More Batteries and Boxes

I also completed a new battery box for the rear cargo area. This 34 1/2 inch x 18 inch box is only 10 inches deep. We cutout the lower section of the cargo area where the spare tire tub is and cutting it out was quite a little task. We use a thin carborundum blade on a little DeWalt 4 inch grinder to do such work. But this hole, complicated by the round tub extending downward to accommodate the tire, had Brian and I taking turns at the terror.

We put some 1.5 inch aluminum angle iron on the front and rear long edges of the box at the top. We pushed the box up against the hole from the BOTTOM, and lipped a piece of aluminum stock over the front edge from the top. This allowed us to drill some holes and basically make an aluminum sandwich across the car at the front lip of the hole.

On the aft end, we could not access the top for a bolt because a member under the door jamb is formed. We drilled a 5/16 hole and tapped in 3/7 bolts – five of them, to hold the back edge flush. Then we took some pipe strapping and ran straps from the structural member on each side, lengthwise under the box. This should be reasonably sturdy.

The box will hold about 265 lbs of batteries, or some 39 cells. That’s a 14 kWh pack just in the spare tire area. And this leaves room for some Brusa chargers on top, and then the standard cargo area floor panel.

At this point, we could almost put the rear seats back in and have an untouched interior, with all cells below floor level. The 24 cells in the rear seat area total 8640 wH so we have 22640 wH in the car entirely below floor level. Not bad. That’s about the size of the Porsche 356 entire pack.

And indeed, that would give us a range of about 75 miles and a working voltage of 226 volts. That voltage is a little lower than we can deal with because as the batteries discharge, these Blue Sky’s are innately lower voltage than the Thunderskays and you can take them down as low as 2.0 v which would be dramatically below the 180 volt minimum for our controller, and coincidentally, our DC-DC converter.

So the plan is to build a 29 inch by 30 inch box that will sit atop the back seat area across the support members. This baby will hold 55 cells for an additional 19,800 watt hours bringing our pack total to 42,400 wH and 424 volts. This is a little high on the voltage, but the Blue Skys come down to 3.4 volts essentially as soon as you remove the charger. That’s more like 401.2 volts or 1.2 volts over our controller maximum. I think that dog will hunt, just barely.

Better, again assuming 300 wH per mile, that’s a theoretical 140 mile range with an 80% DOD at 112 miles. I actually think we’ll average somewhere between 250-280 wH per mile without air conditioning or heat on. So I think we’re looking at 125-130 mile real range, dropping to 110 or so with air conditioning. This should be a serious upgrade from the BMW Mini-E with regards to REAL range (as opposed to press release range). And this makes sense. They have a 34 kWh pack and we’re coming in at about 42.

As Ron Popeil says, but wait there’s more.

I had planned the Porsche originally to use 160 Ah cells. But they were just too big. The car had so many compound curves that we kept getting beat out by a 32nd of an inche everywhere we looked. We dropped to the smaller 90 Ah cell and of course used more of them, but they fit better because of the smaller granularity.

So going to the Mini, I started with an order for 100 of the Blue Sky 100 Ah cells. You have to order these three months ahead of time.

Well, the Mini isn’t a Porsche. In fact, once you get inside, it is a bit boxy in shape. And so we’re winding up with these hugish rectangular boxes.

So while we COULD make our large box 10 inches high, I’m going to fabricate it to 13 inches.

Why?

This is cool. As it turns out, I can put 40 of the larger 180 Ah Blue Sky cells in the same space as the 55 100 Ah cells that way.

Additionally, I can put 20 of the 180 Ah cells in the back seat boxes in place of the 24 100 Ah cells. And indeed, it appears I can put 24 of the 180 Ah cells in place of the 39 100 Ah cells in the aft box – although they will stick up an inch above the box rim. The 180 Ah cells are 11 inches tall.

What this means is that we could ( or you could if you like) replace the 118 cells of 100 Ah each with a scant 84 of the 180 Ah cells. This results in a nearly perfect pack voltage of 285-302 vdc. But it gets better. Because of the much larger current capacity of the 180 Ah cell, our pack storage rises to a whopping 54 kWh and our range is starting to look like something around 180 miles. Yes, that’s right, while BMW suffered some pretty serious embarassment about their puffed up range figures once the car hit the road, the Clubman we’re doing would be able to do a smart 150 mile range WITH THE AIR CONDITIONER GOING FULL BLAST THE WHOLE WAY.

And with a little careful driving on a pretty day with the air con and heat off, I dare dream of a solid 200 mile run.

The only fly in the ointment, weight. Along with the additional power comes more weight. Our 750 lb pack will swell to about 1050 pounds using the larger cells. We should still be below gross, but not by much. The car won’t accelerate as smartly, by about 300 lbs, and so there is a price to pay.

Speaking of prices, our Blue Skys at 100 Ah are justt a little under $14,000 with shipping. Going to the 180 Ah cells will run us up to about $16,500 with shipping. Neither count spares. You want to order spares because you need to have some cells on hand if some go bad, and ideally you want them from the same manufactured lot. A year later, your cells may not be available at all and three years later they probaby won’t be. So we always order a few extra.

So our battery boxes are by design meaured out to accommodate either cell size. Your mileage may vary.

Finally, Brain has something special up his sleeve.

Recall that we are going to mount the largest battery box, 30 inches wide and 29 inches deep, on TOP of the back seat area. Of course, the two smaller boxes reside where the gas tank did, beneath the seats. What this means is that we’ll have to remove ALL the batteries from the top box, all the straps and connections, and the box itself, to access the lower batteries.

As I HAVE been known to make changes after we have the car all put together, Brain has been grumbling about this for a month now. He found a set of rails used by railroads incidentally, to make battery boxes slide out affairs. These rails cost about $500 delivered, but they sport a 1000 lb rating. And he’s dead set on designing a rail system for the upper box to allow it to SLIDE back into the cargo area, over the batteries THERE, revealing the lower battery boxes. Much like a kitchen drawer.

I think is going to be hard. I always think mechanical things are hard. That’s why we mostly have Brain do such things. And he has that thin-lipped look over this one that means he’s pretty set on it. Me? I’ll watch that one on video, thanks.

Jack Rickard