Hmm, I think they are on sale for $2 right now
I use nothing else, I have even checked them against the Fluke 80's at the local community college auto shop, and they are quite accurate.
Looking good, what are the chances that differing temperatures are affecting the batteries on discharge (or charging for that matter)? I know it is impractical to try and keep them the same temp in a DIY setup like this, and I know you have the front batteries in foam (the rear ones too? I forget.)
I hope you can let a camera watch them while you drive, that way your attention isn't divided.