fswiki.us - User contributions [en]

Battery Management Systems

2020-06-15T15:20:15Z

Hmarceau:

==Cell Balancing==
Cell balancing is required due to the inherent differences in the manufacturing of each battery cell. As cells are charged, some might charge sooner than others, reaching their maximum allowable voltage. When this happens, charging must stop to prevent the cell from being damaged. Unfortunately, this means that cells that aren't at their maximum voltage will be left undercharged. Later, when the pack is discharged, the cells that have less energy in them will discharge to their minimum voltage sooner at which point the battery must be shut off to prevent any cell damage. This again leaves many cells that aren't at their minimum voltage with energy still stored within them. This is wasteful and allows poorly performing cells to degrade more rapidly due to their increased charge cycles.
All of this can be mitigated using cell balancing which attempts to ensure equal distribution of energy amongst all cells during the charging and discharging processes.
===Active Cell Balancing===
===Passive Cell Balancing===
==Safety and Monitoring==
As lithium and some other battery chemistries can be quote volatile when outside of their operating range, it is important of a battery management system to be able to sufficiently measure properties like temperature and voltage of individual cells. This data can also be used to feed algorithms that provide insight like state-of-charge estimation.
===Typical Cell Safe Operating Ranges===

{| class="wikitable"
|- style="height: 17.1719px;"
| style="height: 17.1719px;"|Chemistry
| style="height: 17.1719px;"|Min Voltage
| style="height: 17.1719px;"|Max Voltage
| style="height: 17.1719px;"|Discharge Temperature
| style="height: 17.1719px;"|Charge Temperature
|- style="height: 16px;"
| style="height: 16px;"|Li-Po
| style="height: 16px;"|3.0V
| style="height: 16px;"|4.2V
| style="height: 16px;"|10°C ~55°C
| style="height: 16px;"|5°C ~ 45°C
|- style="height: 16px;"
| style="height: 16px;"|LiFePO4
| style="height: 16px;"|3.0V
| style="height: 16px;"|3.6V
| style="height: 16px;"|-20°C ~ 60°C 
| style="height: 16px;"|0 ~ 55°C 
|}

===Temperature Measurement===
As per FS rules and industry good practice, a significant number of cells should have their temperature measured at the negative terminal frequently to ensure safety and preventing events like thermal runaway from occurring.

Typically this is done by placing a thermistor on the negative terminal of the cell with a thermally conductive, but electrically insulating material in-between for purposes of electrical isolation.
===Voltage Measurement===
To ensure cells are not being over charged or discharged, accurate voltage measurement of each cell is needed. The battery management system only needs to measure each series connected cell as parallel connected cells will always have the same voltage potential.
==Note==
[http://fsae.polymtl.ca/ Poly eRacing](Polytechnique Montreal) developed an open hardware BMS, named BMSafe. You can reach out to them in order to get theschematic, code, and layout for free.

[[Category:Electric Vehicle]]

Battery Management Systems

2020-06-15T15:19:54Z

Hmarceau:

==Cell Balancing==
Cell balancing is required due to the inherent differences in the manufacturing of each battery cell. As cells are charged, some might charge sooner than others, reaching their maximum allowable voltage. When this happens, charging must stop to prevent the cell from being damaged. Unfortunately, this means that cells that aren't at their maximum voltage will be left undercharged. Later, when the pack is discharged, the cells that have less energy in them will discharge to their minimum voltage sooner at which point the battery must be shut off to prevent any cell damage. This again leaves many cells that aren't at their minimum voltage with energy still stored within them. This is wasteful and allows poorly performing cells to degrade more rapidly due to their increased charge cycles.
All of this can be mitigated using cell balancing which attempts to ensure equal distribution of energy amongst all cells during the charging and discharging processes.
===Active Cell Balancing===
===Passive Cell Balancing===
==Safety and Monitoring==
As lithium and some other battery chemistries can be quote volatile when outside of their operating range, it is important of a battery management system to be able to sufficiently measure properties like temperature and voltage of individual cells. This data can also be used to feed algorithms that provide insight like state-of-charge estimation.
===Typical Cell Safe Operating Ranges===

{| class="wikitable"
|- style="height: 17.1719px;"
| style="height: 17.1719px;"|Chemistry
| style="height: 17.1719px;"|Min Voltage
| style="height: 17.1719px;"|Max Voltage
| style="height: 17.1719px;"|Discharge Temperature
| style="height: 17.1719px;"|Charge Temperature
|- style="height: 16px;"
| style="height: 16px;"|Li-Po
| style="height: 16px;"|3.0V
| style="height: 16px;"|4.2V
| style="height: 16px;"|10°C ~55°C
| style="height: 16px;"|5°C ~ 45°C
|- style="height: 16px;"
| style="height: 16px;"|LiFePO4
| style="height: 16px;"|3.0V
| style="height: 16px;"|3.6V
| style="height: 16px;"|-20°C ~ 60°C 
| style="height: 16px;"|0 ~ 55°C 
|}

===Temperature Measurement===
As per FS rules and industry good practice, a significant number of cells should have their temperature measured at the negative terminal frequently to ensure safety and preventing events like thermal runaway from occurring.

Typically this is done by placing a thermistor on the negative terminal of the cell with a thermally conductive, but electrically insulating material in-between for purposes of electrical isolation.
===Voltage Measurement===
To ensure cells are not being over charged or discharged, accurate voltage measurement of each cell is needed. The battery management system only needs to measure each series connected cell as parallel connected cells will always have the same voltage potential.

Note
==Note==
[http://fsae.polymtl.ca/ Poly eRacing](Polytechnique Montreal) developed an open hardware BMS, named BMSafe. You can reach out to them in order to get theschematic, code, and layout for free.

[[Category:Electric Vehicle]]

Battery pack

2020-06-15T15:17:13Z

Hmarceau: /* How to properly use a battery pack */

==Introduction==
Safety & reliability should rank pretty high in design criteria as it's impossible to win a competition with a car who caught fire.
==Cell selection==
The selection process of a lithium-ion/lithium-polymer cell is highly dependent on the application it is being used for. Thus, before even starting battery design, it is crucial to understand the battery operating conditions. A light car with 2WD would requires less power than a heavy 4WD with wings. Generally speaking, there is a trade-off between cell energy density (Wh/kg) and cell power density (W/kg). It is then possible to compare the performance of two cell chemistries using lap simulation: there are cases where a higher specific energy cell is optimal, and others where high specific power is.
===Cans vs pouches===
Lithium cells come the most often in cylindrical cans format (i.e. 18650) or in soft metal-plastic pouches. It is easier to design a safe battery pack with cylindrical cells as each cells are protected in a hard shell that protects them from accidental perforation. Cylindrical cells tend to offer higher energy density, while pouches tend to offer higher power density. Cylindrical cells can be spot welded together with fairly inexpensive equipment, while pouches can be connected together by ultrasonic welding, laser welding, or mechanical assembly (rivets, nuts&bolts, ...). While cylindrical cells are typically found in small capacities ( < 4 Ah ), pouches can be found in high capacities ( > 10 Ah), greatly reducing the complexity of assembly of a battery pack, since fewer cells are required to meet the battery pack energy requirement.
====Pouches====
Pouches offer better heat dissipation than other type of cells, but their flexible and thin enveloppe make them prone to perforation or crushing. Some pouches have tabs on the same edge, some, on opposite edges. Opposite edge tabs allow better heat dissipation, but make packaging more complex. They stack easily, using most of the available space. The cells edges are fragile: treat them with care. It is preferable to ask the cell manufacturer to have the edges taped. Most pouches will ignite in case of a nail penetration. One way to reduce the risk it would ignite is to improve the cooling of the cell. As an example, using sheets of metal between cells would spread the heat (and evacuate it out of the cell).
====Cans====
Cans can be cylindrical or prismatic. They are typically safer than pouches, since they are are valve regulated and benefits from a hard shell, reducing their perforation and crushing risk. However, it is generally harder to cool them down. Cylindrical cans also have a lower packing factor than prismatic cells leading to an effectively reduced volumetric density. However, the process of cylindrical cell fabrication is so well honed that battery packs made with state of the art cylindrical cans compare in terms of metrics with battery packs made with state of the art pouches. Cans usually also contain a positive thermal coefficient (PTC) resistor that can protect the cell in case of a short circuit. However, this protection is usually not rated for high voltage packs, can could actually ignite a fire, in a high voltage pack. Cans also can ignite when perforated. Wrapping cells with a phase change material (PCM) is one of the solutions that have been developed to absorb heat in a thermal event, reducing the odds to create a cascade event.
===Cells capacity vs safety===
At the cell level, a smaller capacity cell is safer than a higher capacity cell: a small capacity cell may not store enough energy to generate a thermal runaway. However, it is false to assume that a huge capacity pouch is more dangerous than a couple of low capacity cells put in parallel. High capacity cells are made under extremely high quality control, while low capacity 18650s are not: it's more cash efficient to discard under performing these cells. Discarding a 240 Ah cell would be extremely costly for a manufacturer. Also, high capacity cells include various safety techniques (including internal self fusing) which intent is to prevent a problematic layer of the cell to make the cell enter a thermal runway.
==Packaging==
[[File:ParapostWC.png|right|middle|thumb|Pouch Cell Module Design Example. Credits: Poly eRacing 2014 (Polytechnique Montreal)]][[File:BP.PNG|right|middle|thumb|Cylindrical Cell Battery Pack Design Example. Credits: Poly eRacing 2016 (Polytechnique Montreal)]]A battery pack has to be designed to resists the mechanical loads and impacts it will be subjected to and the thermal conditions it may be exposed in case of a thermal runaway. Also, it must be designed considering electrical safety in mind.

Requirements may differ according to the cell type choice. Pouch cells use a thin flexible shell, which may easily by punctured. Hence, it requires an additional protection and more professional fabrication processes to protect it from being scratched, punctured, teared. Cylindrical and prismatic cells, on the other side, encapsulate the active materials in a hard shell. However, if the shell is made of a conductive material, it may require a dielectric barrier to avoid a short circuit when cells are packaged in series.
===Mechanical loads===
The battery pack must resists to impacts and vibration. Bolts need to have positive locking mechanism. The battery pack should be built strong enough in a way that cells are not crushed in case of a drop on one of its corners.
===Thermal loads===
Lithium-ion cells don't like heat. As per the rules, cells exterior casing shouldn't reach 60 Celsius. Although it's easy to circumvent the rules here, no one should do it.
===Safety===
Sharp edges inside the battery pack should be avoided. Appropriate clearance between wires should be maintained. Ensure that large voltage differences are far from each other. Positive locking mechanisms should ensure that screws don't loosen. It should be impossible to create a short circuit in case of the drop of tools. Think what could go wrong in case of an accident, and try to prevent it.
====Materials====
Dielectric materials and adhesives shall be fire retardant rated and be able to sustain to high temperatures.
====Grounding====
Conductive materials in the battery pack shall be grounded.
====Segregation====
As per the rules, modules need a fire retardant confinement to avoid a fire propagation within the battery pack.
===Interconnection===
Several techniques allow the cells interconnection. They are divided in two categories: the usage of mechanical fasteners and the usage of welding.
====Fasteners====
The concept behind the usage of fasteners is to compress the current tabs together. In this kind of joint, conduction increases with area and with pressure. Hence, the goal is to apply a load as uniform as possible on an area as big as possible.
=====Grease=====
With time, the interface resistance can increase, due to oxidation. One can slow down the oxidation rate with the usage of grease. Grease is actually non conductive but as little impact on overall conduction as it is squished away by the pressure. Some greases contains conductive particles in order to improve their conductivity. Examples are De-Ox or Penetrox.
====Ultrasonic welding====
Ultrasonic welding is one of the best available techniques to joint different materials (as found in lithium cells). However, these welding equipment is expensive. It's the recommended technique for pouch cells.
====Spot welding (also known as resistance welding)====
Resistance welding is speedy and low cost. It doesn't transfer a lot of heat to cell and welding quality is good.However, conductive materials (aluminum, copper, ...) require more power to weld than resistive materials(nickel, ...). It's the recommended technique for cylindrical cells.
====Brazing====
Brazing (soldering) is generally not recommended, because the heat required to melt the brazing alloy can damage the cells components.
====Crimping====
Crimping cell terminal is rare but can be done with extremely simple tools.

==Possible causes of a thermal runway==
Thermal runway (i.e. battery fire) can be caused by an internal short that would be due to a poor cell design and manufacturing quality control. However, it is more likely that it is due to abuse conditions.
===Abuse conditions===

* Charging when too cold
* Charging when too hot
* Discharging when too hot
* Storage at elevated temperature
* Internal damage due to high heat when interconnecting (by soldering)
* Overcharge
* Overdischarge or charging an overdischarged cell
* Overcurrent
* Crush
* Impact or drop
* Nail penetration
* Sharp chips / dust
* Leakage current

==What to do in case of a battery fire==
===Toxicity===
Lithium batteries are toxic. Especially their electrolyte, which is flammable and contains poison salts.
===What should I do?===
In case of a fire, ensure that nobody is in danger of breathing in the smoke. If nobody is in danger, sound the fire alarm and get away. If somebody is in danger, try to save them as fast as possible so you won't get hurt yourself, as people are more important that objects. Lithium caused fires are not easy to put out, even for people that know how. Smother the fire with Sand. ABC extinguishers will not do much except give the cause the fire to slow. If it starts within your car, don't think that you can save it, as its outcome is not good after it starts. Better yet, don't let batteries you own ever get to the point of causing a fire. This is the idea when using Lithium, you must prevent the batteries from getting to that point. too much voltage or too little voltage, too much current running through, or the temperature reaching too high, each have particularly nasty consequences. For any person using lithium batteries, it is wise to read up on this technology before using, so that one can become aware of the challenges of putting out a fire caused by lithium-cell batteries.

==How to properly use a battery pack==
[[Category:Electric Vehicle]]

The proper usage of the battery shall be limited by the battery management system (BMS). Here's a list of advice:
* As per the rules, avoid exceeding 60 Celsius degree.
* Do not overcharge (generally over 4.2V), nor over discharge (generally under 2.7V).
* As per the rules, ensure that the battery pack design does not allow the drop of tools to create a short circuit.
* Ensure that all conductive materials in the battery pack is grounded.
* Ensure that all materials of the battery pack are fire retardant.
The proper storage of the battery shall be:
* In a fireproof container.
* At room temperature or cooler (< 25 Celsius degree).
* In a stable position.
* In a well ventilated area.

2020-06-15T02:59:40Z

Hmarceau:

2020-05-18T13:43:27Z

Hmarceau:

==Function of a Differential==
When turning a corner, the outer wheels will follow a path with a greater radius of curvature. This difference in radius means the outside wheels will have to travel a longer distance, and rotate faster than the inside wheels.in a rear wheel drive car, the front two wheels are allowed to rotate at a different rate, so this does not pose a problem. The rear wheels, however, both have to be driven by the engine, and therefore are mechanically connected to each other, so there is no freedom of movement between them inherent in the design.

[need a pic of cornering to demonstrate wheel travel difference]

Or this youtube video: https://www.youtube.com/watch?v=yYAw79386WI (if anyone sees this before I get around to it, please embed the video -simon)

A differential is a mechanism that allows the two driven wheels to rotate at different speeds to assist in cornering.
==Open Differential==
An open differential is a gear assembly that allows the two rear wheels to spin independently of each other, while maintaining the average rotational speed. If the outside wheel spins faster than it would without the differential, the inside wheel must spin and equivalent amount slower. Similarly, the torque transferred to the ground, but is split in varying amounts between the two rear wheels depending on their relative velocities. The faster spinning wheel will see a proportionally greater amount of torque from the engine.

This torque differential between the two wheels can become problematic in high torque applications where the torque is desired to go to the wheel with the greatest amount of traction, not the greatest rotational velocity, such as corner exit, or corner entry if using an inboard brake. To solve this problem, a differential that can limit the amount of slip is desired.
==Limited Slip Differential==
https://www.taylor-race.com/sites/default/files/DIFFERENTIAL%20ESSAY%20W%20PHOTOS2.pdf

This (this what?) misses the ATV "chicklet diff". It also is a little biased towards ATB since TRE is a supplier of those. It is however a good starting point.

torsen

clutch-type

Even though this is for a game it does a really good job of explained a plated clutch type diff and some basic tuning: <a href="http://www.intothered.dk/simracing/differential.html">http://www.intothered.dk/simracing/differential.html</a>
==Spool==
==Electronic Differential==
It is fairly common to find electric formulas where the [[battery pack]] powers more than one motor on each side of the car. These two motors are then controlled individually and behave like if a virtual electronic differential was separating each other.

[[Category:Internal Combustion]]
[[Category:Electric Vehicle]]

Drivetrain

2020-05-18T13:35:23Z

Hmarceau: /* Final Drive */

A system is needed to transfer mechanical energy from the engine to the ground (are we including tires here? I always think a drivetrain discussion without talking about tires isnt complete -simon). The train takes the rotational mechanical energy from the engine and transfers it to the wheels.
=Gearing=
Gearing determines crawl speed, top speed and torque availability at any vehicle speed in between.

Need to also be mindful of how much tractive for you will be generating in any given gear. This plays with tire selection and helps determine what happens when the driver firewalls the go pedal.

This also plays into forces on the differential and driveline components.
==Motorcycle Gearboxes==
Motorcycle engines have internal sequential gearboxes. This is means that the gearing is located inside the crankcase. The oiling is managed by the engine's oil system. Typically these are 5 (YZ450) or 6 speeds (CBR600RR).
===Clutch===
The clutch on motorcycles is a wet plate clutch. Unlike a car clutch, the motorcycle clutch uses a set of linear springs or a diaphram (less common). When engaged, the springs squeeze a series of clutch plates (explain what these are) tight enough to restrict relative motion. This mechanically links the engine output to the crankshaft. When disengaged, the plates are under no pressure and have the ability to rotate relative to one another. This allows the engine to spin without outputting a torque. [we should get a simple diagram of how the torque flows through the engine here]

Talk about slipper clutches
==Continuously Variable Transmissions==
These allow for an infinite combination of engine to wheel speeds. The benefits of this include allowing the engine to be run at peak power, peak efficiency, or peak torque at any vehicle speeds. The downsides are lower efficiency and a lower peak torque transfer.
==Shifting==
"stick"

electronic - solenoid

pneumatic

automatic (electronic or pneumatic)

not going to touch on driver controls (paddles or whatever)
=Final Drive=
======Chain Drive======
======Belt Drive======
======Driveshaft======
======Gear Drive======
======CVT======
======Direct Drive======

=Differentials=
{{main|Differential}}

=Driveshafts=

* CV Joints
** tripods
* flex plates
* Half-shafts
** material
* lateral play
* do we wanna talk about splines?
* Torque Steer baby - probably make another page for this for details on concept and derivation

=Validation=
In vehicle simulations, including driveline affects become important the more detailed the vehicle model gets. The brake torque and horsepower are not what is seen by the tire's contact patch. A physical measurement of driveline forces should be undertaken at some point. It is not likely needed to do this every year however it should be done at least once to correlate model to reality.

This is typically a rotating measurement and typically involves the use of strain gauges. Wireless amplifiers are often needed. Several companies make them available such as Texense, Izze Racing and Lord Microstrain.

--Can talk more about the measurement in the electronics section. Maybe keep this higher level? You could right a whole section just on measurement techniques, uncertainty analysis, error influence coefficients.I think it is worth calling out a couple companies so teams at least have some direction. In the case of wireless strain gauge amps you can go down a real rabbit hole and not find what you need. As far as the gauge install itself, I think that is a lot easier to self research.[[Category:Internal Combustion]]

[[Category:Electric Vehicle]]