BU-305: Building a Lithium-ion Pack

Examine the requirements for agency approval when building a Li-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 multi-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.

Make and model Cell type Cost per kWh Specific energy
Tesla S 85, 90kWh (2015)* 18650 $260/kWh 250Wh/kg
Tesla 48kWh Gen III 18650 $260/kWh 250Wh/kg
Best practices DoE/AABC) pouch/prismatic $350/kWh 150–180Wh/kg
Nissan Leaf, 30kWh (2016)* pouch/prismatic $455/kWh 80–96Wh/kg
BMW i3 pouch/prismatic N/A 120Wh/kg

Table 1: Price comparison of EV batteries. Mass production allows a low price using the 18650 cell.
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

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

Last Updated 2017-08-03

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Comments (20)

On May 25, 2011 at 2:25am
Kevin Fletcher wrote:

If a NiCD battery pack is exposed to extreme (cold) conditions, during storage / charging.
Is there a possibility that the battery may not charge correctly, and a cell explode during use.

On September 7, 2012 at 12:34pm
pankaj sharma wrote:

sir please tell me which battery is best suited for storing energy from solar panel and how many battery cells are required for 500 MW plant and what is block diagram of battery for simulation matlab

On February 22, 2015 at 3:00am
Patrick Go wrote:

Is there a company that rebuilds lithium-Ion batteries for electric bikes in the Los Angeles, California area?

On April 12, 2015 at 9:40am
milind Bhurat wrote:

Interested batteries news

On December 5, 2015 at 7:57am
Sampath kariyawasam wrote:

I like to lean about lithiyam iron battery.

On March 15, 2016 at 3:28am
Nikhlesh Ranjan wrote:

I am from India. I am working on building lithium ion battery for my e-bike. I want to make commercial pack. But reading this article, little scared to get certificate or license.
can you give some insights?

On March 17, 2016 at 10:20am
George L. Rodis wrote:

Hi there
I wanted to learn how to repair battery bpacks

On May 3, 2016 at 9:45am
ali rajab wrote:

please if you can help
how to repair my laptop sony vaio vgn-fz430e battery back (VGP-BPS8)
and if you please
cells connection diagram or hand schematic for cells connections
thank you and best regards
Ali Rajab

On June 14, 2016 at 1:12pm
Miles wrote:

“there is a trend towards larger formats to reduce supportive electronics that adds 20–25 percent to the finished pack”

20-25% weight? Cost? Could you specify?

On July 26, 2016 at 1:01am
Vaibhav Ingale wrote:

Is it possible to design a battery pack of Li-Ion cells that would provide 80V 200A at the Output (For at least an hour) ?
And How many minimum cells would be required to do the same?

On August 23, 2016 at 10:56am
Ravi Deonanan wrote:

I am from Trinidad I am working on building lithium ion battery for car audio systems I want to make commercial pack. But reading this article, little scared to get certificate or license.
can you give some insights?

On June 28, 2017 at 10:55pm
Jane wrote:

Based on above article the “Reputable battery manufacturers do not supply Li-ion cells to uncertified battery assemblers”, can you share the criteria/ requirement in order to be a certify battery assembler?

On September 20, 2017 at 6:32am
sivasankar wrote:

is the imbalanced cell will cause non-uniform cell discharge in a pack due to different temperature.

On October 19, 2017 at 1:08am
Josh wrote:

The article states strict requirements for Li Ion packs, what about LiFePo4? And if the design is for a large industrial pack that will never be transported via air from assembly to use what is needed for ground certification? Is there such a differentiation?

On December 20, 2017 at 11:43am
Ammon Balaster wrote:

We are introducing a high power portable tool with a .528kWh LiFePO4 battery. What testing is required to sell and transport this product worldwide?  Thank you for your help.

On January 21, 2018 at 1:54pm
Clampamp newton wrote:

The 18650 reputable cells are the leading market format as of 2018, cell balancing is required for charge/discharge.  I want to power a house with a 400 lb battery bank at close to 48v.  Parallell will be around 200, with current controlled with pcb chips and a possible arduno config.  The temp management is further complexed yet lead acid is way too heavy.  I need a group effort to bring the house battery to market.  Post your schematics as open source please to hasten the solar compatibility. 

On March 6, 2018 at 9:28am
Rodney Budell wrote:

How would I find the information (a good starting point) necessary to remanufacture Li ion batteries?

I live in an area that desperately needs good jobs for the residents.  I would like look in to remanufacturing batteries.

Any help would be truly appreciated.

On April 2, 2018 at 1:02pm
Tom wrote:

To assemble battery packs, what certifications would I be required to obtain as the assembler? I understand the pack I build would need to become certified (UN38.3, etc..). What regulatory issues would I encounter as the assembler/builder of battery packs?Would be using cells that are already certified. This would be done in the US, CA.
Want to make sure if there is a UL requirement to become a battery pack builder, I adhere to it. I have been unable to find any regulation. Thank you!

On April 6, 2018 at 3:38pm
Daniel wrote:

Thanks, great starting point. Very interesting and specific, in short good advice. Now I can start researching protection for short circuit batteries which will automatically remove the battery which I’m guessing will remove the battery in a group (a number of batteries in parallel) not individually as trying to protect every individual battery would add excessive electronics especially for these small batteries. My guess is bicycle batteries do not have this protection normally. Also my guess is as lithium ion needs this protection so does Lifepo4 but I’ll continue to research that.

On November 5, 2018 at 4:46am
Miqueias wrote:

Can I use a bathery Li-Po micro controller taken from a Sony bathery in a Tablet bathery with higher mAh and same voltage?