Batteries

Introduction

There are a wide variety of battery chemistries available for use as the main traction battery of an EV. To use each chemistry safely, and to ensure an adequate service life from the battery pack it is important to understand the requirements for the chemistry you are using. Failure to do so may lead to premature or catastrophic failure of the pack.

Good pack design will allow for a nominal amount of abuse. People make mistakes and the pack should allow a margin for safety - and for longevity!

Lithium Iron Phosphate (LiFePO4) Batteries

Lithium Iron Phosphate (also known as LFP, or LiFePO4) batteries offer a good compromise between safety, energy density and ease of use for DIY conversions. They are available in a number of formats, commonly pouch cells and cylindrical cells.

LiFePO4 Pouch cells

The majority of this content is drawn from this thread: https://endless-sphere.com/forums/viewtopic.php?f=14&t=38761 discussing the use of the A123 20Ah pouch cell. However, many of the general points apply equally to other similar pouch cells.

General build requirements

Pouch cells are vulnerable to damage from debris, and must be held in compression (see the datasheet for your battery, but 10-12 psi is recommended for the A123 pouch cells as a guide). A rigid container capable of preventing damage and providing compression is therefore required. Be aware the cells expand and contract in use, so allowance for this must be included in the structure of the case.

The pouch cells should be separated to prevent abrasion between cells, and also to avoid the development of hot spots. Prebuilt modules from A123 systems had thin foam sheets or heatsinks between each cell. Be sure to avoid any debris that could rub on the pouch surface, particularly if using recycled cells.

Mylar, 'Fish paper' or a compliant foam may be appropriate materials to serve this purpose. This material should not be flammable. If the material is heat insulating, it is important to address thermal management.

Compression

Compression is required to prevent premature failure of the cell. Without compression electrolyte will become unevenly distributed, causing current gradients in the cell and uneven heating. Local temperatures can become high enough to form gas formation leading to cells 'puffing up' even when the pack is otherwise held within temperature and voltage constraints. This will be exacerbated in packs with otherwise poor thermal management. Compression forces gas generated to the margins of the cell, outside of the cell stack, minimising its effect cell performance. Gas in the middle cells will create a dead space which does not store or release energy.

There is ~1% expansion through a discharge cycle. As the cell ages, the nominal cell thickness can grow by 3-5%. For A123 cells the ideal pressure is between 4 and 18psi with the ideal pressure being ~12psi. Maintaining 12psi can increase the life by 500 cycles over that of 4 or 18psi

There is some suggestion that in uses where 1C is never exceeded compression may not be required.

Highly rigid endplates with a mechanism to allow for a limited degree of expansion (e.g. steel bands) are considered an effective solution to this challenge.

It should be noted that compression is a challenge specific to pouch cells. Cylindrical cells are designed to maintain their own compression within the cell's electrode stack by their design.

This thread provides more information and experimentation relating to pack compression: https://endless-sphere.com/forums/viewtopic.php?f=14&t=52244

Pouch Cell Pack Design Examples

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Notes regarding recycled pouch cells

Pouch cells are somewhat fragile, and breaching the insulation is not difficult, especially in a cells removed from existing packs and repurposed. If the pouch has had their poly-layers compromised you may see a number of faults:

• Black spots around the perimeter of the cell indicate electrolyte leakage
• Voltage on the outside of the bag. Note that microvoltage between the pouch and the electrode is normal (and due to a capacitive effect).

While the majority of these cells should no longer be in the market, a significant number of faulty cells made it back into the 'greymarket' in around 2013. These cells had misaligned tabs which can also lead to isolation failures between the tab and the pack. These cells should be avoided, particularly in high demand applications.

Cylindrical cells

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Situations likely to cause cell failure

Taken directly from wb9k's post on endless sphere in the A123 thread

1. Overcharge. Any extended time above 3.8 Volts will generate enough heat and electrochemical activity to puff a cell, especially one that is improperly compressed.
2. Overdischarge followed by charge. Any A123 cell that has been pulled low enough to come to rest at <300 mV should be immediately scrapped. The published number for that is 500 mV, but the real figure is closer to 300, so that's a "safety buffer" if you will. Below this Voltage, the Cu electrodes start to dissolve into the electrolyte. When charge is applied, the Cu forms dendrites that puncture the separator layer, forming an internal short in the cell. This can puff a cell in a hurry---the more charge current on tap, the worse it's prone to be.
3. Driving a cell negative. I've neglected to mention this before, but it is a possibility. I don't know much about the specific mechanism at this time.
4. Malfunctioning or misinformed electronics. This is the most common cause of all of the above in my experience. At this stage of the game, it is critical for YOU to understand how your BMS functions on at least a cursory level. Choose your BMS very carefully and periodically verify that it is operating properly. They're not all created equal. Make sure V sense lines are securely connected and free of corrosion. Just because your BMS says there was never a problem doesn't necessarily make it so. Avoid harnesses or ribbon cables between multiple modules if possible--they are problematic wherever they are used in any mobile electronics.
5. Exposure to or generation of sufficient heat. I don't know exactly at what temperature gas formation begins in the electrolyte, but we spec a max storage temp of 80 (or 85?) degrees C and I suspect this is the reason. The hotter, the puffier--to a point. This is why soldering tabs poses a real hazard to cell health. If you feel you must solder, sink or blow the heat away from the body of the cell. Use a big iron that can make sufficient local heat quickly, before the whole mass of the cell gets hot. You might even get the cell warm enough to melt separator if not careful.
6. No compression, not enough compression, improperly distributed compression. This is a pack/module design issue. Apply 10, maybe 15 psi to your cell stack end to end and then band snugly and evenly. Use hard endplates of some sort--never wrap cells directly or allow their shape to become distorted. Protect all areas of the pouch from impact damage. This obviously does not apply to cylindrical cells.

Cell aging for LiIoPO4 cells

Derived (barely paraphrased) from wb9k's post on endless sphere in the A123 thread

Capacity loss is caused by the Lithium that was available for storage becoming permanently plated on the cathode. Being unable to move within the cell it is no longer available to store energy. The impact of this plating is greater than the amount of lithium 'lost' to plating because not only is the lithium no longer available, it is also preventing access to that part of the cathode meaning Li that can still move has to take a longer path to reach the cathode. Lithium plating is one cause of increased cell resistance (there are others), a sign of worsening cell health.

There is no linear relationship between actual capacity loss and impedance rise. However some cell defects will also increase impedance.

Increasing cell resistance may cause a number of symptoms which may be confused with High Self Discharge.

1. Elevated Peukert Losses. As more energy per amount of current through the cell is lost as heat, the cells useable capacity decreases. So the apparent capacity loss is higher than the actual capacity loss of cycleable lithium. When used in low current applications (e.g. solar energy storage) the actual and apparent decrease in capacity will be small. In high current draw applications (like EV traction packs), the Peukert loss increases proportionally, so the apparent capacity loss increases much faster than the actual capacity loss.
2. Greater voltage excursion under the same load. Due to increased cell resistancethe voltage will sag further under the same load than a cell in optimal condition. The inverse is also true, the voltage will be higher for the same amount of charging current applied. The cell will then rebound to a voltage further from the loaded and charging voltages. This, obviously, can look like high self discharge but is a different phenomenon.
3. Absolute maximum current decrease.

Elevated impedance causes a more complex constellation of symptoms, some of which may be easy to confuse with High Self Discharge (HSD). Ohm's law (E=I/R) holds the key to understanding here.

1) Elevated Peukert losses. Because more energy per unit of current through the cell is lost as heat, less of the cell's capacity is actually USABLE. Thus, apparent capacity loss can be significantly greater than actual capacity loss caused by the loss of cycleable Li alone. In low current applications, the two numbers will be close together. In high current applications, Peukert losses increase in proportion, so apparent loss of capacity breaks further and further away from actual capacity loss as current increases.

2) Greater voltage excursion under the same load. Elevated resistance across the cell means that voltage will sag more under the same load than it did when the cell was healthier. Conversely, voltage will rise higher with the same amount of applied charge current than it did when it was healthier. At the same time, rebound/settling voltages will be further away from loaded/charging voltages. In other words, the cell will rebound to a voltage further away from loaded voltage, all else being equal. Similarly, voltage will settle farther from the charge voltage with the same charge applied. This can give the illusion of elevated self-discharge, but the phenomenon is actually not the same thing. Again, the greater the charge and load currents, the greater the effect becomes.

3) Absolute max current decreases. Because the cell's series resistance is elevated, the maximum possible current through the cell is decreased.

Just to confuse things further, there can be many factors that lead to impedance rise. Some are related to Li plating, others are not.

Lithium Ion Batteries

Lithium Ion (LiIon) batteries have a greater energy density than Lithium Iron Phosphate batteries, but have more challenging needs to use safely. The ideal operating range of LiIon batteries is between +15 and +45°C. The upper limit of temperature is particularly important as LiIon batteries experience thermal runaway - an unstoppable chain reaction that can occur in milliseconds releasing the stored energy in the cell. This can produce temperatures of 400°C and a fire that is extremely difficult to put out. Thermal runaway can start as low as 60°C and becomes much more likely at 100°C

Risk factors for thermal runaway:

• Short Circuits - either internally or externally
• Overcharging
• Excessive current draw or when charging

LiIon Pouch cells

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18650 and other cylindrical cells

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