Lithium-Ion Batteries for Electric Vehicles

In 2007, I converted a GMC Sonoma from its original gasoline propulsion to pure electric, using flooded lead-acid (FLA) batteries (see “Born to be Wired” in HP122). The type of FLA batteries most commonly used for EV conversions, golf cart batteries, have three 2 V cells and a capacity ranging from about 200 to 260 amp-hours (Ah). Moving the truck’s 3,200 pounds required a higher voltage than the 96 or 120 volts commonly used for lighter-weight vehicles, so I used 24 batteries for 144 V and an energy capacity (at 100% discharge) of about 37 kilowatt-hours (kWh).

However, the battery weight (approximately 1,800 pounds) brought the vehicle very close to its maximum gross weight of 5,000 pounds. I expected my batteries to have a five-year life, but in the third year, they started to show signs of failure.

The short life boiled down to maintenance. I knew that the best practice for FLA batteries is to re-water them monthly if they are being cycled frequently (as they usually are in an EV). The charging process causes evaporation through electrolysis. I was usually good at watering the batteries, but on a few occasions, I postponed it, only to find that enough of the electrolyte had evaporated to expose the top of the lead plates to air. Exposed lead oxidizes, making it harder for the plates to interact with electrolyte and, thus, reduces their capacity.

This undoubtedly contributed to a shorter life, but the nail in their coffin occurred when I was unexpectedly called away for several weeks during the summer. In my original design, a daily timer was set on the battery charger to ensure the batteries were fully charged before I left on my morning commute. Without daily driving, the charger was excessively charging the batteries. This boiled off a significant portion of the electrolyte and overheated the batteries, causing them to swell.

I didn’t realize this until I tried to drive my vehicle and heard a “bang” in the battery box, and the vehicle lost power. Several of the batteries in the middle of the pack (those that got the hottest) had swollen—one had swollen enough to cause an internal short circuit, which ignited the gasses at the top of the battery. I replaced the worst of the batteries, hoping that the remaining ones still had some life. But after testing, I found that all of the remaining batteries had a significant reduction in capacity—the only solution was to replace them all.

Battery Shopping

Because I wanted my next battery pack to give me better service, I started investigating lithium iron phosphate (LFP) batteries, which had dropped in price significantly—from $75,000 for a 31 kWh pack in 2006 to $12,000 in 2010 (since then, prices have remained fairly constant). A comparable FLA bank was more affordable (about $5,000), but I was convinced that Li-ion batteries would improve vehicle performance (power, acceleration, range, and energy economy) and render a long-term payoff.

Switching to LFPs shaved almost 1,000 pounds from the vehicle, more than doubled the vehicle’s range, gained back the vehicle’s original acceleration, and nearly halved its energy use per mile. To see why I experienced such a dramatic improvement, we need to compare the batteries themselves.

The FLA reference battery is a standard 225 Ah, 6-volt golf cart battery (e.g., Trojan T105). The LFP reference batteries are high-capacity (180 to 200 Ah), 3.2 volt prismatic batteries. There are a variety of manufacturers and distributors for this type of battery, the most popular and available being the CALB SE180AHA, the Sinopoly LPF200AHA, and the FluxPower BATVXLFP200AH. In comparing, keep in mind that a golf-cart battery has three cells for about 6 V, whereas the LFP prismatics come as single cells (packs of 4 cells for 12 V are also available). The comparison table values based on watt-hours (Wh) provide an apples-to-apples comparison because they relate to stored energy. Except where noted, the table characteristics come from manufacturers’ specification sheets. The LFP column is the average of the specifications for the three popular LFP models, whose values differ by 10% or less.