[sabahtom]

This box takes 42 Hipower 100Ah. Brackets (in red) to be 25x2mm MS. Angle bar 25x2mm on the corners (not shown, no time). Air gap 10mm top, back, sides and between the two rows of cells. Box 2mm aluminium.

Motor and gearbox are at the front. Its a Kia Pregio minivan.

I can probably squeeze in another 8 to make the wiring easier, but the box won't fit so neatly between the chassis crosspieces.

In this drawing I've packed the cells between the chassis crosspieces. If I drop the box so that it sits under the chassis crosspieces I lose 50mm of road clearance and can fit in another 6 cells without making the box much bigger. I can get in all 50 if I put a triangular end on the box and narrow it down to hold another 4 cells standing upright.

How much work will I save myself by having two big boxes of cells with 50 cells in each? If I do 50 cells per box my most positive and most negative cables will each be about 1.5 metres long.

Thanks
Thomas

[sabahtom]

Hi everyone - if I could mention, the welder is waiting for a drawing but he hasn't done a battery box before, so I was hoping for some feedback on it before I do something expensive and wrong.

Cheers
Thom

[rwaudio]

Originally Posted by sabahtom

Hi everyone - if I could mention, the welder is waiting for a drawing but he hasn't done a battery box before, so I was hoping for some feedback on it before I do something expensive and wrong.

Cheers
Thom

I can't comment on the box much, but do you live in a climate where heating or cooling the battery pack is required? If so you would need to make room for insulation and a heating/cooling system.

Is the pack serviceable once installed even if it's just to measure cell voltages?

[sabahtom]

Originally Posted by rwaudio

I can't comment on the box much, but do you live in a climate where heating or cooling the battery pack is required? If so you would need to make room for insulation and a heating/cooling system.

Is the pack serviceable once installed even if it's just to measure cell voltages?

Hi rwaudio

Thanks, it will have a lid that opens outwards. Will have about 300mm of elbow room between bodywork and terminals.

I do need to put a fan in for cooling, but due to the deep puddles (roads department here is not good at making things flat) I'll probably have to feed it with cabin air.

I've been advised not to drop the battery box below the level of the bodywork halfway along the wheelbase (100mm was my plan), in case I have to go over someones driveway and it scrapes in the middle.

[dougingraham]

Originally Posted by sabahtom

I do need to put a fan in for cooling, but due to the deep puddles (roads department here is not good at making things flat) I'll probably have to feed it with cabin air

Unless the side opens so you can gain access to all the terminals I dont think I would do it this way. If there is room why not stand them upright. Then you would have good access to the terminals.

I have not made a box out of aluminum so I can't comment on that part of it. I chose to use fiberglass instead of metal for the battery boxes because fiberglass is not conductive and I could incorporate the insulation as part of the structure.

From what I have heard there is normally no need to cool the batteries even in places that are very hot in the summer. If your current is below the continuous discharge rating the battery should not overheat. On the other hand if you live in a place where the batteries can get colder than freezing then you might need to have the ability to warm them up.

Making the battery boxes has been the most difficult part of my build. I suspect that is almost always the case.

[sabahtom]

Originally Posted by dougingraham

Unless the side opens so you can gain access to all the terminals I dont think I would do it this way. If there is room why not stand them upright. Then you would have good access to the terminals.

Originally Posted by dougingraham

Thanks Doug. It will open out sideways. Putting them upright would mean cutting up the floor of the van, and pulling all the seats out if I want to take off the battery box lids.

I have not made a box out of aluminum so I can't comment on that part of it. I chose to use fiberglass instead of metal for the battery boxes because fiberglass is not conductive and I could incorporate the insulation as part of the structure.

I hope to split the pack in two, making 160kg per box. I was expecting to use 2mm x 25mm brackets to hold that up. Could you let me know the thickness of fibreglass and what kind of frame you used, and any suggestions for sizing mine?

From what I have heard there is normally no need to cool the batteries even in places that are very hot in the summer. If your current is below the continuous discharge rating the battery should not overheat. On the other hand if you live in a place where the batteries can get colder than freezing then you might need to have the ability to warm them up.

Most of the time the discharge will be well below the continuous rating of 3C, except for 3 minutes at a time twice a day when I climb the hill I live on.

Making the battery boxes has been the most difficult part of my build. I suspect that is almost always the case.

I'm such a novice at this that the best thing seems to be the most simple option, so if that is fibreglass with a steel frame and no ventilation I'll do that (I need to check the cost of it here). Then I can get it on the road and see how things work and save up for a round of upgrades.

Thanks
Thomas

[dougingraham]

Originally Posted by sabahtom

I'm such a novice at this that the best thing seems to be the most simple option, so if that is fibreglass with a steel frame and no ventilation I'll do that (I need to check the cost of it here). Then I can get it on the road and see how things work and save up for a round of upgrades.

I am not an expert in composite structures so I am feeling my way through this. And now that I am almost done with the rear battery box there are things I would do differently. One mistake I made was I removed too much material from the car so I have a hole in the rear deck of the car that is several inches larger than the outside dimensions of the battery box that must be bridged with something. It would have been better if this was the steel of the car.

For the battery box itself I used the high density blue foam insulation material that can be found commonly in the construction industry. This is 1/2" thick material with a moisture barrier bonded on both sides. I removed the moisture barrier and then cut the pieces to size with a hobby knife or utility knife and a straightedge. I was concerned that this material would not be strong enough to support the batteries but even bare it supports the weight of a single layer of my 100 AH cells without deforming. Since the box was to be supported from a lip around the top I covered the floor area with wax paper and with the box upside down glassed the outside of the box first with enough excess cloth so that there is a 4 inch lip on the floor. I used alternating layers of 7.3 ounce (per sq yard) unidirectional cloth, two in each direction. Then another layer for each direction of vertical side of 8.8 oz bidirectional cloth. So the outside verticals all have three layers of cloth and the outside bottom has six layers of cloth. I then reinforced the outside vertical corners with a bias glass tape. Then I turned it over and did the same thing on the inside, extending the glass over the lip that was formed when glassing the outside of the box. This ends up with approximately 1/8" thickness of glass on the lip of the box. What I would do differently next time is use only the bidirectional cloth since it drapes better over corners. I would probably go to four layers of cloth on each side to increase the stiffness that would be lost by using the bidirectional cloth instead of the uni.

I am guessing it weighs less than an equivalent strength aluminum box and it is insulated. I can turn it upside down and stand on it. I have used it as a stool (although not very comfortable.)

If this sounds like a lot of work, it is. It is messy, a little smelly and itchy. It is not cheap either. Probably similar in cost to having someone cut, bend and weld aluminum. And it certainly took longer. There were a couple of points where I thought about throwing it all away and paying someone to do the sheet metal work. Would I do it this way again? I thought I knew what I was getting into when I started. I had done some smaller work with the materials before. In my situation I would do it this way again. A lot of people would choose some other way.

[sabahtom]

Is there any point having an air gap if I'm not going to ventilate the cells? I expect no point. If I have no forced cooling there will be better heat transfer if the cells make good contact with the steel battery box (of course insulated near the terminals). That will make it a much simpler design.

I understand the only time I might want cooling is when the BMS is shunting. I think this heat will be generated inside the data collection module for each cell group, which I can place outside the battery box.

I'm not doing any long trips on the highway, probably 70kmh max for 10 mins at a time, infrequently.

Thanks
Thomas

[dougingraham]

Originally Posted by sabahtom

Is there any point having an air gap if I'm not going to ventilate the cells? I expect no point. If I have no forced cooling there will be better heat transfer if the cells make good contact with the steel battery box (of course insulated near the terminals). That will make it a much simpler design.

I understand the only time I might want cooling is when the BMS is shunting. I think this heat will be generated inside the data collection module for each cell group, which I can place outside the battery box.

The latest generation of Calb cells has gone to a flat outer surface case eliminating the ribbed case design. Presumably this would be because no cooling is needed and no need to strap the cells either.

I am not using a BMS. With all the cell testing I've done and the anecdotal evidence it seems like a waste of money. For the cost of even the cheap ones you can buy quite a few extra cells or some pretty decent test gear. The data supplied is interesting but doesn't help you drive the vehicle. An initial bottom balance and using a charger that will reliably do a CC/CV with the cutoff voltage at a point you can set seems adequate. Adding a BMS adds a lot of complexity that just isn't necessary. For example you will have a fuse on every wire at the cell connection that goes back to the BMS right? if not you are asking for a fire in the wiring when the insulation rubs through or the shunt balancer has an issue.

The LiFePo4 cells are about the easiest cells to deal with out there. Since you have enough range with them you can slightly undercharge and if you bottom balance and you have an overdischarge event you don't usually even ruin anything. You almost can't charge them too quickly from a wall outlet. A wall outlet can put out about 1500 watts. To exceed a 3C charge rate on a single cell would mean the cell would have to be smaller than a 160AH cell. With two cells in series it would have to be smaller than 80AH. With 40AH cells your pack would have to be less than 4 cells. A dryer outlet only increases that by a factor of 5 and a maxxed out 70A J1772 only doubles that so it is difficult to charge them too quickly. As long as you shut off when the voltage gets up there you are set.