Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories
Ken, Micro Wizard sells battery packs for their timers. The pack holds 6 - "C" cell batteries and they're so cheap they're not worth your time making them. We start with new batteries each year and run them all day. Never had a problem. I keep 2 extras on hand in case a wire would get kicked but it hasn't happened in 20 years. I highly recommend them. I also use them in my garage on my timer for my new Best track.
I know you probably went with the C cell battery pack, but I wanted to mention something I've been doing this year. I use a USB battery bank with a special cable that converts the 5v from the USB to the 9v my timer needs. It has a 2.5mm barrel jack, so I also needed an adapter to 2.1mm. I am able to recharge it between races if needed, but since I'm using a 20,000mah battery bank, it only ever got down to 75%. That was after running 9 races, each lasting around 3 hours with the timer turned on and an average of 40 cars run per race.
You can find everything on that big website named after a river. There are also battery banks that come with the correct cable connector for a timer and can choose the output voltage. I just used what I already had and I can use the battery bank for other stuff too. The whole setup cost me about $40.
For me, I just didn't want to deal with batteries that couldn't be recharged.