I run my ct-1/2 pack with sla batteries - one internal, and two in an external pack that I connect via a frankenmod to a parallel the main battery.

I don't have the proper Metz charger. I have adapted an old 6V ac plug pack from an old desktop calculator to feed AC to the charger terminals, and figured out that it needs about 7.2VAC in to properly charge the SLA battery ( yes, the three position swiths is set to dryfit ). So I feed the plug pack from an autotransformer feeding at higher than 120V to pump 7.2 Vac out of what was meant to be a 6V pack.