Wednesday, August 12, 2009

Passenger Seat Battery Box

The passenger seat battery box is quickly growing into a "battery module". The more we chew on this one, the more work this one becomes.

We received the Blue Sky 100 Ah batteries from EVcomponents last week.

A lot of good news and some not so good. The specs for the cells are all off size wise. They list very different sizes from the Thundersky 90Ah cells. They are not different at all. They are the same case, and EXACTLY the same size as the Thunderskys. I have no idea why the discrepency.

This doesn't pose MUCH of a problem. The listed specs are larger than the Thundersky's. For example, the THundersky 90Ah cell is 61 mm thick while the Blue Sky is listed at 67 mm. It's not. It's 61mm. Since we had gone ahead and sized for the 67 mm, our box is too large. But not by much. In fact it's kind of nice to have some slop. We can stuff it with foam padding.

The cells tested out very nicely. I pulled a random cell and charged/discharged it twice. I got 106 Ah the first time and 111.83 Ah the second. If anything, the curve is sharper at the two ends than the Thunderskys, and flatter across the middle. It is supposed to be a lower voltage. You charge at 3.6 volts (odd, we were doing that with the 4.25 volt Thunderskys), and you can discharge all the way down to 2.0 volts.

In practice, not so much difference. The curve is from 3.3 to 3.0 volts, almost identical to the Thunderskys. There is nothing there from 3.0 to 2.0 volts. Two amp hours maybe.

But we did graph temperature. At a 99 Ah discharge rate over the course of a little over an hour, the Blue Sky because noticeably warm - 40C. Warmer than the Thunderskys.

Our largest box, which will now be on rails over the passenger seat, holds 55 cells.

This makes for a pretty "dense" pack of cells. In the Speedster, almost all our cells were in small boxes, and by far the majority hung out in the airstream below the car. So we didn't need to worry about cooling.

On this box, with this density of cells, note that the cells in the center of the box are quite insulated from the edge of the box by the other cells. And, the box is in still air in the car interior, no exterior air flow to carry away heat.

Heat is a funny thing. These cells start to suffer at 55C. With 40C with an hour of 100 Ah discharge, this sounds like we are in fine shape. We aren't. With the packing of the cells close together, and no air flow, we have just built a perfect way for heat to build up in the center of our box and it is quite cumulatiive. If you don't give it a way to escape, it just keeps building up.

With a number of cells in smaller boxes where the gas tanks were and where the spare tire is, THEY get plenty of cooling from the air passing over the surface of the box. And no cell is very far from the aluminum surface of the box.

What this sets up is a huge thermal differential and it is a RECIPE for cell imbalance.

So we have to cool the cells somehow.

The solution is to add a cooling fan to the box.

On the BMW version, they added cooling fans to the top of the battery pack. I'm not happy with that. First, we have a LOT of batteries in the gas tank area and spare tire area that BMW didn't use. This will give us a much lower center of gravity than they enjoy, and I assure you a HUGE impact on vehicle handling thereby. But more importantly, our box over the passenger seat then doesn't stack nearly as high as theirs. Even with the rail system, it will sit about a foot above the rear seat bottoms.

We are going to have a large flat surface to the top of the box, basically a LID on it. This will actually work fantastically to augment our cargo space. We are going to retain all the rear cargo space we already had with the Clubman, but we lose the additional space you get when you fold down the rear seats. This is partially offset by having this flat sturdy lid 10 inches above where the back seat bottoms would have been. This makes a perfect shelf for a brief case, small purchases, jackets, purses, wine bottles, notbooks, etc etc. And it allows us to load longer items like a two by four or something from the hardware store from the rear of the car. So I don't want a fan there. I want it flat and strong.

We have rails on both sides of the box. I don't want a fan at my elbow on the front edge of the box. So the ideal placement would seem to be in the rear cargo area at the rear face of the module.

And the fan needs to be substantial. We selected a COMAIR model JQ12BA available on eBay

This is not a little computer fan. It is a 25 watt unit drawing about 2.5 amps at 12 vdc. It puts out 235 cubic feet per minute and is about 150 mm square. It has some nice bearings in it resulting in a fairly quiet operation at 54 dBm. They run a little over $20.

We'll mount this on the aft end of the box by building a little diamond plate enclosure with some aluminum screen over it. This is mounted to the box and a hole cut in the box end panel to allow flow. We mount the fan so it sucks air INTO the box.

I'm not really sure how much heat will be generated, and under what operating conditions. I know it won't generate any while sitting at a stoplight, and quite a bit while accelerating, so we're guessing here. Let's not guess.

We'll put it on a thermostat. Again we found a very nice Hayden Automotive Model 3653 fan switch on eBay

This unit is actually a bit more expensive than the fan itself. But it allows us to set the turn on temperature anywhere between 32F and 240F and it features a remote sensing probel. Designed to be put on the radiator of a car, and to drive a radiator electric fan, this unit is well able to handle the 2.5 amps current we need to drive the fan. Better, we can mount this unit on our fan enclosure, and drill a small hole to route the PROBE into the battery box. We can then put this temperature probe dead in the center of the battery pack.

In this way, we can set the thermostat to turn on at about 100F, and when the battery case temperature reaches this, it will switch on the fan, which will then PRESSURIZE the box.

Why are we doing it this way? Well, the next step is to drill some very small escape holes in the aluminum bottom of the box. And by clustering these holes mostly in the center of the box, we direct the cooling flow past the center cells more than to the cells on the edge. We'll sprinkle a few along the edges as well.

The cells themselves feature a ribbed structure that creates channels just for the flow of such cooling air.

By having the fan blow in, the heated air will be expended DOWN into the cavity where our gas tank boxes are. While this may heat those cells a bit, probably not. This area is open to the underneath of the car, and those batteries are in the airstream.

In this way, we avoid bringing heated air into our passenger compartment, when we very well may be spending more energy to run the air conditioner. And by clustering more holes in the center of the pack, we can modulate which cells get how much flow to some degree.

Finally, the temperature probe is metallic and offers the potential to short some of our cells. So we have to sheath it in heat shrink tubing and put a hot air gun on it to create an insulating area over the probe piping. We'll leave the end probe itself bare, and stick it down between the battery cases in one of the channels formed by the cell ribs.

In this way, we can draw air from our passenger compartment, circulate it between the cells, and exhaust the heated air DOWN and out of the car. Our rail system still works. Our flat top cargo area on top of the box is intact, and the system only uses power if cell heat becomes a problem. If I was wrong and heating just isn't an issue, it will never come on.

But I'm not usually wrong. The 235 cfm may be over kill. The entire box is 9 cubic feet. But we are going to obstruct the fan with an aluminum screen, and the batteries themselves. It's doubtful it will actually put out 235cfm in operation. And overkill is always appropriate.

Finally, I'm still not onboard with the shunt BMS issue and most of the monitoring circuits out there. But we have a LOT of cells in this pack - 112. And not only can my thinking change, but the available BMS options seem to be growing and some show some promise. So we've decided to go ahead and wire the car from the get go with 16 gage wire to every cell in the pack. What we are going to do you may not want to deal with. It's a bit expensive. But we are using some 19 pin MILSPEC Amphenol connectors. We'll mount these to the boxes with the wiring running to each cell within the box This will allow us to PLUG INTO the box with an external maintenance shunt balancer, future BMS circuitry, or just a measurement box. My initial plan is to build a small box with a huge Soviet rotary switch and Digital Multimeter with a couple of connection terminals. This wll allow me to quickly cycle through the cells checking voltages, and If get to one with two little or too much in it, manually connect a load to bleed off some energy, or manually connect a small 12v charger to add some energy to individual cells.

I'll make this box with a plug to connect to these heavy duty amphenol connectors. Amphenol 97-3102A-22-14S to be specific? Why this one? We got a deal on 60 of these at $55 on eBay. They run about $24 apiece on Digikey and indeed we had to buy the plug full price. There is no magic on 19 pins. I suppose if you get to a size that would accommodate 56 pins, to do it in one connector, the pins get too small to carry any current. And if you have a smaller number of pins, you need a lot of connectors. It's just a judgement call. But in truth, the eBay deal on a sack full of them was what drove the decision.

Jack Rickard

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