It's not just capacity to charge..
But it's also about battery accepting the charge
When you get to 80% or above the batteries any battery will take hours to get to 100%
More batteries mean more reserve and less depletion
Even having two would mean starting out around 70% maybe higher and would take about THREE hours to get to 90%
Many people due 50~90% then plug in and charge to full charge
You did not include info on the converter..charger
Please post model number etc..
If your converter is a low power model you can replace it, or buy a portable charger to use
Many of us keep a portable charger for backup use, or extra charging capacity
