Battery Molecube Design

From molecubes
Jump to navigation Jump to search
Figure 1. Battery Molecube module with the battery taken out.

The battery Molecube enables wireless operation of Molecube robotic systems. Several factors were considered in the design of this module, including power density, ease of manufacturing, and cost. After initially considering a custom Lithium Ion pack from Rose Batteries which had optimal power density, associated fabrication complexity and cost proved to be significant drawback of this option. We could not find any commercial prismatic Lithium Polymer cells in the 2000mAh range that would be available in the dimensions required (50mm x 50mm x 50mm).

Figure 2. Power On/Off switch is placed on edge of the battery Molecube.

Electrical Design Overview

Commonly available video camera batteries happen to have very similar requirements as what is needed for Molecubes. Power density is critical, smaller is better, and due to much aftermarket competition, these products are priced very reasonably, roughly $20 each. After testing several at various discharge rates and identifying the limits of the protection circuitry internal to the battery a Samsung SB-L220 2200mAh pack was selected at 7.2v. Notably, a pack rated at 3000mAh showed a tested capacity of 2000mAh while the selected Samsung pack consistently performed in the 2400mAh range. This suggests these ratings should be taken with a grain of salt.

Figure 3. Battery voltage boost regulator, assembled PCB.

Performance Characteristics

Following is the data collected on the performance characteristics of this pack:

  • Low Discharge Rate, Measured Capacity 2487.7mAh, Discharge Time 6.13hrs, Average discharge current 0.41A
  • Medium Discharge Rate, Measured Capacity 2482.8mAh, Discharge Time 3.2hrs, Average discharge current 0.78A
  • High Discharge Rate, Measured Capacity 2368.8mAh, Discharge Time 1.15hrs, Average discharge current 2.06A
  • Max Discharge rate of about 4A could be maintained for roughly 3min before the protection circuitry inside the pack shuts down.

See this spreadsheet on GitHub for complete battery test data.

Battery Regulation

To achieve the required bus voltage (12v-24v), a boost regulator was determined to be necessary. With some assistance from National Semiconductor an appropriate design was created conforming to the volume limitations. The regulator based around the National LM3478 achieves roughly 91% efficiency. The regulator has Vin = 6.0V – 8.5V and a Vout = 14V @ 2A.

The maximum discharge rate of the battery is estimated to be in the 4A range for short bursts. Estimates place maximum regulator load in this range of battery load conditions. This battery pack should support between 6 and 10 cubes under medium to heavy load for between 30 minutes and 1 hour. The pack has been tested under light load with 2 to 4 cubes for several hours of continuous use.

Regulator Board Design

Cadsoft EAGLE was used for the board design. Pack design files and Bill of Materials available in this GitHub folder. This wiki also provides the instructions on the Battery Regulator Assembly

Mechanical Design

The Solidworks files for all the components of the battery Molecube are available in this GitHub folder. The exploded component view in Figure 10 below shows that the battery Molecube includes one assembled voltage regulator PCB, two sets of interface PCBs: one triplet and one duet, five interface PCBs in total. This arrangement leaves space for a large opening in one of the cube faces for the battery insertion and removal. Besides these electronic components, the mechanical design is fairly simple. It only includes one 3D-printed internal bracket and two slightly modified exterior shells.

Design Notes

A plug was included in the final battery assembly enabling the battery to also provide a power interface for powering Molecubes from a power supply between 12-24V. A power switch accessible on the side turns the pack on and off. Because these packs are regulated, they cannot be safely connected in parallel (multiple packs on the same bus). Future ideas include a 2 cube batter y, essentially 2 Molecubes printed as a single unit. These two battery cubes would be configured in series no longer requiring a regulator. This would also allow multiple of these dual packs to be connected on the bus resulting in higher capacity and higher maximum discharge rate configurations.

Battery Image Gallery