Lithium Ion Battery Pack

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Lithium-ion cells form the battery pack that provides secondary power for the EPS. The pack is necessary to sustain the satellite's critical continuous loads during eclipse regions in which the primary power source provides negligible or no power, as well as to serve as an energy buffer to save up for brief high-power activity bursts exceeding the power rating of the primary power source.

DIETR Requirements

  • As per DIETR-0175, batteries shall maintain charge for a minimum of six months between launcher integration and deployment
  • As per DIETR-0190, the total stored chemical energy shall not exceed 100Wh
  • As per DIETR-0180, the battery cells are held in open circuit by a Remove-Before-Flight (RBF) pin prior to integration with the P-POD launch dispenser. This is consistent with the CalPoly CubeSat specification.
  • As per DIETR-0160, the battery cells are held in open circuit until the deployment switches close. A minimum of three series switches are required.

Redundant mechanical switch sits in parallel to reduce the susceptibility of the overall system to mechanical failure may be good practice.


Cell Format

An anode, cathode, and electrolyte-soaked separator are layered over an insulating material and rolled up into a "jelly roll", which is then slid into a steel canister. The standard canister is 18mm in diameter and 65mm long, and is called an "18650" lithium-ion cell. The nominal voltage of a single cell is 3.7V. Capacity and lifetime of a cell vary widely from model to model and make to make, due to the different trade-secret electrolyte formulations and manufacturing methods used by manufacturers.

These cells were selected because lithium ion chemistry has significantly higher energy density compared to older technologies like nickel cadmium, and lithium batteries have become common, relatively inexpensive, and easy to acquire. Though lithium-polymer film pouches have comparable energy density, the 18650 lithium ion cell was favoured due to the more structurally robust steel casing, as well as the longer lifetime.

Lithium ion cells usually hold a charge for a year or more in storage if initially stored around their nominal voltage. Attempting to store at high voltages degrades the shelf life. Ultimately the shelf life is a function of both the self-discharge and the small continuous load of the BMU, if not separated by a remove-before-flight pin.

The model currently owned by SFUSat is the NCR18650B.

Pack Configuration

To allow easy step-down regulation to 3.3V and 5V, it is ideal to have the pack configured with two cells in series yielding a nominal pack voltage of 7.2V. To increase pack capacity, cells in parallel could be added. This could be configured in one of two ways: either by having a set of N 2S sub-packs in parallel, or by having a set of 2 NP sub-packs in series. Since each series branch requires a cell balancer, and the former configuration has N series branches whereas the latter only has one, the latter configuration is preferable. Additionally, this opens up the possibility of taking full advantage of cell-level switching (see below).

As per the power budget, the worst-case intermittent power draw is 18.7248W. Therefore, assuming the worst-case of deeply depleted batteries at 2.8V/cell, the 2S series pack must be capable of discharging at 18.7248W/5.6V = 3.3437A. The common 3400mAh NCR18650B Panasonic cells have a max discharge of 4.8-6.8A, so even just one parallel branch (a 2S1P configuration) would be sufficient to serve the worst-case load spike - and with over 40% margin of safety too!

The battery pack must be sufficiently sized to at least support the continuous load of 0.4719W for the maximum eclipse time of 2165s. This requires 1,021.53J of energy. The satellite should also be able to execute an imaging pass at any time throughout a given orbit, which requires 1,236.5J of energy. Together these loads require the battery pack to have 2258J. A single 3.7V, 3400mAh cell holds 45,288J nominal, so with just two cells in series this basic requirement is already exceeded by over 4000%. Therefore a basic 2S1P configuration is enough to meet all the basic capacity and peak load requirements - but parallel cells will be added to both increase the fault tolerance and lifetime of the pack as well as to increase the energy buffer for multiple image bursts. The limit is a 2S3P configuration, which would store roughly 75Wh of energy. Any additional cells would push the pack capacity over the 80Wh threshold beyond which NanoRacks requires much more rigorous thermal runaway suppression structures (see: NR-SRD-139). A 2S4P configuration would also slightly exceed 100Wh and violate DIETR-0190.

Cell-Level Fuses

If a pack has parallel cells but only pack-level current monitoring, it would not be protected from a failed cell potentially shorting out its parallel cells in a pack-internal short. A low-cost way to protect against this cascading failure is to add a PTC fuse in series with each cell. PTC fuses are commonly used for this purpose as well as general overcurrent protection in lithium ion packs and some cells even ship with PTC fuses pre-installed inside their casings.

Cell-Level Switches

Some designs use a pack-level bilateral switch, which is logical if the pack is purely a series configuration since any one cell failure results in an overall pack failure. However, if the pack has cells in parallel with the failed cell, cell-level bidirectional switches allow the failed cell to be disconnected and the rest of the pack could then continue to operate at reduced load and capacity. The BMU HAL would provide the status of cells and modified load and capacity limits for Computing to allow the appropriate modifications to the task scheduling. This increases the fault tolerance of the battery pack, at the expense of additional efficiency losses, additional cost, and additional component count.

An alternative to a bilateral power MOSFET switch is a solid-state relay. They are slightly more expensive than an individual MOSFET but less expensive than multiple, and take up a lower footprint. However, the on-resistance is higher than a MOSFET implementation, and most importantly an SSR does not offer four-state switching.

Characterisation

NanoRacks "Flight Acceptance Test Requirements for Lithium-ion Cells and Battery Packs" (NR-SRD-139) outlines the characterisation testing required on each individual cell for flight. Cells should be assigned serial numbers for associating test data and recording the cell's history, as some tests are harmful and disqualify a cell from flight (see: Cell Level Protection: Verification).

All test data must be timestamped, saved, and marked with the cell's serial number. For this purpose instruments with data logging and file export features are needed. Usually HP/Agilent/Keysight instruments have these features. Pictures must also be taken throughout the test process. Ensure that the serial numbers are clearly marked on each cell and visible in the pictures.

Reported test data includes:

  • Visual inspection for external case imperfections
  • Length, width, and height to 0.1mm precision (use digital calipers)
  • Mass to 0.1g precision (use a digital kitchen scale)
  • Open-circuit voltage
  • Closed-circuit voltage under 0.5C load (use a rheostat)
  • Current, voltage, and temperature during three full charge/discharge cycles using CC/CV

Acceptance testing for self-discharge must be performed. Cells should be discharged to manufacturer minimum and left on the shelf for 14 days while logging the open circuit voltage. Any cells that self-discharge more than 2.0mV are to be failed.

No specific accept/reject criteria is specified for the other test results.