Battery Interconnects

After procrastinating for quite some time (not to mention healing my injured arm...) it was time to figure out battery interconnects. I had originally been planning on using 3/4" wide 1/16" copper bar stock, stacked two high, as the interconnects. But putting the bend in it was a pain. And I hate drilling through thin copper. I got some hole punches, but the 1/16" copper laughed at them.

So, on to Plan B. The local metal supply store had sheet copper 0.020" thick. This turned out to be perfect - I can cut some of the weird shapes right out of the sheet. Cutting straps 3/4" wide, 7 of them would stack up to form the same cross-sectional area as 2/0 cable (0.105 in^2). After much careful measuring (and a first failed prototype - good thing I tried cutting the paper out and fitting it first!) I created a layout that fit on a 8.5" by 14" legal piece of paper:


You can find the PDF here. I printed out 7 of them, and then cut out around the outside edge and used spray adhesive to stick them to the copper sheet:


I cut around each one to make for easier handling:


Next up - the holes. This hydraulic punch did not do much with 0.0625" copper, but it sails through 0.020" copper like butter. I call it Mr. Punchy:


Here are the holes punched in one section. To reach the interior holes, I cut each piece in half. Mr. Punchy can reach pretty far in, though. It is *much* easier to line up the hole punch than it is to drill through thin sheet:


My assistant, Naiche, came by and together we used aircraft shears to cut out all 105 pieces:


Now for the tricky part. To make sure that the straps don't pry the battery posts out of the batteries, I designed a bend in each one - basically, a 3/8" by 3/8" triangle. I tried just using pliers to bend it, but it was very difficult to get a precise bend. So, time to design a tool. This is the bending jig. It is basically just a piece of plywood with holes drilled to align with each different piece. A 1/8" deep inset is routed into the surface, and a welded metal piece is screwed to the base. This piece has a small angle welded to it, on top of which a welded stamp fits:


You can find the sketchup file here. I extracted a PDF of the base and rotated it so the holes would fit on 8.5" by 11" paper (did not want to make another trip to the copy shop):


This PDF can be found here. After a pleasant afternoon of metal cutting and welding, plus wood routing and drilling, I had my bend jig:

You can see the stamp on the upper right. To bend a strap, all you do is put screws through its end holes into the holes on the jig. This keeps it aligned. Then you place the stamp over the ridge in the metal base and pound it down with a hammer:


Remove it all, and you have a nice bent piece of copper strap:


And, finally, after bending all of the pieces and stacking them up, I put heat-shrink tubing over all of them. All done:


Next up, attaching the straps, hooking up the PakTrakr and the regulator harnesses, and then finishing the wiring. So close I can almost taste it...

Warmers Installed

EDIT - DO NOT ATTEMPT THIS TECHNIQUE - IT DOES NOT WORK. See this post for why.

My assistant Naiche came by yesterday and helped me install the warmers. First step is taking most of the batteries out - the warmers fit pretty tightly and it is very difficult to shove them between the batteries. So, out they come, and in goes a warmer:


Here is the rear box, all complete (you can see the pink foam peeking out between the batteries):

I had to drill a new hole and install a grommet in order to get the cord for the middle box to come out in a useful location:


All three warmers are hooked up to this terminal strip, with the input 120V AC line hooking up as well:


Finally, installed an inlet just like the 240V inlet (except with a 120V 15A plug) and wired it up. Here it is, plugged in and warming:


Using a clamp-on AC ammeter, I determined that it is pulling about 5A, which is about 600W, which is about what I expect from the warmer cable I'm using. Not a lot of heat - but hopefully enough to maintain the batteries at a warm temperature. It's very cold out right now, so I'm warming the whole garage with a 4000W electric heater right now. I'll let the batteries stay at this temperature for about 48H and then see if the warmer can maintain them in the cold.