Batteries for electric bikes: The ultimate guide

Batteries always play an important role in the range and performance of electric bicycles, but also, in many cases, the price of it is 40% of the total cost of the bicycle. That is why it is important to have the most information possible before deciding.

Which type of battery do electric bicycles carry?

The first electric bikes were powered by heavy lead batteries in two very similar variants called AGM-Lead and Gel-Lead. They differ from those of cars because these  are not starter batteries, but are controlled by deep charge and discharge charge cycles run; then it is the alternator that is responsible for providing power to the system. We can still see these AGM batteries in many Chinese electric bicycles, in vehicles for people with reduced mobility and in affordable electric scooters.

A leap in quality and lightness was provided by the next generation of batteries: Nickel Cadmium (NiCd) and Nickel Metal Hydride (NIMH). They are currently so disused that it is very difficult – if not impossible – to find cells to replace degraded cells. If your neighbor sells you his old bike with this type of battery, think that you may never pay off the investment. Replacing this battery with a Lithium battery can double or triple the price you paid for the bike.And then lithium came. And he did it to stay.Most stores specialized in medium and high quality  e-bikes are based on this chemical compound.

The advantages of lithium

Among the multiple advantages stands out the lower energy density per cubic centimeter, the absence of a “memory effect” and a greater number of recharge cycles. In other words: they are batteries that weigh 4 times less than lead batteries, you can always charge them at all times without waiting for them to discharge, and they last many, many more years.

The Chemistry of Lithium

We can find three types of lithium compounds: Lithium ION, Polymer Lithium (LiPo) and Lithium Iron Phosphate (LiFPO4). The latter have the advantage of having more recharge cycles, but they are very heavy. Polymer Lithium is quite versatile and cheap, but at this time, the industry clearly bets on Lithium ION cells in a single format: 18650. This number refers to the dimensions of the cell, so they can be adapted according to the demands of space and capacity regardless of who makes them.

What magnitudes come into play?

The energy capacity that a battery can store is expressed in Watts per hour (Wh), which is the result of multiplying the System Voltage (V) by the Amps at the time it is capable of providing (Ah). There are batteries ranging from 300 Wh to 700 Wh. The most widespread batteries from almost all manufacturers are around 400 Wh.

What is inside a Lithium battery?

Electric energy emanating from individual cells connected to each other. Depending on the configuration and the number of cells, the desired storage capacity is obtained. Each cell has a voltage of 3.6V. If we connect 10 of them in series, we will obtain 36V, which is the typical voltage with which most of the electric bikes on the market operate.

On the other hand, the most used cells have an electric charge of 2500 mAh (2.5 Ah). Therefore, if we use 10 cells in parallel, we would have a 36V x 2.5Ah = 90 Wh battery. Not bad, but not enough to do the Camino de Santiago. If we want to double the capacity, we only have to add one more cell in parallel to each of the 10 that we have connected in series, and so on. Large capacity batteries can have 6 in parallel and 10 in series, thus achieving more than 500 Wh. Easy isn’t it?

Apart from the casing, the battery level indication LEDs and the lock keys, you will see a very discreet but decisive component above the cells: the BMS (Battery Management System). This device has many tasks; some of them related to the safety and integrity of the battery, but the most important is that it is in charge of equalizing the charge and the discharge of the groups of cells, preventing the closest ones from degrading before, as happens with mobile batteries.

How many kilometers can I travel?

Among all the factors that determine the range of an electric bicycle, the capacity of the battery is as decisive as the rest, so we invite you to read this article in which we introduce you to the different characters that come onto the scene, from the engine efficiency up to what you had for breakfast this morning. So important are they, that the range can vary up to 80%.

If we focus on the batteries, it is obvious that the more watts an hour, the more kilometers you can do. Our experience tells us that with 400 Wh in Madrid or circulating on forest tracks and in medium assistance, many of our us exceed 80 km with a central system such as Bosch, Yamaha or Brose. If the motor is integrated in the wheel, the range is reduced by 30%. If you want to make routes that exceed 100 kilometers, we recommend a central system and a 500 Wh battery.

If you use the electric bike for rides in the city, it is not profitable to invest in more battery. A 280 Wh battery will allow you to travel 30-40 kilometers through urban traffic without sweating.

The useful life of Lithium batteries.

All batteries degrade without exception. Manufacturers insure their batteries for two years against manufacturing defects and also against more than 20% degradation, but the actual life span extends far beyond that. For example, using the great and affordable Urban Biker Mini around 7 years already and still there has been no need to replace any batteries due to extreme degradation.

In general, and if we stick exclusively to the battery, the life  span depends largely on the quality of the cells that are installed, so it is important to know which manufacturer is behind it. Another factor is the stored energy: the rate of degradation of the battery decreases with the capacity of the battery; the larger ones take longer to degrade.

Degradation by time and by recharge cycles

Some manufacturers such as Bosch or Yamaha ensure that these batteries are capable of withstanding 500 charge cycles (in their laboratories) while retaining 80% of their capacity.

If in each cycle it can make you run 80 kilometers, this gives us a life span of 40,000 kilometers. Is it true that they will be able to make a complete tour of Earth, or will the batteries die before completing their charge cycles due to the elapsed time? . We will be very pending.

Also, most of the time you charge the bike, the battery is not completely discharged (this is one of the advantages of lacking a “memory effect”). How do manufacturers compute these partial cycles? We promise to find out.

Idle discharge rate

Batteries at rest usually have a discharge rate that varies from 5% to 10% per month. Many battery models go into “hibernation” mode after several days without use thanks to the protection offered by their BMS, but despite everything, discharge due to inactivity always occurs.

Let’s say you take a long route and completely melt the battery. On the screen of your handlebar (or on the battery LED indicator) you will see that the battery indicates 0%. The bike stops working, but in reality, there is still 15% battery capacity left. Why? To avoid a deep discharge.

Let’s continue with the story: You get home and leave the bike parked without load in a corner for four months. In this time frame, if you haven’t charged it before, the battery has had time to fully discharge.

When the battery reaches a deep discharge, it deteriorates completely. The cells begin to swell (some of them give off a whiff of a compound similar to acetone) and the battery, simply, will die. It is irreversible.

What solution is there? Very easy: No problem leaving it unused for many months, but you have to leave it with some charge. According to many manufacturers, the best way to store a battery that is not being used is at 60% of its capacity.

Lithium is affected by temperature.

As we have already noted on some occasions, lithium also gets constipated. The batteries reach optimum performance with a service temperature of around 20ºC and decrease as the temperature drops. In other words: in winters you can travel up to 30% less kilometers than in summer.

When is a battery considered to have degraded?

If you already begin to suspect that you are not doing the kilometers you expected, there are gadgets that will tell you in a precise way the useful life of your battery: These are professional battery analyzers.

This appliance simulates a complete discharge cycle under actual conditions of use. The extracted capacity is divided by the nominal capacity and the percentage of degradation of the battery is calculated. As we have already mentioned, most manufacturers guarantee their batteries for 2 years, but in order to enforce this guarantee, many times the manufacturer requires that the battery be sent to its facilities with the consequent loss of time.

Life span and “economic” life span

There are specialists who consider the “economic end of the battery” when it reaches 70% of its initial capacity. Degradations of less than 70% are possible without substantially changing the speed and power performance of the propulsion system, but it all depends on the energy that the controller is demanding at specific times. Perhaps you can move without problems on a flat surface or dowhill, but when reaching unevenness, the BMS will stop sending current to the controller to protect the cells against extreme discharges.

But do not worry: when your battery reaches this state, you will have traveled thousands of kilometers for many years and you will have fully profited from your investment.

And the future?

It seems that the lithium-based cylindrical cell format has been implemented, but the industry is restless looking for higher energy densities, lower weight and lower costs, therefore energy storage systems such as graphene have appeared, a compound that promises to meet these requirements.

R&D is underway but unfortunately, the practical results can only be seen in the very long term.

For now we will just have to continue enjoying the wonders of Lithium, a white powder that makes electric bikes travel millions of kilometers every day.

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