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Hi there,

I am new to EV conversion, or rather, new to engineering as a whole, although I have a reasonable background in physics.

Would the space left behind after the fuel tank of an ICEV is removed be a possible place to store lithium ion battery packs?

I was thinking that the cavity could be covered by welding some sheet metal on, and mounting batteries to that so that they sit between the original undercarriage
and the piece of metal. Of course there would need to be a good amount of insulation foam to absorb any shocks and keep the batteries being jostled too much/at all.
This could then be covered with some sort of plastic cover to make it look less out of place and at least help protect the metal from corrosion.

The only problem might be getting cooling systems to reach the batteries in this location, as from what I've read, lithium ion batteries definitely require some cooling to improve their
lifespan and mitigate any fire hazard.

Am I completely barking up the wrong tree here?
I haven't been able to find any mention of the above idea online, and most people will use any remaining space under the bonnet, and space in the trunk or where the spare tire sits.

Many thanks.
 

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The fuel tank space is rarely large enough to contain much of the battery, so additional spaces are needed. Fuel tanks in most vehicles are shaped to fit the available space, working around other components; battery modules are generally rectangles that won't fit well into irregular spaces. So, use that space, but don't expect to fit much there unless you're converting a large truck.
 

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The bigger issue is weight even if you could get it to fit. A full gas tank is only about 50kg, which is not much battery capacity. It'll handle like it's pulling a large caravan with a 350kg tongue weight

Take a look at Damien McGuire's Youtube channel. One of his BMW's, the E39, has a split pack with one in the "boot" and one in the former engine's bay.

 

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To give you a practical (but hypothetical) example, I've got an 80% designed split pack design for my conversion aimed at maintaining the front to back weight ratio of my donor vehicle. Removing the engine but re-adding motor and accessories from the front removing ~200lb from the rear (read: the gas tank and support structures), I need a 9:5 ratio of battery split front to rear. That means putting a pretty small amount of my battery pack in the back and adding BMS, contactors, fusing, and box structure before putting the bulk of the batteries up front. This is still worth it to me as it maintains the original vehicle balance front to back, and leaves a lot of engine bay space up front.

tl;dr: It helps to get 5/9ths of my batteries in the gas tank areas but depending on your range requirements you probably can't only use the former gas tank area.
 

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The fuel tank space is rarely large enough to contain much of the battery, so additional spaces are needed. Fuel tanks in most vehicles are shaped to fit the available space, working around other components; battery modules are generally rectangles that won't fit well into irregular spaces. So, use that space, but don't expect to fit much there unless you're converting a large truck.
A good example of this. Without cutting, this is what I came up with for stashing 5 VDA-355 batteries and very rough supporting hardware in the former gas tank area. Apologize for the indistinct view but point is a simpler arrangement would be preferable if I weren't trying to tuck these batteries in the former gas tank area, but I'd prefer them to be tucked up and out of the way of collisions from the rear or bottom side so the extra complexity in the battery box is worth it to me. (Ignore the battery module polarity, just a quick mockup)

Yellow World Slope Font Urban design
 

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Yep, I'm putting 3 Tesla modules in where my gas tank was. Needed the space. It will only be about 50lbs heavier than a full gas tank, so weight shouldn't be an issue. I have 7 more modules going in elsewhere.
 
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