What to Look Out for in an e-Scooter Battery?

 

Introduction

Lithium battery production is complex. The next most challenging thing to manufacture would probably be a computer chip.

 

The battery cells are packed in the most stringent cleanroom environments to ensure that not a single dust particle enters. After that, it goes through a series of activation, testing, and rejections before preparing to ship. It is a costly and hazardous process to ensure the batteries in our e-scooter today are safe to ride.

 


This machine handles only one of the many processes.

 

 

Most e-scooters on the market use a widespread format call 18650 (refers to the dimension of 18mm by 65mm); The biggest manufacturers for the 18650 are Samsung, LG, and Panasonic, with Chinese companies like BYD, EVE, LISHEN, and BAK gradually taking more market share. 

In addition, batteries that often fail strict quality control, such as those rejected from the above companies and counterfeit, are smuggled out and sold at a lower price. Such cells can cause increased internal resistance, reduced performance, expansion, easily build up heat, and even combust into flames.

 

Therefore it is vital to get the cells from a reliable supplier. The cell itself does not hold much energy, they have to be packed into a battery pack to store enough energy and output enough voltage. Here are some examples of how a battery pack is properly assembled:

 

  • Cells are structurally secured and protected.

  • Properly welded (ultrasonic welding) and not soldered (some rare design uses novel material for conduction).

  • Adequate spacing for cooling.

  • Battery Management System (BMS) should carry out standard protections such as over/under voltage, over/under current, balancing etc.

  • The pack is protected from short circuit.

 

Unfortunately, unless the pack can be safely opened, most people will be unable to tell if the cells are genuine and the pack is properly assembled. Thus end-user has to place faith in the product brand itself.

 

Lithium battery has a life cycle and requires replacement after some period of time. A typical Samsung 35E will drop to about 60% of charge after 500 cycles. So a 10km range brand new battery will last about 6km after 500 cycles of use. Therefore the used e-scooter battery has a poor resale value. Towards the end of life, the lithium should be properly disposed of. There are companies such as NEU Battery Materials that offers recycling solution for lithium ion batteries such as lithium iron phosphate. 

 

Portable e-scooter’s smaller battery pack

 

Most consumer e-scooter uses a battery pack about 6~10Ah, 1~2 kg. which ranges from 20 to 30 cell. Most of the design are inbuild in the scooter, only removable for professional servicing. Few e-scooters are designed with a swappable battery pack for user. A small battery pack with about 30 cells is optimum in keeping the structural integrity while easily hand-carried.

 

Left: Battery for e-scooter Right: Battery for e-bike 

 

The cost of such battery pack would be a few hundred dollars, which is still affordable to replace. They are relatively easy to design and safe to operate.

 

Taiwan’s electric motorcycle company Gogoro wanted to design their swappable battery pack to be as light as possible while keeping to the structural integrity, they took 1 year to design the battery pack and came up with their swappable design with high reliability.

 

High power e-scooter’s big battery pack

 

Large battery packs for some high-end e-scooter can consist of 150~200 cells or more, more than 6 times the cells in a smaller pack. Although it is not as complicated as designing an electric car like the Tesla with more than 7000 cells, it is still moderately complex to design for more than 100 cells. Worst, if some scooter designer decided to pack more cells into a small form factor, they can neglect some structural design and impose higher risk for the consumer. Here is an example of how electric car lithium cells are packed:

 

Battery pack assembly by the TUM CREATE EVA team (two of the engineers now work with MIMO Motor), every cell is structurally supported with active cooling and battery monitoring system.

 

Properly designing the large battery pack requires careful engineering and handling, it often contributes additional volume/weight to keep the battery safe from vibrations and impact. In some case the total battery pack weight will be double of the cells combined weight. Therefore, some hands-on enthusiasts and few e-scooter manufacturers would want to pack more cells into the limited compartment and likely ended up like this:

 

A poor example of large e-scooter battery pack: cells are not supported, packed closely to each other with tape, and ends are soldered, which will change battery chemistry and high probability of failure and causing severe fire.

 

 

Such a battery pack can cost thousands of dollars, and the owner should expect a costly running cost in replacement after the life cycle.

 

However, there is one advantage of using the large battery for short distance (~10km) first/last-mile commuting. Each short distance trip consumes little of the lithium cycle, which will lengthen the total life span.

 

A good application for a larger battery in personal commuting is the cargo bicycle. In total volume and weight is less of a problem, they can afford to design a better protected large battery pack. 

 

MIMO's take

 

Whichever you choose, it all voices down to the purpose of your vehicle. If it is for personal transport, your vehicle is advised to generally not weigh more than 20kg in total to be easily carried up or down the staircase. As such, a smaller battery pack will be more than sufficient for first/last-mile commuting. At the same time, it is also safer and affordable for replacement.

 

On the other hand, although a large battery is undeniably suitable for long-range, the downside is that it is expensive for replacement and poses unnecessarily high risks if not properly designed. MIMO ardently believes in your safety, well-being and convenience, hence for all our products, we stand firm on using the smaller battery pack for e-scooter applications.

 

 

 

2021-09-06 - Edited by Witono and Adeline