This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
The Estes, Quest and other model rocket launch systems are fine for most
model rockets. If you do a lot of flying there have been some suggestions
posted to the net. If you are trying to launch cluster models with solar
igniters you will need more 'juice' than 4 AA batteries can provide. This
is also true of clustered Copperhead type igniters.
From: email@example.com (C. D. Tavares)
A motorcycle gel cell, however, will last a long, long time.
Our club uses a gel-cell the size of three VHS tapes to launch 120
rockets over six hours, and it comes home at about 80% charge.
From: firstname.lastname@example.org (Bill Nelson)
I bought a 12 volt motorcycle battery for about $20. I only need to
recharge it 3 or 4 times a year. I have adapted all my launch
controllers to allow usage of the battery.