All you need to know about rechargeable batteries

R1 - 2017-04-04 08:36

All electric vehicles have to be continuously supplied with a source of electricity. Had only the energy for electrical vehicles been supplied by means of cables, they would have to be very long and, thus, susceptible for tangling... Before any roads with induction-based technology are constructed and prior to any dreams of Nikola Tesla of wireless power transfer with regard to considerable distances coming true, batteries are the only solution we have.

In the case of electric unicycles, Li-Ion batteries are the most frequent solution applied. Unlike to what characterizes lead / acid batteries, nickel-cadmium or nickel-metal hydride ones, they are much less toxic (especially with regard to direct contact of a human with what they contain), being nearly 100% recyclable.

To make things easier, such terms as a battery and cells will be used as synonyms.

Energy density

Lithium-Ion batteries are characterised by relatively high energy density.

Currently, lithium-polymer batteries are known for their capability of reaching the highest energy density possible. Why aren't they applied in electric vehicles then? It turns out that lithium-polymer batteries are much less resistant. The way they have been designed makes those cells only placeable inside soft packages which consecutively makes them more susceptible to mechanical damage. In the case of any damage made to the inner part of a cell, a short circuit is created, leading to the release of an enormous amount of energy. It is accompanied by a short circuit, being a process one cannot really hold. The chemical reaction taking place there is not only rapid, but also very dangerous. Lithium-polymer batteries are also somewhat more expensive than lithium-ion ones, being very popular at the moment. It is possible that there will be the prototypes of unicycles equipped with lithium-polymer batteries to appear soon. There are also the works on the enrichment of lithium-ion batteries with grapheme being conducted now which – according to Tesla company (previously known as Tesla Motors) – will contribute to the increase in their capacity by approx. 40 % (namely – almost a half of its current one!).

Yet another option of lithium-ion batteries – namely lithium-manganese cells that are used e.g. in the electric unicycles of Solowheel company or the cars manufactured by Chevrolet Volt – is available even now. First of all, they are much more resistant to high temperatures – and this is mostly related to any mechanical damage or cases of short-circuit they cause as leading to ignition in an extremely rare way. Its lower energy density (lower by approx. 2/3, when compared to classic Li-Ion batteries), as well as a considerably higher price, are definitely the downsides of this solution that also makes the products of Solowheel respectively more expensive.

Still, there is nothing to worry about. Classic and well protected lithium-ion batteries are successfully used in Tesla cars – the safest and fastest mass-manufactured cars in the world – among others.

While deciding what electric unicycle to buy, it is not only worth spending a bit more money on it, but also to pay attention to the right brand in order to make sure that the batteries of our choice have been well protected. Renowned manufacturers who take care of the assembly process of their batteries (as well as of the quality of cells themselves) are e.g.: Kingsong, IPS, Gotway, InMotion or Ninebot (Segway). If it is still not enough for some, even a higher safety level (with regard to mechanical damage) is what Solowheel ensures.

Battery life span and how to extend it

Lithium-ion batteries are characterized by a decent life span that is determined with the use of the number of charge/discharge cycles.

The life span of lithium-ion batteries depends on a few factors. It turns out that if successive charging takes place when the battery still holds over 50% of the energy it accumulated, the number of charging cycles becomes nearly three times higher.

Discharge	Maximum number of charge cycles (approx.)
100%	500
50%	1400
25%	2500
10%	4600

This is the reason why it is worth charging one's unicycle after each ride. Lithium-ion batteries do not have any memory effect that is present in some older battery types (NiCd, NiMH). It forces the user to nearly totally discharge their devices prior to successive charging.

Other aspects that may influence the life span of a lithium-ion battery are the working and storage temperatures. The optimal temperature range is between 10 and 30 °C. In the case of storing one's unicycle for a longer time, e.g. throughout winters, its batteries should be charged to the level of approx. 50% and stored in the temperature of about 15 °C.

What is more, the storage time of lithium-ion batteries that exceeds a month means one needs to charge them up to the level of more than 50% at least once a month.

It is worth knowing that lithium-ion batteries undergo two-phase charging. The first phase relies on charging through the "injection" of energy in the form of a constant current into the battery. The maximum charging current is determined by the multiplicity of C – where "C" stands for the battery capacity measured in amp-hours (Ah) – with its value depending on the battery type and quality. "1C" stands for one-hour charging current which means that a battery charged with such current should be fully charged within an hour. Each manufacturer of batteries indicates what the maximum current to charge a given battery is, while its value usually falls into the range of 1C to 5C. For example, if a given battery of 10 Ah capacity needs to be charged and it may be treated with the use of 5C current, the maximum charging current could be set to the level of 50 A, thus making the battery fully charged after approx. 12 minutes. At the same time, such high charging current considerably shortens the battery life and it is not applied in practice. The lesser the charging current, the longer the battery life span is. 1C level makes for a reasonable compromise between the charging time and the life span of a battery.

Standard chargers used in unicycles have their current efficiency set below the level of 1C, mostly for cost-related reasons. In the case of KingSong KS16 unicycle, its charger allows for the use of current up to 2A level as the maximum value.

After the level of 80% of charging of a given lithium-ion battery is reached, which also concerns its target voltage, the second charging phase activates – charging with constant voltage. It turns out that it is possible to extend the battery life even twice if we interrupt the charging process at this precise moment. While 80% charge clearly means decreased range, most manufacturers found a "sweet-spot" stopping charging somewhere between 90-100% of the real charge, to offer the best possible range together with good battery preservation.

The impact of temperature

The electric unicycle is excellently suitable for rides and the pleasure it provides is not only limited to summertime. Winters are also great seasons for this means of transport – and not so only in the case of the most ardent unicycle riders. However, one needs to be aware of the fact that the nominal parameters of batteries get decreased at lower temperatures. Worry not though – taking winter rides does not make damage to batteries and after a winter tyre has been applied, it is still possible to use one's electric unicycle.

At the same time, one can't really count on reaching the maximum range their unicycle offers. Temporary capacity drops appear once the temperature reached 10 °C, while under -10 °C it begins to dive down. Kingsong have determined the decrease in the range of its devices at 50% to occur not before the temperature drops below -20 °C.

Riding one's unicycle at low temperatures also implies a considerable decrease of battery energy efficiency.

After the temperature drops under -10 °C, only 50% of one's battery power becomes available. In such an event, one needs to reduce rapid acceleration and braking, as it may turn out that the battery is not capable of supplying the device with sufficient power that is needed by its motor; that may consecutively lead to a sudden shutdown of a unicycle while riding.

Simultaneously, it needs to be indicated that it will all get back to normal after the winter is finished.

BMS - Battery Management System

The Battery Management System is an electronic circuit placed inside all battery packs of lithium-ion batteries that are applied in electric unicycles. This circuit-based system is that a lot of myths and conceptions have arisen with regard to.

A considerable part of unicycle users think that it is the BMS that is responsible for the cases of sudden shutdown of wheels while riding. Many would like to remove it from their devices for good, claiming that it is unnecessary and only endangers their safety. Reality is different, though. It may occur that some manufacturers have failed to properly set the limit parameters of their BMS's, due to which they are too restriction-sensitive and disconnect their devices from power supply even at the cases of slight overloads. In the case of electric wheels, each BMS should be properly adjusted to cutting off the power supply only when the overload reaches its critical level, as exceeding it might permanently damage the battery. Such shutdowns, taking place at certain overload levels, are indispensable – the reason is that if they have not appeared, batteries could be subject to permanent damage (it would not be possible to use one's unicycle any longer without the necessity for replacing it), also causing the risk of a short circuit affecting the device.

Cell protection is only a part of functions performed by the BMS. The most significant task of a BMS is the maintenance of proper parameters in the course of charging, as well as balancing the energy from various cells in order to optimise the efficiency of a whole battery. The BMS system monitors and transfers such parameters as current, voltage and temperatures to a communication bus on an ongoing basis.

Summing it up, there is no way a battery could properly operate without the BMS.

Ecology

The supporters of combustion engines complain that the swap of a traditional vehicle for an electric one is not the right solution, mainly because of the manufacture and recycling of batteries. This misconception has its roots in a number of old myths that are repeated on the media that do not tend to verify their sources.

Would you like to know why lithium-ion batteries has a beneficial impact on environment? Click here.

Summary – the devil is not as black as it is painted

Everyone being concerned with the question of battery life related to their unicycles should approach it in the following way: the minimum number of charging cycles of a unicycle amounts to 500; is it really not that much? Lithium-ion batteries are used in the majority of smartphones and netbooks. These devices need to be charged practically every day. 700 charging cycles are correspondent to about 2 years of use; after the period passes, the operating time related to the battery becomes shorter than in the beginning. The initial 500 charging cycles should make the battery working with no considerable decrease in its capacity. If a given unicycle has the range of 30 km coverable per one charge, it is possible to calculate its life span on the level of 15,000 km (counted as 500 cycles multiplied by 30 km). If we charge our unicycles more frequently, the battery life may only undergo an increase (see: the life span table above). It really does not happen very often for anyone to be able to cover 1,000 km a year. In the case of such extreme use of a unicycle, a well-maintained battery should be available for use even for 15 years. There's nothing left to do but riding!

As it is perfectly well known, the market of electric unicycles is developing in a very dynamic way. I cannot imagine riding a single wheel for 15 years only to change its battery and ride it for the next 15 ones. And you? Have your say in the comment section!