BU-305: Building a Lithium-ion Pack

Building a Li-ion battery pack begins by satisfying voltage and runtime requirements, and then taking loading, environmental, size and weight limitations into account. Portable designs for consumer products want a slim profile and the choice is a prismatic or pouch cell. If space allows, a cylindrical cell such as the 18650 often provides the lowest cost and best performance in terms of specific energy, safety and durability. (See BU-301a: Types of Battery Cells)

Most battery packs for medical devices, power tools, e-bikes and even powertrains for electric cars (EV) are based on the 18650. This appears impractical but the small cell works well because it is one of the most mature Li-ion formats available, is produced in high volume and enjoys a low cost per Wh.

The cylindrical cell is not ideal as it leaves empty spaces in a mul ti-cell configuration. This disadvantage turns into an advantage when considering flexibility and cooling. The Tesla S85 EV uses over 7,000 cells, switched in parallel to boost the current and in series to increase the voltage. Should one cell in series open, the total power loss is minimal; if one in parallel shorts, fuse protection removes this cell from the circuit. Failing cells can thus be eliminated without bringing the battery down.

EV manufacturers are not united on the choice of cell, but there is a trend towards larger formats to reduce supportive electronics that adds 20–25 percent to the finished pack. With a larger cell, however, the electronic components get dearer because of higher current handling. According to 2015 reports, the Tesla S 85 has the lowest cost per kWh using the 18650. Other EVs have larger prismatic cells at higher kWh costs. Table 1 compares the kWh cost.

* In 2015/16 Tesla S 85 increased the battery from 85kWh to 90kWh; Nissan Leaf from 25kWh to 30kWh.

Batteries should be designed to permit failure without a catastrophic event. All energy sources will fail eventually and the battery is no exception. After an unwanted event, the FAA mandated to place the Li-ion ship-battery of the Boeing Dreamliner 787 into a metal container with venting to the outside. Tesla reinforced the EV battery by adding a heavy-gauge steel plate on the bottom that provides extra protection against projectiles from the road.

Large batteries for power applications are cooled. Some use a rod system to bring the heat to the outside, others deploy forced air or use liquid cooling. Liquid cooling is superior and although more expensive, EV batteries gravitate towards this form of cooling.

Meeting Safety Approvals

Reputable battery manufacturers do not supply Li-ion cells to uncertified battery assemblers. This precaution is understandable, considering that Li-ion cells could be charged and discharged beyond safe limits with inadequate protection circuits.

Authorizing a battery pack for the commercial market and for air transport can cost $10,000 to $20,000. Such a high price is troubling, knowing that cell manufacturers discontinue older cells in favor of higher capacity replacements. A pack with the new cell, even if specified as a direct replacement, requires new certifications.

The common question asked is, “Why are additional tests needed when the cells are already approved?” The simple answer is that cell approvals cannot be transferred to the pack because regulatory authorities place the safety confirmation on a finished product and not the components. The completed battery must be tested and registered to assure correct assembly and compliance with safety standards.

As part of the test requirements, the finished battery must undergo electrical and mechanical assessment to meet the Recommendations on the Transport of Dangerous Goods on lithium-ion batteries for air shipment, rules set by the United Nations (UN). The UN Transportation Testing (UN/DOT 38.3) works in conjunction with the Federal Aviation Administration (FAA), the US Department of Transport (US DOT) and the International Air Transport Association (IATA)*. The certification applies to primary and secondary lithium-based cells.

The UN 38.3 test includes:

• T1 – Altitude Simulation: Low pressure simulates unpressurized cargo hold at 15,000 meters.

• T2 – Thermal Test: Temperature extreme by keeping batteries for 6h at -40°C and then +75°C.

• T3 – Vibration: Simulates vibration during transportation at 7Hz to 200Hz for up to 3 hours.

• T4 – Shock: Simulates vibration during transportation at given G-forces relating to battery size.

• T5 – External Short Circuit: Short circuit with <0.1Ω at 50°C. Case cannot exceed 170°C.

• T6 – Impact: >20mm cylindrical cells are impact tested; <20mm cell types are crush tested.

• T7 – Overcharge: Charge at twice the recommended current for 24 hours (secondary batteries only)

• T8 – Forced Discharge: Same as T7, forced discharge with primary and secondary cells.

The test batteries must pass the tests without causing harm, but the packs do not need to function thereafter. The test is strictly for safety and not consumer endurance. The authorized laboratory needs 24 battery samples consisting of 12 new packs and 12 specimens that have been cycled 50 times. IATA wants to ensure that the batteries in question are airworthy and have field integrity; cycling the packs 50 times before the test satisfies this requirement.

The high certification cost discourages small manufacturers from using Li-ion for low-volume products and entrepreneurs may choose nickel-based systems instead. These batteries do not need to be tested to the level of lithium-based products for air transport. While reputable companies follow the instructions, rules are being broken and the penalties are stiff. ( See BU-704: How to Transport Batteries)

Simple Guidelines for Using Lithium-ion Batteries

• Exercise caution when handling and testing lithium-ion batteries.

• Do not short-circuit, overcharge, crush, drop, mutilate, penetrate with foreign objects, apply reverse polarity, expose to high temperature or disassemble packs and cells.

• Use only lithium-ion batteries with a designated protection circuit and approved charger.

• Discontinue using a battery and/or charger if the pack temperature rises more than 10ºC (18ºF) on a regular charge.

• The electrolyte is highly flammable and battery rupture can cause physical injury.

* IATA (International Air Transport Association) works with airlines and the air transport industry to promote safe, reliable, secure and economical air travel.