Ford awarded patent for rooftop EV backup battery

10 June 2023

Although it sounds like an obvious and unusual idea at the same time, the latest patent awarded to Ford raises more questions than it answers. USPTO released Ford’s patent application after granting it to the company and the file makes an interesting reading. The whole contraption was invented by a team of Ford engineers from Mexico and the application was filed in December 2021 - it took the USPTO quite some time to get to this one.

So what is it? In essence, it’s a whole EV battery on the roof rack. Before you start pointing out the obvious issues with that idea, let’s see how Ford imagines it could work. The premise is quite simple - a removable battery in an aerodynamic box that acts as a backup battery. It doesn’t provide power to the electric motors, it is only used to recharge the main battery pack, making the power electronics much simpler.

The pack can be recharged from either a domestic socket or a public car charger - nothing unusual here. It can be used to provide extra power for tools, lights - anything that needs electricity can be powered by this invention - again, that’s a straightforward idea. The team behind this application went as far as designing the enclosure to ensure proper thermal management - cooling when needed and heat protection from external sources. After all, a battery pack sitting on the roof exposed to the Sun all day may not be the best idea.

While the idea is great, the elephant in the room is greater - or should we say the elephant on the roof? Yes, the weight is such an issue that it’s a surprise a team of engineers wouldn’t consider this a problem. Obviously, they never tried loading up a roof rack with a canoe or a spare full-size wheel wrapped in a 35” tire. Those are light things - a 50 kWh battery pack can weigh anything between 772 lb and 1,323 lb depending on its chemistry.

Loading it up would require either a team of 20 people or a crane. But that’s not even it - you’d have to drive a vehicle with around 1,102 lb load on its roof - half a metric ton load {{2 meters}} (at least) off the ground. Going around any bend at more than 6 mph would result in scraping the road with side mirrors. There is a reason why all road-going vehicles have a roof load limit of 165 lb - it’s not because the roof would buckle in as a tin can, it’s because laws of physics can be unforgivable.

What were they thinking? Surely, they knew what they were doing, right? Well, nowhere in the patent filing do we see a mention of the type of battery. The usual lithium-ion is simply out of the question because it’s too heavy. Maybe a small, 10 kWh pack? That would still weigh over 88 lb, and it wouldn’t be sufficient enough to make much difference - and it wouldn’t be as large as the battery in drawings. How about an aluminum-air battery? Magnitude lighter, with great capacity, and perfect as a backup.

Ford would have to first fix the inherent issues with recharging this type of battery - they are called primary for a reason. Despite that, they are used as backup batteries in the military and telecoms and offer a reliable source of power. As a traction battery, they have a long way to go, and more development is needed, but as a backup source, they could just work. Is Ford hiding something? Or have the engineers simply decided to ignore this tiny detail?

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