Lithium-Ion Batteries for Electric Vehicles: Page 2 of 4

Of the many advantages of LFP over FLA, the most important may be the difference between their cycle life versus depth of discharge. The LFPs can be discharged more deeply, which means more usable energy. A typical FLA golf-cart battery has a life of about 1,000 cycles when discharged to 50% of its capacity each cycle. An FLA’s life cycle drops rapidly when the depth of discharge is increased above 50%. LFP batteries have a life of about 3,000 cycles when discharged by 70% each cycle (about 2,000 cycles when discharged by 80%). This factor plays significantly into the total payback of these batteries.

Other LFPs vs. FLA Advantages

Lighter weight. For about the same usable energy capacity, LFPs are about one-third the weight. The reduction of weight contributed significantly to my vehicle’s increased acceleration and range, and decreased the amount of energy used per mile.

Less space. LFPs are about half of the volume of FLAs—I was able to consolidate all of my batteries in the bed of the pickup (instead of putting some under the hood), while retaining the pre-upgrade cargo space. This left more room under the hood for future enhancements, such as regenerative braking.

Improved capacity at low-temperatures. In cold (say, -4°F), the capacity of FLAs drops to about 50%. LFP capacity only drops by about 8% at that temperature. Although winters where I live aren’t that cold, I still had to reduce my winter driving range expectations by about 25% with FLAs—with LFPs my range reduction is less than 10%.

Steady discharge voltage & low impedance. An FLA’s discharge voltage tapers significantly as its state-of-charge decreases, whereas an LFP’s remains fairly constant until the battery is close to empty. Also, with one-quarter of the internal resistance (impedance) of FLAs, LFPs supply more power to the motor and lose less to heat. The steady discharge voltage and the lower impedance, along with the weight reduction, also improved the vehicle’s acceleration and range.

Higher charge & discharge current. LFPs can be safely charged and discharged at a much higher current than FLAs. A suitably large charger is capable of charging a nearly empty pack within about one-third of an hour (based on a 3C charge rate). To keep costs down, I kept my 30 A Manzanita Micro PFC-30 charger. The higher discharge capability allowed me to increase the battery amp setting on my Zilla controller, adding a few more horsepower to improve acceleration.

Less self-discharge. When not being charged, FLAs can lose 4% to 15% of their energy per month (depending on temperature), compared to 1% to 3% for LFPs. I can now let my EV sit for long periods without having to worry about recharging the batteries.

In addition to these measurable ones, there are some less tangible advantages that also make a big difference:

No idle memory. Although a phenomenon that is not well-documented, experienced EVers know that FLAs temporarily lose capacity when left idle. Prior to the battery upgrade, if the EV was idle for several days, the apparent capacity on the first drive/charge cycle was reduced by up to 25%. LFPs don’t experience this.

No maintenance. LFP batteries need no regular maintenance, eliminating the risk of damage that can result to FLAs if they are not watered—the reason that I got only 60% of the useful life out of my original batteries. There are “zero-maintenance” sealed lead-acid batteries, but these have a lower cycle life and a higher cost than vented FLAs, and they can still lose capacity if left in even a partially discharged state.