Battery box

[CapriV]

Hi all.
I'm attempting to build a battery box, well, 2 boxes that will be linked in series with 2 batteries in each box.
This will be a summer car so I would like to add a cooling function, (36c here today)
For cooling I was going to sandwich two 3/8" plates together, one with a coolant path hogged out of it and two hose barbs tapped into it.
The plates would be held together with countersunk bolts around the perimeter and sealant applied to make it water tight.
The upper plate would be the floor of the box itself and the walls would be made from 3/16 Aluminum, fully TIG welded.
Should a battery box be air tight?
I was considering some gore vents, these will let air pass through but not moisture.
Also want to add a Burst Mechanism for safety.
I also want to install the HV contactors and battery fuses inside the boxes.
Safety is something that needs to be high priority.
Anyone got any tips on battery boxes?
Thanks in advance.

[Bratitude]

allow for adequate dead space between the box walls and the modules, especially if the box will be exposed to the elements.


This also allows for some separation of coolant and modules if a leaks occurs, a drain plug is a good simple addition.

mid pack disconnects to enable safe handling and assembly.

As you said at least one fuse and contactor per box

[Zieg]

The SCCA has a set of rules for battery boxes, you could reference that in your build if you like. A pair of 3/8 plates sounds like a bit much for cooling though. Do you know what batteries you will be using? You may be able to adapt their oem cooling solution. And no it shouldn't be air tight, some kind of moisture sealed vent should be good.

Bottom half of the second page of my build thread has some pics of an SCCA-compliant battery box, and how I re-used the OEM cooling plates on my pacifica battery. viewtopic.php?t=3225&start=25

[CapriV]

Thanks for the input.
I'm using 4x a123 packs and the Thunderstruck BMS.
There's no built in cooling.
I'd like to avoid putting any mount holes through the base of the box so there's zero chance of coolant getting into the box.
This might make securing batteries a little more tricky.
I could have a center divide welded into the box and both batteries get secured to that somehow, surface mount tabs welded inside will probably be the route taken.
Only other idea might be to avoid the coolant and use fins.
The battery up front will have space underneath for fins. The battery in the trunk could have a hole cut in the floor to allow fins to be in fresh air flow.
The coolant system could be utilized when not in motion though, ie when charging, unsheltered in the blistering heat, during work hours.
Next, I need to work out what size cabling and fuse rating for each step of the system.
Anyone have any idea on what's a good starting point with wiring specs? Cheers

[Zieg]

Well, bigger is better. Do you know your peak and suatained amp draw? Length of the runs? I went with 2/0 in my Lotus 7 replica, drawing up to 350~375 amps at peak.

There's also a table of recommended wire sizes in that document I mentioned.

[CapriV]

Max amp draw with the lexus inverter is 450amps but it will only pull a fraction of that on cruise. I was considering 1 gauge wire.
My plan was to use a positive contactor inside each battery box but this will make the precharge system in a series setup more complex.
Could the negative contactor control also control the secondary positive contactor in the rear mount battery?
This way the main positive contactor in the front battery pack will have all the volts present for the precharge circuit to do its work.
Then key to start position will only be closing a single positive contactor in the front battery pack.
Next potential issue I see is charging.
When batteries are charging the contactors will be closed too? This will make the charging circuit more complex having to be wired pre contactor on both battery packs or have the secondary rear mounted battery contactor remain closed during charging.
Hmm, I may scrap this idea.