Lithium-Ion Batteries for Electric Vehicles: Page 3 of 4

Longer life. While this can vary widely depending on factors such as daily depth of discharge and LA battery type (marine, golf-cart, AGM, industrial, etc.) regularly used and properly maintained EV-type FLAs have a typical life of about five years; LFP batteries have a typical life of 10 years. With only 12 years of data on LFP technology, their true longevity is uncertain.

The Main Disadvantage

While Li-ion batteries offer many benefits for EV applications, the main disadvantage (other than cost) compared to FLAs is the need for a battery management system (BMS), particularly while being charged. The job of a BMS is to monitor the voltage and temperature of each cell to protect them from excessive charging and discharging. While any battery system, whether it be FLA or Li-ion, can be improved with a BMS, they are not typically used with FLA cells because, as long as all the batteries in a pack are of the same manufacturer, model, and age (ideally from the same manufactured batch) and have been treated equally, the individual cells tend to behave the same while being charged. However, LFP cells, even of the same manufactured batch, can vary in capacity, leading to dangerously elevated voltages on the full cells as the others are still being charged.

FLA cells tend to be fairly tolerant of brief periods of high charge voltage (it is recommended to periodically elevate charge voltage, known as an equalization charge, which gasses the electrolyte vigorously in an effort to remove water/acid stratification and bring weaker cells up to full). With Li-ion batteries, even a fraction of an hour at elevated voltage can cause damage. Highly overcharged cells will swell and create internal gas pockets that prevent electrolyte contact with the electrodes. This usually permanently damages the cell (cells can be damaged and not show swelling). In extreme cases, the swelling can cause a cell case to rupture, releasing volatile organic solvent gases, which can be caustic and flammable. It should be noted that LA batteries have their share of dangers, including explosions, and a very hazardous electrolyte.

A BMS protects individual cells from over-voltage by shunting current around the full cells when they reach their “full” voltage, which allows the other cells to continue to charge. A good BMS can also detect when a cell is beginning to overheat (another sign of pending danger to cells) and shut off charging to the pack to protect all of the cells.

A BMS can also help during discharge by signaling for disconnection of the load when individual cells drop below their minimum voltage. Cells discharged too deeply can be permanently damaged or, at minimum, have their capacity or cycle life permanently reduced.

All factory EVs, such as the Leaf and the Volt, have a BMS that performs these functions. Some also manage air or liquid cooling of cells, both during charge and discharge, to prevent thermal runaway (a condition where one overheated cell causes its neighbors to begin generating heat), and to help maintain peak battery efficiency. Typical EV conversions using prismatic, hard-cased LFP cells usually don’t need active cooling for the cells because the cases have separators and air gaps built into them. This helps spread out the heat, and LFP chemistry (unlike some lithium-oxide-based chemistries) does not contain much internal oxygen, which can be a catalyst to thermal runaway.

The bottom line is that the risk of damage or danger is far too high to use Li-ion batteries without a battery management system. My upgraded conversion uses a relatively high-end BMS manufactured by Manzanita Micro (about $35 per cell). An optional display shows the state of each cell—both its voltage and temperature—in real time. Being a data-geek, I like to keep an eye on the whole system.