I've replace my own batteries, both in the crank and the PCV. I have a Cannondale SI spider so different from yours, just one battery.
Get your battery specs and Google it. I found this, first hit:
http://www.batterybob.com/product.asp?intProdID=166006
You need two. You also need a soldering iron (I use a butane one). I would recommend a voltmeter thing to verify that the battery is making the right voltage AND that once you solder it in that the SRM spider is receiving the same voltage, meaning you touch the voltmeter leads to the leads going to the circuit board and make sure the battery's full power is going through to the board.
Finally I'd recommend using a brush on electrical tape. I used it to seal the solder joints. I have heat shrink tubing also but there's no room to work with that stuff. The brush on electrical tape is great.
My own procedure detailed below. Now it's a 30-40 minute process, but the first time I was really hesitant and it took maybe 2 hours. I did both cranks I think and one, which I think I did 3 years ago? needs to be done again.
http://sprinterdellacasa.blogspot.co...r-battery.html
I also replaced the PCV battery. Much more straightforward:
http://sprinterdellacasa.blogspot.co...placement.html
Remember that the volts number is absolutely critical, at least I treated it that way. I don't understand the implications of changing the voltage but my understanding is that it screws everything up.
The mAh is a measure of how long it lasts, sort of. With the PCV I used a lower capacity battery because the full capacity one is not readily available, at least I couldn't find one. Since I plug in the PCV after every ride the lower capacity has not been an issue.
As far as measuring the slope (the number you program into the PCV head, like 20.9 or something) I found that the slope didn't change when I changed the battery. If you want to measure it there's a process for doing so. I haven't detailed it in the blog but it's here:
http://weightweenies.starbike.com/fo...hp?f=8&t=77383
I used two 25 pound weights (weighed each one separately on a digital scale allegedly good to 1g), some chain (ditto on the weight), hooks, etc. The idea was to have a non-stretching set up to hang the weights. It takes some doing to balance everything with just one person but I got my bike on my workbench, hung the weights so they hung below the workbench, and the numbers on the PCV head magically stabilized. So it works.
A week and a half is not a lot of time to get everything into the house and soldered up. Depending on your commitment to Nationals you may want to try and source another PCV head.
Hope this helps and good luck,
cdr