In the past 20 years, the best ebike batteries used lithium-ion 18650 batteries. Lithium ion occupies a dominant position as the best technology, and advanced chemical technology continues to increase capacity and reliability. Lithium ions provide the most energy for size. It is reliable, portable and economical.
Over the years, many alternative battery chemistries have shown promise. Lithium polymer (LiPo), nickel metal hydride (NiMH) and lithium iron (LiFe) have all been tried, but none of them can improve lithium ions.
However, heavy investment in a series of new technologies has expanded the range of electric vehicles before they need to be charged. The most promising technology that began to appear was the rechargeable hydrogen battery.
Rechargeable hydrogen batteries are now on the market, although they are still in the early stages of production. In 2021, this new form of electric bicycles will begin to be promoted in Australia. If this technology proves to be successful, hydrogen batteries for electric bicycles are expected to reach Europe and the United States in 2022.
Since the introduction of cost-effective hydrogen batteries is new to the market, it is still too early to say whether they will start the electric vehicle power revolution.
Ebike battery replacement
You usually have one or two years of good use before the bicycle’s battery starts to fail.
For most electric bike owners, most ebike replacement batteries may be more than half the price of replacement bikes with the latest batteries, which is a shock. In addition, the rapid redesign of ebikes often means that it is difficult to obtain replacement batteries for your model bike.
When the battery is dead, ebike owners are forced to find low-cost alternatives, or completely scrap the bicycle. The problem is that the cost of a good replacement battery itself may be as high as an electric bicycle that contains the battery.
Why are cheap ebike batteries unreliable?
Battery Management System
BMS protects lithium-ion battery packs from a series of potentially damaging problems. The battery may be damaged if it is overcharged, overdischarged, discharged too quickly, or overheated. The BMS ensures that the individual cells in the battery pack operate within their prescribed limits.
There are two types of BMS. The most important battery management system is an overall management unit that manages all individual batteries in the battery pack. Each battery cell in a larger battery pack can also have a separate protection circuit.
A battery with an independent battery protection circuit can help further protect each battery. If the main BMS does not provide adequate protection for the battery, they provide secondary protection.
Low-cost batteries rarely use secondary protection in the battery because they are usually more expensive and slightly consume battery power. Ultimately, secondary protection within a single battery cell is unnecessary, because the main BMS should provide good protection.
Low-cost ebike battery packs use three-layer printed circuit boards, which are more prone to failure and cannot be repaired cost-effectively. Production line experts suggest that due to the warpage effect, three-layer printed circuit boards have a 30-40% failure rate when they leave the assembly line.
Three-layer printed circuit boards save a small amount of cost compared to four-layer technology, but when price is critical, reliability will suffer.
China is the world’s largest manufacturer of cheap batteries. Multilayer printed circuit boards (4-layer or 2-layer) are more reliable, but difficult to obtain, and the quantities produced in Chinese factories are different.
4-layer printed circuit boards are more complex, and the design and production costs are higher. In addition, although 2-layer printed circuit boards are generally cheaper, circuit boards are larger and heavier than printed circuit boards with more layers.
Electric bicycle battery explained
In order to correctly understand the problems we encounter in low-cost lithium-ion battery packs, it is necessary to quickly explain the structure of the battery pack.
Almost all ebike battery packs build larger storage capacity with higher voltage by connecting some smaller lithium-ion batteries together.
Electric bicycle battery explained
In order to correctly understand the problems we encounter in low-cost lithium-ion battery packs, it is necessary to quickly explain the structure of the battery pack.
Almost all ebike battery packs build larger storage capacity with higher voltage by connecting some smaller lithium-ion batteries together.
Lithium-ion battery pack structure
Introduction to 18650 cells
Most electric bicycle battery packs use 18650 lithium-ion batteries. These small cylindrical batteries are slightly larger than the 1.5-volt AA batteries that have been used in flashlights at home for decades. However, the 18650 battery is more impactful than the average AA battery.
The rated voltage range of each 18650 battery is 3.7 volts, although the storage capacity of different brands and models of batteries may vary greatly.
Build a higher voltage and capacity battery pack
Series connection
To make a 36-volt battery, we connected 10 3.7-volt batteries end to end.
18650 cells wired in series
18650 batteries in series
Although the nominal voltage of 10 lithium-ion batteries connected in series should be 37 volts, the market promotion of lead-acid technology in the past still exists. Sealed lead acid (SLA) batteries are manufactured in 12 volt increments, and 12v, 2v, 36v, 48v and 72v lithium ion equivalents can replace the old SLA unit.
The nominal voltage obtained by connecting 10 lithium-ion batteries in parallel is 37 volts. The voltage is rounded to 36 volts to maintain consistency and price/power/weight comparison with similar voltage lead-acid products.
Parallel connection
In order to increase the capacity, we can connect several batteries in parallel. Two 3600 mAh batteries are connected in parallel to provide a 7200 mAh battery.
36v ebike battery
Using 3600 mAh 18650 batteries, a 36 volt 10 amp battery requires a total of 30 batteries. (3 3600 mAh batteries are connected in parallel, 10 of which are connected in series).
48v ebike battery
Similarly, if we use 3600 mAh 18650 batteries to make a 48-volt, 10-amp battery pack, this will use a total of 39 batteries. (3 3600 mAh batteries in parallel, 13 deep).
In the past two decades in the sales and maintenance of lithium-ion electric bicycle battery packs, we have found that many low-cost battery packs have insufficient battery supply.
There can only be two possible reasons for this. Either the quality control on the assembly line is not good (or more likely it is non-existent quality control), or the assembly plant deliberately reduces costs. Using low-cost batteries, ebike batteries, we found missing batteries in about 1/5 of the test kits.
The numerous problems of low-cost battery packs force us to dismantle each battery pack and carefully inspect a long list of products. We usually encounter 1/2 battery problems, so we have to carry out quality inspections and fix the faults ourselves.
This can lead to unbalanced power and charging, which can quickly lead to battery failure.
Changes in battery cells
We often find that the battery cells in the battery pack are different. Some lithium-ion batteries have higher voltages or higher rated energy.
The rated voltage range of each 18650 battery is 3.7 volts, however, the storage capacity has been increasing over the years. Each battery can store 1800 to 3600 mAh of power.
In addition, the 3.7 volt battery is the rated voltage. The voltage of a 3.7 volt battery varies from 4.35 volts when fully charged (the latest lithium ion chemistry) to 3.0 volts when discharged.
In both cases, the vehicle management system cannot adjust the charging current correctly. Some batteries are over-driven, while others are under-charged. Batteries are sensitive to overcharging, overdischarging, and overheating. The mixing of battery cells can cause rapid deterioration of battery performance and early failure.
Incorrect wiring
We have several batteries with reverse wiring, the black ground wire is positive and the red wire is negative.
We have a batch of 100 batteries from a Chinese manufacturer, and the battery connection plugs are randomly connected. In 50% of batteries, the positive and negative connections are reversed.
In addition, the connector leads are also randomly connected, so 50% of the connecting leads are also reversed.
In addition, we also encountered the problem of reverse wiring of the battery charger connector.
There are many advantages to making your own battery pack. Manufacturers use rigid metal frames to hold individual batteries. Rigid battery frame cannot absorb shock or vibration well, and tight packaging will cause poor heat dissipation. Both heat and vibration are detrimental to battery life. Patience and thinking when placing battery cells and BMS can provide opportunities for looser packaging with rubber cushioning. A little consideration on the placement of the battery and BMS can have a great impact on the expected life of the battery pack.
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