Hi Larry,
I've been looking for ages for a battery that will adequately replace the one in the JBL Charge! After some research, I discovered the battery model is AEC982999-2P, which is a 3.7V 6000mAh battery. A quick Google search did not return a replacement that is the same size as the JBL Charge's.
I know it's not a very common site to use in the US, but Alibaba.com is a good place to look! Hope this helps!
Thanks
Joe