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The Dark Wizardry of the Toyota Prius Battery

EVs, hydrogen, hybrids, or plug-in hybrids?
In recent years, few discussions have become as entrenched as the alternative powertrain debate. Some argue that EVs are the only way forward. Others propagate the virtues of hydrogen. And that’s not to mention hybrids and plug-in hybrids.

A car for every need

There have been many misunderstandings about this, but at Toyota, we don’t really have a preference. That’s to say, we do, but not exclusively. Rather, we believe that each technology has its own merits, and that the best choice depends entirely on the usage. For last-mile-mobility in busy cities, EVs are hard to beat. 

For large vehicles that cover hundreds of miles on a daily basis, hydrogen (aka fuel cell) holds the most promise. And for a mixed use – sometimes in town, sometimes on long distance drives in the open country, hybrids and plug-in hybrids will prove the winning bet. And what’s more, this is a technology that is already within the reach of thousands today. In 2016, about 28% of the cars that Toyota sells in Europe will be hybrids.


A balanced symbiosis

So, for a hybrid, the EV-range is the single most important element that determines its efficiency, right? Wrong! A hybrid, and by extension a plug-in hybrid, stands or falls with the cleverness of its entire hybrid system, and how the engine, the motors, the battery, and the regenerative brakes work in balanced symbiosis to squeeze that extra meter out of every drop of fuel, or every kWh of electricity.

A study conducted by the University of Rome shows exactly this. 20 drivers went out in a Toyota Prius, to conduct the same 37km commute and back through Rome’s notorious city traffic, repeatedly and at different times of the day. Together, they accumulated 2,200km. Now, anyone who reads through the technical spec of Prius, will find that its EV range is only about 2km – hardly enough to make a dent in a 37km journey. Yet the experiment showed that on average, the drivers managed to cover 62.5% of the test course distance with the combustion engine switched off – or 23km.


Size isn’t everything

So, what is this dark wizardry that allows the Prius battery to multiply its range ten-fold? Well, the principle is pretty straight forward: the battery is constantly topped up by the hybrid system, using two different power sources. The first one is regenerative braking – a motor/generator is used to slow the car down. The energy that is otherwise wasted as heat during braking, now gets recycled into electricity and stored anew in the battery.  In the experiment, this accounted for about 1/3 of the electricity ultimately used to drive the car. 
The second way of replenishing the battery, is through the thermal engine. Rather than allowing it to run at engine speeds where efficiency is low, it is kept as much as possible near the peak of its efficiency – any excess power produced is transferred back into the battery. So, hybrid and plug-in hybrid batteries subscribe to that general truth in life – size isn’t everything. Sometimes, it is simply better to be clever.