Charging a production electric car these days is relatively easy: you pull up at a charging station, open the charge port hatch, and plug a compatible charging lead into your car. It’s a simple action which takes at worst 20 seconds to accomplish if the charging station is free to use, and not much more if you have to first swipe a payment or authentication card before plugging in. While you’re off doing other things, your car — and its on board computer — charge the battery pack until it hits a predetermined charge level, power or time limit, or the battery pack is full.
Although the act of plugging your car in to charge is a really simple one, the science behind getting energy from the charging station into your car’s battery pack is the complete opposite. As well as the age of the vehicle and its battery pack — something we’re discovering ourselves with our aged Transport Evolved Toyota RAV4 EV staff car — the chemistry of the battery pack, its temperature, and the capabilities of the charging station are all significant factors. Which is why automakers spend a lot of time researching and developing new charging profiles for their electric cars in an attempt to ensure each battery pack lasts for as long as possible.
Traditionally, automakers have been particularly cautious about the upper and lower charge limits they set for their electric cars in order to facilitate a long battery life. While a battery pack may have a theoretical capacity or 24 kilowatt-hours, for example, the automaker may use just 19 or 20 kilowatt-hours, sacrificing a bit of storage (and thus range) to ensure long life.
As Automotive news (subscription required) detailed on Monday, Japanese automaker Nissan has figured out how to cram extra electrons into the battery pack of its LEAF electric car without changing the upper and lower capacity limits of its battery management system: treat electric car battery packs a little like a pint of beer.
Or rather, to treat the charging of a nearly-full electric car battery pack the same way as you would a nearly-full pint of beer being filled from a tap.
As you charge a lithium-ion battery pack, the current flowing through the battery from the charger gives ions at the positive side of the battery enough energy to move to the negative side of the battery, where they stay until discharging takes place. Then the ions make the opposite journey back to the positive electrode, giving up their gained energy in the process and producing an electrical current.
For the charge to be stored however, there need to be tiny microscopic free ‘slots’ at the negative electrode. When the battery is reasonably empty, there are plenty of spaces on the negative electrode for those free energy-filled ions to attach themselves to. Thus, it’s possible to charge at a reasonably high current, moving lots of ions across to the negative electrode rapidly. When the battery nears full, and all of those spaces are taken up, the ions already at the negative electrode push back on any new ions trying to occupy the same space, heating up the battery and making it harder for other ions to pile on. The more ions there are trying to pile onto an already crowded electrode at the same time, the harder the push-back (resistance) becomes.
Like a beer glass being filled quickly from the tap and half-full of bubbles, the battery gives the impression of being full even if it there’s still space on the electrode for some extra ions.
Just as a good barkeeper waits for the bubbles to subside before topping off the pint glass, Nissan’s engineers decided to see if giving a nearly-full battery a short break during the final few percent of charging would allow it to encourage more ions to make their way onto the negative electrode. It set up three short micro charges spaced five-minutes apart at the end of the charging cycle, alternating brief low-current charging with short breaks.
Those rest periods — where the battery’s voltage will naturally sag slightly as a small amount of self-discharge and ion realignment happens — makes it easier for fresh ions to find a place on the electrode.
As Nissan explained to journalists earlier this week, its new charging profile allows Nissan’s engineers to squeeze and additional 0.7 percent extra energy into the battery pack without changing cell chemistry or capacity. More stored energy translates to longer range. And while it’s not exactly a huge amount — just 0.75 miles — it all counts, especially on limited-range electric cars like the Nissan LEAF.
At this point we should probably note that Nissan isn’t doing anything particularly new: we’ve seen battery experts advocate just the same kind of behavior over the years with certain types of battery chemistries — but it’s good to see the automaker work hard to squeeze every extra kilowatt-hour out of a battery that it can.
Should you try something similar yourself? It’s probably not a great idea: Nissan’s new charging behavior will be carefully controlled by its battery management system. Trying to replicate it without the proper equipment is likely to do more harm than good.
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