deep cycles shorten cell life.

C-rating is current relative to capacity. On a 5 Ah pack, 1C is 5 A and 10C is 50 A. Pack labels, especially on hobby lithium-polymer packs, often overstate it. Use the cell maker’s continuous rating and multiply by the number of cells in parallel. High-drain cylindrical cells are commonly rated for roughly 15 to 45 A each, depending on size and chemistry.

Voltage sag is the drop across the pack’s internal resistance: ΔV = I × R. Pack resistance is cell resistance times cells in series, divided by cells in parallel. Resistance rises in the cold and as cells age. Design for an old pack, cold and nearly empty.

Worked example: a 15 kg quadruped

Assume a 12-joint quadruped on a 13-cell lithium-ion pack. Estimates or logs give these loads:

• Capacity for 30 minutes of trotting: 300 W × 0.5 h = 150 Wh. At 80% usable, 150 ÷ 0.8 ≈ 190 Wh.
• A 13S2P pack of 2.5 Ah cells holds about 13 × 3.7 V × 5 Ah ≈ 240 Wh, which leaves margin for aging.
• Peak current at nominal voltage: 2,000 W ÷ 48 V ≈ 42 A, or 21 A per cell.
• With cells of 20 mΩ each, pack resistance is 13 × 20 ÷ 2 = 130 mΩ.
• Near empty, at 3.3 V per cell, open-circuit voltage is 13 × 3.3 ≈ 42.9 V. Solving 2,000 = (42.9 − 0.13 × I) × I gives I ≈ 56 A, at about 42.9 − 0.13 × 56 ≈ 36 V.

Sag lowers the voltage, so the same power needs more current. At 56 A the pack delivers 28 A per cell, near the limit of many high-drain cells, and the joints see only about 36 V. A 13S3P pack lowers resistance to about 13 × 20 ÷ 3 ≈ 87 mΩ and current per cell to under 20 A, at the cost of 50% more cell mass. The other option is to limit jump power when the pack is low. Either way, check each joint’s top speed against 36 V, not 48 V.

Bench supplies

A bench supply is convenient for development, but it differs from a battery in two ways.

• Current limit. When a jump or fast move exceeds the limit, a constant-current supply drops its voltage, and the drivers may hit undervoltage and reset. Size the supply for peak power, or put a battery or a large capacitor bank on the bus to cover peaks.
• No reverse current. Most bench and switching supplies can only source current. When joints brake, the returned energy has nowhere to go, the bus voltage rises, and the supply may shut down on overvoltage or be damaged. Add a brake resistor circuit or use a supply that can also sink current.

Long, thin leads add sag of their own. Use short, heavy leads, and connect the supply’s remote sense leads, if it has them, at the robot’s power input.

Wiring and connectors

Drop and heat in a wire follow from its resistance. Copper values at 20 °C:

Count both conductors: a 1 m run from pack to robot is 2 m of wire. In the example, 56 A through 2 m of 14 AWG drops 56 × 2 × 0.0083 ≈ 0.93 V, about 2.6% of a 36 V bus. Safe current also depends on insulation temperature rating and bundling, so treat the table as a drop and heat check, not a current rating.

• Distribution. Feed each leg or arm from a central point, or daisy-chain within a limb. In a daisy chain the first segment carries the current of every joint after it, so size it for the sum.
• Fuses. Fuse each branch to protect its wire, and put a main fuse or breaker at the pack.
• Pre-charge. Connecting a charged pack to the drivers’ bus capacitors draws a large inrush current that pits connector contacts. Use a pre-charge resistor, an anti-spark connector or a pre-charge circuit.
• Connectors. Rated for peak current at your temperature, keyed against reverse polarity, and locking where the robot vibrates. Cables that flex need strain relief and high-flex wire; see hollow-shaft joints and cable routing.
• Emergency stop. Cutting bus power stops the joints, but it lets the robot collapse and leaves returned energy with nowhere to go. Decide stop behavior together with the drivers and brakes.

Checklist

• One bus voltage, with every joint and driver rated from empty to full pack voltage.
• Power listed for standing, a typical task and the hardest move, summing only joints that work together.
• Capacity from average power, with about 20% left unused.
• Peak current per cell within the cell maker’s continuous rating, checked on a nearly empty pack.
• Joint top speed checked at the sagged voltage.
• Bench supply sized for peaks, with a path for returned energy.
• Wire drop and heat checked over both conductors, with fuses, pre-charge and locking connectors.

Browse power and control cables and motor drivers, or estimate joint loads first with the joint torque calculator.