RCexl states that the CDI module draws about 650ma when it is running at speed, So you can plan for around that for power consumption when deciding how much battery capacity you need to power the ignition module with. They say a 4.8v nicad pack of about 800mah capacity provides a good hot spark with the system.
<cite>ref http://www.cncengines.com/rcexelcdi.pdf</cite>

here is a discussion about power requirements with the ignition modules too.
http://www.rcuniverse.com/forum/m_10...m.htm#10527705